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International audience; We continue studies of the uncertainty quantification problem in emission tomographies such aspositron emission tomography (PET) or single photon emission computed tomography (SPECT)when additional multimodal data (anatomical magnetic resonance imaging (MRI) images) are avail-able. To solve the aforementioned problem we adapt the recently proposed nonparametric posteriorlearning technique to the context of Poisson-type data in emission tomography. Using this approachwe derive sampling algorithms which are trivially parallelizable, scalable and very easy to implement.In addition, we prove conditional consistency and tightness for the distribution of produced samplesin the small noise limit (i.e., when the acquisition time tends to infinity) and derive new geometricaland necessary condition on how MRI images must be used. This condition arises naturally in thecontext of identifiability problem for misspecified generalized Poisson models with wrong design. Wealso contrast our approach with Bayesian Markov chain Monte Carlo sampling based on one dataaugmentation scheme which is very popular in the context of expectation-maximization algorithmsfor PET or SPECT. We show theoretically and also numerically that such data augmentation sig-nificantly increases mixing times for the Markov chain. In view of this, our algorithms seem to givea reasonable trade-off between design complexity, scalability, numerical load and assessment for theuncertainty

International audience; Statistical test analysis has proven itself to be versatile tool in various scientific and technical fields, following either a frequentist approach based on a $p$-value, or a Bayesian approach evaluating a Bayes factor. In this study, the authors adapted a Bayesian approach to a radiation-detection application in the industrial context of nuclear decommissioning. The detection of a weak uranium signal on concrete, under the constraint of a very low signal-to-noise ratio, represents in particular a major challenge in this application area. For this purpose, we developed an original Bayesian statistical hypothesis test based on a bivariate gamma distribution of Kibble. Said test allows merging the absolute and relative characters of two Bayesian tests developed in the same context, as well as providing better performance tradeoff in both cases of stationary and non-stationary radiological backgrounds. The simulation-based study showed that the proposed Bayesian test should meet the abovementioned expectations, and allow the detection of a relatively low surface activity uranium contamination, while ensuring a competitive tradeoff between statistical sensitivity and specificity.

International audience; BACKGROUND AND PURPOSE: Image-guided radiotherapy (IGRT) involves frequent in-room imaging sessions contributing to additional patient irradiation. The present work provided patient-specific dosimetric data related to different imaging protocols and anatomical sites. MATERIAL AND METHODS: We developed a Monte Carlo based software able to calculate 3D personalized dose distributions for five imaging devices delivering kV-CBCT (Elekta and Varian linacs), MV-CT (Tomotherapy machines) and 2D-kV stereoscopic images from BrainLab and Accuray. Our study reported the dose distributions calculated for pelvis, head and neck and breast cases based on dose volume histograms for several organs at risk. RESULTS: 2D-kV imaging provided the minimum dose with less than 1 mGy per image pair. For a single kV-CBCT and MV-CT, median dose to organs were respectively around 30 mGy and 15 mGy for the pelvis, around 7 mGy and 10 mGy for the head and neck and around 5 mGy and 15 mGy for the breast. While MV-CT dose varied sparsely with tissues, dose from kV imaging was around 1.7 times higher in bones than in soft tissue. Daily kV-CBCT along 40 sessions of prostate radiotherapy delivered up to 3.5 Gy to the femoral heads. The dose level for head and neck and breast appeared to be lower than 0.4 Gy for every organ in case of a daily imaging session. CONCLUSIONS: This study showed the dosimetric impact of IGRT procedures. Acquisition parameters should therefore be chosen wisely depending on the clinical purposes and tailored to morphology. Indeed, imaging dose could be reduced up to a factor 10 with optimized protocols.

Data are available in the Zenodo repository at https://zenodo.org/ record/6630369 and https://zenodo.org/record/6630479; International audience; From 1985 onwards, South America has undergone a major expansion of agriculture at the expense of native vegetation (e.g. native Pampa grassland). As an emblematic crop, the surface area cultivated with soybeans has increased by 1000% between 1990 and 2020 in Uruguay. The environmental consequences of this massive land use conversion on soil degradation remain poorly documented although the agriculture expansion is projected to continue to increase in the coming years in South America. In this study, sediment cores were collected in reservoirs located downstream of two contrasted agricultural catchments draining the Rio Negro River (Uruguay) for reconstructing the sediment dynamics and the sources of erosion associated with this expansion. Results demonstrated the occurrence of two periods of acceleration of sediment delivery since the 1980s. The first period of acceleration was recorded in the mid-1990s and was related to afforestation programs. The second and larger acceleration phase was recorded after 2000 during the soybean crop expansion. This period has been marked by a greater supply of sediment from the native grassland source highlighting the impact of agriculture expansion at the expense of native vegetation. Conservation measures should therefore be urgently taken to preserve biodiversity and soil functions in this region.

International audience; Background: EOSedge™a (EOS Imaging, Paris, France) is an X-ray imaging system using Automatic Exposure Control (AEC) with tube current modulation, in order to optimize dose deposition in patients.Purpose: This study aims at characterizing EOSedge organ dose deposition in comparison to a Digital Radiography (DR) system and the previous EOS system (EOS-1st generation), in relation to their respective image quality levels.Method: Organ doses were measured in an anthropomorphic female adult phantom and a 5 years old pediatric phantom using Optically Stimulated Luminescence dosimeters (OSL), which were carefully calibrated within the studied energy range. Organ doses were recorded on the EOSedge and the Fuji Visionnary DRF (Fujifilm Medical Systems U.S.A., Inc, Lexington, MA). The resulting effective doses were compared to the EOS-1st generation values present in the literature. Image quality assessment was carried out on end-user images. Quantitative image quality metrics were computed for all tested modalities on a quality assurance phantom. Qualitative assessment of EOSedge image quality was based on anthropomorphic phantom acquisitions against the EOS-1st generation system, and on clinical images against the tested DR system.Results: For a Full-Spine exam, and on the female adult phantom (resp. the pediatric phantom), an effective dose of 92 μSv (resp. 32 μSv) was obtained on EOSedge, and 572 μSv (resp. 179 μSv) on the DR system; these values were compared to effective dose values of 290 μSv (resp. 200 μSv) from the literature on EOS-1st generation, leading to an effective dose reduction factor of 6 with respect to the DR system, and of 3 to 6 with respect to EOS-1st generation. EOSedge provides the best compromise between contrast-to-noise ratio (CNR) and dose, with more consistent CNR values than the other tested modalities, in a range of attenuation from 10 cm to 40 cm of poly(methyl methacrylate) (PMMA). Within this range, EOSedge is also comparable to DR for 10 cm and 20 cm of PMMA, and better than DR for 30 cm and 40 cm of PMMA, both in terms of spatial resolution and low-contrast detection. The anatomical landmarks of interest in the follow-up of spinal deformities can be detected in all tested modalities.Conclusion: Results showed that EOSedge provides significant dose reduction factors for full spine imaging in both adults and children compared to the other tested modalities, without compromising image quality. We believe that this work could help raise awareness on the capabilities of modern X-ray systems, when equipped with appropriate AEC strategies, to perform ultra low-dose, long-axis images.

Modern external radiotherapy techniques, such as stereotactic radiotherapy which uses small radiation beams, enable to deliver precisely high doses to the tumor, while sparing adjacent healthy tissues. However, this improvement goes with increasing complexity of the treatment plans delivered and it is, therefore, necessary to ensure the safety of these treatments through adapted quality assurance. End-to-end quality assurance is a unique tool to test the whole treatment chain encountered by patients in hospitals. Dosimetric gels, which are still in the research stage, could enable a better implementation of this type of quality assurance as they are currently the only dosimeters able of measuring 3D dose distributions with high spatial resolution. Fricke-Xylenol orange-Gelatin (FXG) gel read by optical computed tomography has already been used in the literature to perform end-to-end quality assurance, however on a dose range restricted to 4 Gy, which is not adapted to stereotactic treatments. In this study, the composition of the FXG gel has been optimized and a dual-wavelength reading method has been developed in order to span an absorbed dose range representative of stereotactic treatments (0.25 - 10 Gy). This method has been characterized and validated in small irradiation fields. Then, end-to-end quality assurance tests have been implemented in a clinical setting for the treatment of intracranial tumors by stereotactic radiotherapy on the Novalis TrueBeam STx (Varian) and the CyberKnife (Accuray) accelerators. The planned and measured dose distributions have been compared using different analysis tools, such as gamma-index and superposition of isodose curves, dose profiles and dose-volume histograms. Point and planar measurements with other dosimeters have also been performed to extend this comparison. The results obtained have shown that the 3D end-to-end quality assurance method using FXG gel and dual-wavelength optical readout is accurate, appropriated for stereotactic radiotherapy and provides additional information to the quality assurance currently performed in clinical routine with more conventional dosimetric methods.; Les techniques modernes de radiothérapie externe, telles que la radiothérapie stéréotaxique qui utilise des petits faisceaux d'irradiation, permettent aujourd'hui de délivrer précisément de fortes doses à la tumeur tout en épargnant les tissus sains adjacents. Cette amélioration s'accompagne cependant d'une complexification des plans de traitement délivrés. Il est alors nécessaire de sécuriser les traitements par la mise en place de contrôles qualité adaptés à ces nouvelles techniques. Parmi ceux-ci, les contrôles qualité end-to-end permettent de tester l'ensemble de la chaîne de traitement rencontrée par les patients à l'hôpital. Les gels dosimétriques, encore à l'état de recherche, pourraient permettre une meilleure mise en place de ce type de contrôles car ils sont actuellement les seuls dosimètres capables de mesurer des distributions de dose en 3D avec une haute résolution spatiale. Le gel Fricke-Xylenol orange-Gelatin (FXG) associé à une lecture par tomographie optique a déjà été utilisé dans la littérature pour réaliser des contrôles qualité end-to-end, cependant sa gamme de dose d'utilisation était limitée à 4 Gy et ne permettait pas son utilisation pour des applications stéréotaxiques. Dans cette étude, la composition du gel FXG a été optimisée et une méthode de double lecture optique a été développée afin d'envisager l'utilisation de cette méthode dosimétrique sur une gamme de dose représentative des traitements stéréotaxiques ([0,25 ; 10] Gy). Cette méthode a été caractérisée et validée dans des petits champs d'irradiation. Plusieurs contrôles qualité end-to-end ont ensuite été mis en place en milieu clinique pour le traitement de tumeurs intracrâniennes par stéréotaxie sur les accélérateurs Novalis TrueBeam STx (Varian) et CyberKnife (Accuray). Les distributions de dose mesurées par gel et celles planifiées ont été comparées en utilisant différents outils d'analyse tels que le gamma-index et la superposition de courbes isodoses, de profils de dose et d'histogrammes dose-volume. Des mesures ponctuelles et planaires avec d'autres dosimètres ont aussi été effectuées pour compléter cette comparaison. Les résultats obtenus ont montré que la méthode de contrôle qualité 3D end-to-end par gel FXG et double lecture optique est précise, adaptée à la radiothérapie stéréotaxique et apporte des informations supplémentaires aux contrôles qualité actuellement effectués en routine clinique avec des méthodes dosimétriques plus conventionnelles.

Resume La radiotherapie guidee par l’image est devenue une technique standard d’irradiation permettant d’ameliorer le devenir clinique des patients en termes de toxicite et de controle local du fait d’un meilleur ciblage du rayonnement lors de la seance d’irradiation. Les systemes d’imagerie de positionnement, embarques ou non, tels que les kV pour les acquisitions bidimensionnelles et surtout les tomographies coniques de basse energie pour les acquisitions tridimensionnelles sont cependant irradiants dans un large volume comprenant le volume cible mais aussi du tissu sain, avec un risque theorique de majoration de toxicite et de second cancer. Il apparait donc tres important a la fois d’optimiser la dose delivree par l’imagerie de controle de type tomographies coniques de basse energie mais aussi de rapporter cette dose. Le rapport de l’AAPM (Association of Physicists in Medicine) publie en 2018 (« Image guidance doses delivered during radiotherapy : Quantification, management, and reduction ») propose une demarche d’optimisation et de gestion de ces doses. Ce rapport constitue la base de cette mise au point dont les objectifs sont de donner les ordres de grandeur des doses delivrees par l’imagerie de positionnement et de proposer une methodologie de gestion de ces doses dans la pratique clinique. Une tomographie conique de basse energie delivre approximativement 0,5 a 2 cGy a l’isocentre selon la localisation anatomique. Tant que les algorithmes de calcul ne sont pas disponibles dans les systemes de planimetrie pour les faisceaux de positionnement, il semble adapte d’utiliser au moins les ordres de grandeur publies. Cette estimation doit permettre au clinicien de decider de la strategie therapeutique en cas d’accumulation de sessions d’imagerie de positionnement.

International audience; Image-guided radiotherapy (IGRT) has become a standard irradiation technique to improve the clinical outcome of patients in terms of toxicity and local control due to better targeting of radiation during the irradiation fraction. Positioning imaging systems, whether embedded or not, such as kV for 2 x 2D acquisitions and especially kVCBCT for 3D acquisitions are however irradiating in a large volume including the target volume but also healthy tissue, with a theoretical risk of increased toxicity and second cancer. It therefore appears very important both to optimize the absorbed dose due to IGRT practice but also to report it, especially in case of kVCBCT. The AAPM report published in 2018 (<< Image guidance doses delivered duringradiotherapy: Quantification, management, and reduction >>) proposes a management of image guidance doses delivered during radiotherapy. This report is the basis of this focus article that aims at giving orders of magnitude and proposing a management of image guidance doses delivered during radiotherapy inclinical practice. The dose delivered per kVCBCT is about 0.5 to 2 cGy at isocenter according to treatment site. As long as the calculation algorithms are not available in the treatment planning systems, it seems appropriate to use at least the published dose orders of magnitude. This estimate should ultimately allow the clinician to decide on the therapeutic strategy in the event of accumulation of positioning imaging sessions. (c) 2020 Societe francaise de radiotherapie oncologique (SFRO).; La radiothérapie guidée par l’image est devenue une technique standard d’irradiation permettant d’améliorer le devenir clinique des patients en termes de toxicité et de contrôle local du fait d’un meilleur ciblage du rayonnement lors de la séance d’irradiation. Les systèmes d’imagerie de positionnement, embarqués ou non, tels que les kV pour les acquisitions bidimensionnelles et surtout les tomographies coniques de basse énergie pour les acquisitions tridimensionnelles sont cependant irradiants dans un large volume comprenant le volume cible mais aussi du tissu sain, avec un risque théorique de majoration de toxicité et de second cancer. Il apparaît donc très important à la fois d’optimiser la dose délivrée par l’imagerie de contrôle de type tomographies coniques de basse énergie mais aussi de rapporter cette dose. Le rapport de l’AAPM (Association of Physicists in Medicine) publié en 2018 (« Image guidance doses delivered during radiotherapy : Quantification, management, and reduction ») propose une démarche d’optimisation et de gestion de ces doses. Ce rapport constitue la base de cette mise au point dont les objectifs sont de donner les ordres de grandeur des doses délivrées par l’imagerie de positionnement et de proposer une méthodologie de gestion de ces doses dans la pratique clinique. Une tomographie conique de basse énergie délivre approximativement 0,5 à 2 cGy à l’isocentre selon la localisation anatomique. Tant que les algorithmes de calcul ne sont pas disponibles dans les systèmes de planimétrie pour les faisceaux de positionnement, il semble adapté d’utiliser au moins les ordres de grandeur publiés. Cette estimation doit permettre au clinicien de décider de la stratégie thérapeutique en cas d’accumulation de sessions d’imagerie de positionnement.

International audience; Determination of actinide isotopic composition is one of the most important tasks accomplished under IAEA safeguards treaties for verification and inspection activities. Spectrometers based on HPGe detectors have traditionally been the detectors of choice for such systems with a large array of various data analysis codes developed specifically to suit the performance of these detectors. Over the past decade in this domain there has been interest in using medium resolution room temperature detectors based on CdZnTe and LaBr$_3$(Ce) crystals. Moreover, progress in analytical tools that could be applied to analyze worsened statistical quality spectral data has opened a wide range of possible applications where such compact detectors and absence of cryogenics are advantageous. In this paper we aim to provide a review on the recent advances in the domain of isotopic composition determination systems used as with traditional HPGe detectors as well as with medium resolution ones. We limit ourselves to the achievements in the European Union and divide our review in three parts. First, we provide an update on the current status of the French code IGA aimed at applications with traditional HPGe systems. Second, we present the recent results in the domain of isotopic composition determination codes development for medium resolution detectors, namely the codes MCSIGMA and CAMILA developed in the framework of a PhD project in Belgium. Third, we give an outlook on the current research activities conducted at CEA List Saclay in the domain of isotopic composition determination algorithms applied to medium resolution detectors.

International audience; The MAGIX code (a French acronym standing for Automatic Gamma and X-ray Measurement) is a software developed to analyze γ/X spectra on the topic of severe accident diagnosis. Indeed, the gamma spectra obtained after a severe reactor core accident are complex because they are composed of hundreds of lines of short-lived fission products and Fukushima accident demonstrated a lack in robustness of data interpretation during a crisis. MAGIX allows a complete and entirely automatic analysis of the spectra, with identification of radionuclides and calculation of activities. It can analyze spectra measured by detectors with excellent resolution such as HPGe detectors as well as detectors with medium resolution (e.g. CZT and LaBr3). For most detectors, the analysis of the spectra can be done without a detection efficiency curve because its process can include the calculation of a relative detection efficiency. MAGIX accepts spectra corresponding to any experimental setup (energy slope, energy range, resolution, absorber, etc.). However, these experimental conditions can have an impact on the quality of the results. Results on spectra simulated in different configurations showed that the analysis of the HPGe spectrum with the user defined efficiency and with the MAGIX detection efficiency were close. Furthermore, they also showed that the accuracy of activities was similar with increasing energy slopes but decreased with resolution degradation, with fewer correctly identified radionuclides in this case.

International audience; The pseudo-deterministic (or simply deterministic) transport method is used in Monte Carlo particle transport problems to increase the sampling of a region in space that particles have a low probability of reaching. Although it has been used by many authors over the years, mainly due to its implementation in the code MCNP which itself has been extensively validated, to our knowledge, a proof of the unbiasedness of the method has never been published.This article thus provides a comprehensive mathematical description of the pseudo-deterministic transport method, built from simple transformations of a given random variable representing an arbitrary physical quantity of interest. Some considerations are made for the handling of potential secondary particles created during the interaction process, and of specific estimators such as pulse height or energy spectrum.This description should allow for a better understanding of the technique, including its possible uses and limitations, and can be used as a reference by both simulation code users and developers looking to implement pseudo-deterministic transport in their code.

The uncertainty of reconstructed PET images remains difficult to assess and to interpret for the use in diagnostic and quantification tasks. Here we provide (1) an easy-to-use methodology for uncertainty assessment for almost any Bayesian model in PET reconstruction from single datasets and (2) a detailed analysis and interpretation of produced posterior image distributions. We apply a recent posterior bootstrap framework to the PET image reconstruction inverse problem and obtain simple parallelizable algorithms based on random weights and on existing maximum a posteriori (MAP) (posterior maximum) optimization-based algorithms. Posterior distributions are produced, analyzed and interpreted for several common Bayesian models. Their relationship with the distribution of the MAP image estimate over multiple dataset realizations is exposed. The coverage properties of posterior distributions are validated. More insight is obtained for the interpretation of posterior distributions in order to open the way for including uncertainty information into diagnostic and quantification tasks.

In the context of reducing the patient dose coming from CT scanner examinations without penalizing the diagnosis, the assessment of both patient dose and image quality (IQ) with relevant metrics is crucial. The present study represents the first stage in a larger work, aiming to compare and optimize CT protocols using dose and IQ new metrics. We proposed here to evaluate the capacity of the Non-PreWhitening matched filter with an eye (NPWE) model observer to be a robust and accurate estimation of IQ. We focused our work on two types of clinical tasks: a low contrast detection task and a discrimination task. We designed a torso-shaped phantom, including Plastic Water® slabs with cylindrical inserts of different diameters, sections and compositions. We led a human observer study with 13 human observers on images acquired in multiple irradiation and reconstruction scanning conditions (voltage, pitch, slice thickness, noise level of the reconstruction algorithm, energy level in dual-energy mode and dose), to evaluate the behavior of the model observer compared to the human responses faced to changing conditions. The model observer presented the same trends as the human observers with generally better results. We rescaled the NPWE model on the human responses by scanning conditions (kVp, pitch, slice thickness) to obtain the best agreement between both observer types, estimated using the Bland-Altman method. The impact of some scanning parameters was estimated using the correct answer rate given by the rescaled NPWE model, for both tasks and each insert size. In particular, the comparison between the dual-energy mode at 74 keV and the single-energy mode at 120 kVp showed that, if the 120 kVp voltage provided better results for the smallest insert at the lower doses for both tasks, their responses were equivalent in many cases.

In vivo dosimetry for patient radioprotection is a real challenge concerning new radiotherapy treatments. In this study we aim to develop a dosimeter based on tissue equivalent material, flexible and adaptable to the patient morphology in order to perform in vivo dosimetry for complex irradiation beams. Here, we report the evaluation in standard beams of highly flexible dosimeter composed of a silicone elastomer and containing leucomalachite green as radiochromic dye and give a first estimation of LMG-micelle hydrogel response. All results are compared to a commercially available PRESAGE® dosimeter.

International audience; In this article, we aim at measuring a low-activity uranium contamination deposited on concrete surfaces of a nuclear facility and presenting varying enrichment levels. Considering this application field, CEA LIST, has developed an original approach, combining gamma-ray spectrometry based on high-purity germanium (HPGe) measurements and specific Bayesian algorithms. This methodology gives access to an indirect surface activity estimation, assuming that the ratio between the number of alpha particles to be quantified and the number of detected gamma rays is known. The Bayesian approach characteristic property is able to inject a representation of the physical context in the form of a probabilistic a priori. It enables to improve the tradeoff between the true detection rate (TDR) and the false alarm rate (FAR) at low count rates and takes benefit of a richer time-energy information structure than the algorithms used in the conventional detection procedures. The performance evaluation and characterization of Bayesian statistical tests are performed using classical receiver-operating characteristic (ROC) curves by comparison to frequentist hypothesis tests. Results indicate that the Bayesian approach has a superior detection performance for low-activity uranium contamination compared to the frequentist approach. The estimated gain is contained between 30% and 50%, considering a variable or stable radiological background. The Bayesian approach offers the best tradeoff between the TDR, FAR, and the response time and is compatible with the user's requirements.

International audience; The constant increase of computed tomography (CT) exams and their major contribution to the collective dose led to international concerns regarding patient dose in CT imaging. Efforts were made to manage radiation dose in CT, mostly with the use of the CT dose index (CTDI). However CTDI does not give access to organ dose information, while Monte Carlo (MC) simulation can provide it if detailed information of the patient anatomy and the source are available. In this work, the X-ray source and the geometry of the GE VCT Lightspeed 64 were modelled, based both on the manufacturer technical note and some experimental data. Simulated dose values were compared with measurements performed in homogeneous conditions with a pencil chamber and then in CIRS ATOM anthropomorphic phantom using both optically stimulated luminescence dosimeters (OSLD) for point doses and XR-QA Gafchromic® films for relative dose maps. Organ doses were ultimately estimated in the ICRP 110 numerical female phantom and compared to data reported in the literature. Comparison of measured and simulated values show that our tool can be used for a patient specific and organ dose oriented radiation protection tool in CT medical imaging.

International audience; Nuclear decommissioning takes places after dismantling, and consists of studying the possible presence of any residual contamination. To do so, every surface inside the facility must be radiologically characterized, and every contamination removed. In the case of an alpha contamination linked with a low gamma activity, using a High-Purity Germanium detector (HPGe) combined with hypothesis tests on the count statistics can be helpful to assess said contamination. Gamma-ray activity enables to determine the total contamination activity provided both are related, like in the case of an uranium-based contamination. Hypothesis tests determine the statistical power of the measurement, allowing decision making when dealing with countings below the detection limit. A representation of the tests results using Receiver Operating Characteristics (ROC) curves allow the user to select an appropriate time of measurement, true detection and false alarm rates, in accordance to the required specifications. This paper presents the feasibility study of such a method, applied to the detection of a low-activity gamma surface contamination of uranium on concrete, with varying enrichment levels.

Introduction This study compares the performances of different dose calculation algorithms in heterogeneous medium. Then, the properties of two algorithms on clinical cases are considered for the thoracic cases in conformal 3D radiotherapy (RC3D). Methods (1) A study on the CIRS phantom was performed by comparing the dose distributions measured with Gafchromic EBT3® with those calculated by the AAA [Analytical Anisotropic Algorithm v13, Varian], AXB [Acuros XB v13.7, Varian], PB [Pencil Beam v4.5.5, Brainlab], MC [Monte Carlo v4.5.5, Brainlab] and CCV [Collapsed Cone Volume v2.0.1, Mobius Medical System]. The performances of these different algorithms were tested using an experimental design of Taguchi L 18 to demonstrate the influence of the following factors, in 18 tests: energy, collimator angle, gantry angle, X and Y field size. A comparison by subtracting the dose distributions between those measured and computed was achieved for the 18 tests. (2) The impact of the selection of the algorithm was performed on treatment plans initially calculated with AAA, on a set of 15 patients with thoracic cancer in RC3D. By keeping the same UM number, these treatment plans were compared in terms of coverage of the 95% PTV, a conformity index, doses to the PTV and organs at risk. If necessary, the planimetry was modified in AXB (weighting, filtering and normalization point) in order to validate the treatment plan clinically. Results (1) In heterogeneous medium, the average dose differences between the measured and calculated doses are, respectively, for the AXB, AAA, CCV, MC and PB: 0.01.8%, 1.22.2%, −0.12.0%, 1.21.9% and 7.04.1%. (2) For treatment plans with similar MU numbers, an average dose difference of −1.2% in the level of the 95% coverage of the PTV and a modification of conformity index from 1.10 (AAA) to 0.76 (AXB) are obtained, attesting to a lower coverage of the PTV with AXB, related to a difference in the evaluation of heterogeneities. Once these treatment plans were modified, the resulting planimetry was similar to the clinically validated one, with a mean difference of MU greater than 1.5% against treatment plans calculated with AAA. Conclusions The best performances are shown with the MC and the AXB algorithms. The experimental design allows reliable comparison of the algorithms with 18 tests. The PEX highlights bigger deviations for small field modelling. Regarding the dosimetry study, AXB can be used for RC3D thoracic cancer treatment. A complementary study for the same localization is ongoing for the VMAT treatment technique.

International audience; Radionuclide identification and quantification are a serious concern for many applications as for in situ monitoring at nuclear facilities, laboratory analysis, special nuclear materials detection, environmental monitoring, and waste measurements. High resolution gamma-ray spectrometry based on high purity germanium diode detectors is the best solution available for isotopic identification. Over the last decades, methods have been developed to improve gamma spectra analysis. However, some difficulties remain in the analysis when full energy peaks are folded together with high ratio between their amplitudes, and when the Compton background is much larger compared to the signal of a single peak. In this context, this study deals with the comparison between a conventional analysis based on “iterative peak fitting deconvolution” method and a “nonparametric Bayesian deconvolution” approach developed by the CEA LIST and implemented into the SINBAD code. The iterative peak fit deconvolution is used in this study as a reference method largely validated by industrial standards to unfold complex spectra from HPGe detectors. Complex cases of spectra are studied from IAEA benchmark protocol tests and with measured spectra. The SINBAD code shows promising deconvolution capabilities compared to the conventional method without any expert parameter fine tuning.

International audience; In this article, we present some specific aspects of symmetric Gamma process mixtures for use in regression models. First we propose a new Gibbs sampler for simulating the posterior. The algorithm is tested on two examples, the mean regression problem with normal errors, and the reconstruction of two dimensional CT images. In a second time, we establish posterior rates of convergence related to the mean regression problem with normal errors. For location-scale and location-modulation mixtures the rates are adaptive over Holder classes, and in the case of location-modulation mixtures are nearly optimal.

International audience; Purpose or Objective: While in-room Magnetic Resonance Imaging starts becoming part of radiotherapy (RT) treatments, the use of X-ray imaging equipment in Image-Guided RT (IGRT) is still growing and with it the need to evaluate the additional dose-to-organs it delivers. This study aims atverifying the accuracy of Monte Carlo (MC) calculation of the patient dose-to-organs delivered by four commercially available kV imaging systems: the XVI CBCT (Elekta), the OBI CBCT (Varian), the ExacTrac 2D-kV system (Brainlab) and the 2D-kV CyberKnife imaging system (Accuray). Simulations were validated against OSL measurements in the pediatric anthropomorphic phantom Grant (CIRS, ATOM) performed in three different clinical sites.Material and Methods: Each of the four kV-imaging systems was modeled as a Virtual Source Model (VSM) using the Penelope MC code. Such models were validated as part of a previous study using ionization chambers in water phantoms [G. Boissonnat et al., ESTRO 2017 Vienna]. In a second step, CT images of the phantom Grant were used to generate a voxelized phantom by converting the HU value of each voxel into the appropriate biological tissue (chemical composition and density). Then for each system, photons produced by the corresponding VSM were propagated in the voxelized phantom in order to obtain the 3D relative absorbed dose-to-medium map for three localizations (head, thorax and pelvis). MC-calculateddoses were calibrated in amplitude using the ratio between the air kerma measured with an ionization chamber at the isocenter and the corresponding simulated value. After calibrating OSLs in air kerma at every beam quality, OSL measurements were performed in the anthropomorphic phantom at three localizations (head and neck, thorax and pelvis). After verifying that beam quality inside the phantom was impacting OSL corrections factors of less than 5%, they were neglecting. Therefore measured air kerma values were converted into absorbed dose-in-medium values using the incoming beam quality before being compared to simulated dose values.Results: MC calculations were performed in 2 hours on a cluster of 40 CPUs with a MC uncertainty better than 5% in 1mm3 voxels. The current study highlights the possibility to reproduce absolute dose measurements using VSM-driven MC simulations with an overall agreement better than 20 % (inside the irradiation field) for all four kV imaging systems and for the three anatomical localizations as presented in Table 1.Conclusion: This study demonstrates that MC calculations based on VSMs allow obtaining reliable absolute doses for kV imaging protocols in a reasonable computing time. All these developments are currently integrated into a dedicated software for imaging dose prediction, which will also include the Tomotherapy MVCT imaging system [V. Passal et al., MCMA 2017 Napoli].This software will enable to study the magnitude of additional doses delivered by in-room X-Ray imaging positioning units during the course of a complete RT treatment.

International audience; Introduction: The delivery of complex plans is usually included in an image-guided radiotherapy (IGRT) process that is based on a daily in-room imaging session in order to adjust the tumor position. IGRT strategies lead to deliver additional absorbed doses to treated patients due to these imaging sessions. The aim of this study was to calculate the tridimensional dose distribution due to Mega Voltage Computed Tomography (MVCT) imaging sessions with Monte Carlo (MC) simulations for Tomotherapy (Accuray) courses.Material and methods: MC modelling of the MVCT with the code GATE/GEANT4 was extensively evaluated by comparing measurements and simulations for percent depth dose and off-axis profiles in water and water-equivalent material for static and rotational irradiations, using two fan beam widths. In the isocenter plane, beam dimensions were 4x400mm2 and 50x400mm2. In addition, measured and MC computed absorbed doses were compared for a helical irradiation delivered to the STereotactic End-to-End Validation anthropomorphic phantom (STEEV, CIRS). Depending on the irradiation settings and the material, dose measurements were performed with an ionization chamber, a diamond detector, radiochromic films or optically stimulated luminescent dosimeters (OSL).Results: For depth dose profiles acquired with static irradiations, simulations and measurements agreed within 3% for depths varying from 0 to 12cm for both beam widths, but reached 6% at a 20cm depth for the 4mm wide beam. For off-axis profiles acquired with static irradiations, simulations and measurements agreed within 3% in high dose region the longitudinal axis. Discrepancies were slightly higher in the lateral axis. Simulated and measured profiles obtained for a 360° irradiation were also in good agreement. Absorbed doses measured with 27 OSL dosimeters in the STEEV phantom for a 12cm long helical irradiation varied from 13 to 16mGy according to the position. MC calculated doses in the same conditions differed by less than 10%.Conclusion: The MC modelling of the Tomotherapy MVCT allows to calculate tridimensional absorbed dose distributions in patients. This model can now be used to quantify the absorbed doses due to imaging sessions during Tomotherapy courses and to evaluate the dosimetric impact of different IGRT strategies.

International audience; The scope of this work was to develop a synthetic single crystal diamond dosimeter (SCDD-Pro) for accurate relative dose measurements of clinical proton beams in water. Monte Carlo simulations were carried out based on the MCNPX code in order to investigate and reduce the dose curve perturbation caused by the SCDD-Pro. In particular, various diamond thicknesses were simulated to evaluate the influence of the active volume thickness (e(AV)) as well as the influence of the addition of a front silver resin (250 µm in thickness in front of the diamond crystal) on depth-dose curves. The simulations indicated that the diamond crystal alone, with a small eAV of just 5 µm, already affects the dose at Bragg peak position (Bragg peak dose) by more than 2% with respect to the Bragg peak dose deposited in water. The optimal design that resulted from the Monte Carlo simulations consists of a diamond crystal of 1 mm in width and 150 µm in thickness with the front silver resin, enclosed by a water-equivalent packaging. This design leads to a deviation between the Bragg peak dose from the full detector modeling and the Bragg peak dose deposited in water of less than 1.2%. Based on those optimizations, an SCDD-Pro prototype was built and evaluated in broad passive scattering proton beams. The experimental evaluation led to probed SCDD-Pro repeatability, dose rate dependence and linearity, that were better than 0.2%, 0.4% ( in the 1.0-5.5 Gy min(-1) range) and 0.4% ( for dose higher than 0.05 Gy), respectively. The depth-dose curves in the 90-160 MeV energy range, measured with the SCDD-Pro without applying any correction, were in good agreement with those measured using a commercial IBA PPC05 plane-parallel ionization chamber, differing by less than 1.6%. The experimental results confirmed that this SCDD-Pro is suitable for measurements with standard electrometers and that the depth-dose curve perturbation is negligible, with no energy dependence and no significant dose rate dependence.

International audience; Purpose or Objective: Monte Carlo (MC) simulations were shown to be a powerful tool to calculate accurately 3D dose distributions of kV-CBCT scans for a patient, based on planning CT images. However, this methodology is still heavy and time consuming, preventing its large use in clinical routine. This study hence explores a method to derive empirical functions relating organ doses to patient morphological parameters, in order to perform a fast and personalized estimation of doses delivered to critical organs by kV-CBCT scans used in IGRT protocols.Material and Methods: Doses to critical organs were first computed using a PENELOPE-based MC code previously validated [H. Chesneau et al., ESTRO 2016], for a set of fifty clinical cases (40 children and 10 adults) covering a broad range of anatomical localizations (head-and-neck, pelvis, thorax, abdomen) and scanning conditions for the Elekta XVI CBCT. Planning CT images were converted into voxellized patient geometries, using a dedicated tissue segmentation procedure: 5 to 7 biological tissues were assigned for soft tissues, whereas ten different bone tissues were required for accurate dosimetry in the kV energy range. Correlations between calculated mean organ doses and several morphological parameters (age, weight, height, BMI, thorax and hip circumference …) were then studied for each anatomical localization to derive appropriate empirical fitting functions.Results: As expected, results on the paediatric cohort show dose variations highly correlated with the patient morphology, varying in the range 3:1 between a 17-y old teenager and a 2-y old baby, for the same CBCT scan. Except for the head-and-neck localization, for which the mean organ doses show no significant variations with the morphology, doses to all major organs at risk can be predicted using linear or exponential functions for thorax, pelvis and abdomen scans. The use of morphological parameters directly measured on the planning CT allows to reach better correlations than global parameters such as BMI, because they represent most relevant indicators of the patient morphology at the scan time.Conclusion: This study demonstrates that it is possible to derive mathematical models predicting the doses delivered to major critical organs by kV-CBCT scans according to morphological parameters. This method allows a fast and personalized estimation of imaging doses usable in clinicalroutine.

International audience; Purpose or Objective: With the growing use of X-ray imaging equipment in Image-Guided RadioTherapy (IGRT), the need to evaluate the dose-to-organs delivered by kV-CBCT imaging acquisition increases. This study aims to propose accurate Monte Carlo (MC) calculations of the patient dose-to-organs delivered by two commercially available kV-CBCT systems: the XVI from Elekta’s VERSA HD accelerator and the OBI from Varian’s TrueBeam system. Simulations are to be validated using in phantom OSL measurements.Material and Methods: For both kV-CBCT systems, the kV irradiation head geometry was implemented in the MC simulation code Penelope. As a first step, the resulting photon distributions were expressed as Virtual Source Models (VSM) for every standard irradiation condition (kVp,filtration, collimation); it was then validated and adjusted using in water-phantom measurements performed with a calibrated Farmer-type ionization chamber. In a second step, the validated VSMs were used to simulate the dose delivered by both the XVI and OBI systems in anthropomorphic phantoms, using standard clinical imaging protocols. Simulated dose-to-organs were then confronted to dose measurements performed using OSL inserted into the same phantoms, following a dosimetric protocol for OSLs previously established [1]. In addition, VSM results were confronted to their direct MC counterparts in order to evaluate the benefit of using such technique.Results: The current study highlights the possibility to reproduce OSL dose-to-organ measurements using VSM-driven Monte Carlo simulation with an overall agreement better than 20 %. In addition, the use of VSM in the MC simulation enables to speed-up the calculation time by a factor better than two (for the same statistical uncertainty) compared to direct MC simulation. Nevertheless, if direct and VSM calculations are in agreement inside the irradiation field, outside, VSM results tend to be significantly lower (10-30%).Conclusion: The use of a VSM was demonstrated to simplify and fasten MC simulations for personalized kV-CBCT MC dose estimation. In addition, OSLs enable to perform the low dose measurement in the kV range needed for in phantom X-ray imaging equipment dose QA. This study is to be completed in the near future by the addition of other standard X-ray imaging equipment dedicated to IGRT.

Purpose Nanoparticles appear as a novel tool to enhance the effectiveness of radiotherapy in cancer treatments. Many parameters influence their efficacy, such as their size, concentration, composition, their cellular localization, as well as the photon source energy. The current Monte Carlo study aims at comparing the dose-enhancement in presence of gadolinium (Gd), either as isolated atoms or atoms clustered in nanoparticles (NPs), by investigating the role played by these physical parameters at the cellular and the nanometer scale. In parallel, in vitro assays were performed in presence of either the gadolinium contrast agent (GdCA) Magnevist® or ultrasmall gadolinium NPs (GdNPs, 3 nm) for comparison with the simulations. Methods PENELOPE Monte Carlo Code was used for in silico dose calculations. Monochromatic photon beams were used to calculate dose-enhancements in different cell compartments and low-energy secondary electron spectra dependence with energy. Particular attention has been placed on the interplay between the X-ray beam energy, the Gd localization and its distance from cellular targets. Clonogenic assays were used to quantify F98 rat glioma cell survival after irradiation in the presence of GdNPs or GdCA, using monochromatic X-rays with energies in the 30 keV-80 keV range from a synchrotron and 1.25 MeV gamma photons from a cobalt-60 source. The simulations that correspond to the experimental conditions were compared with the experimental results. Results In silico, a highly heterogeneous and clustered Gd-atom distribution, a massive production of low energy electrons around GdNPs and an optimal X-ray beam energy, above the Gd K-edge, were key factors found to increase microscopic doses, which could potentially induce cell death. The different Gd localizations studied all resulted in a lower dose enhancement for the nucleus component than for cytoplasm or membrane compartments, with a maximum dose-enhancement factor (DEF) found at 65 keV and 58 keV, respectively. In vitro, radiosensitization was observed with GdNPs incubated 5h with the cells (2.1 mg Gd/mL) at all energies. Experimental DEFs were found to be greater than computational DEFs but follow a similar trend with irradiation energy. However, an important radiosensitivity was observed experimentally with GdNPs at high energy (1.25 MeV), whereas no effect was expected from modeling. This effect was correlated with GdNPs incubation time. In vitro, GdCA provided no dose-enhancement at 1.25 MeV energies, in agreement with computed data. Conclusions These results provide a foundation on which to base optimizations of the physical parameters in Gd radiation-enhanced therapy. Strong evidence was provided that GdCA or GdNPs could both be used for radiation dose-enhancement therapy. Their in vivo biological distribution, in the tumor volume and at the cellular scale, will be the key factor for providing large dose-enhancements and determine their therapeutic efficacy. This article is protected by copyright. All rights reserved.

International audience; In this study, the impact of radiochromic films (XR-RV3) calibration on PSD measurements was investigated under various peak kilovoltage (kVp) and additional filtration conditions. Films were calibrated free-in-air for six beam qualities with Allura Xper FD20 system (Philips). Six calibration curves (CCs) were constructed. Each beam quality was characterized in terms of mean energy (ME) in the air, with table, with table and water phantom using Monte Carlo simulations. A cohort of 155 patient films from cardiology (37) and vascular (118) procedures were read with each CC. Routine calibration beam quality was taken as reference (DoseNorm). Overall, it was observed that for a wider ME difference between the exposed film and the CC used, a larger deviation (from -28% to +41%) was observed. The choice of beam quality for the calibration is a key point when additional filtration and kVp are automatically controlled in clinical conditions.

International audience; We estimate the quantum state of a light beam from results of quantum homodyne tomography noisy measurements performed on identically prepared quantum systems. We propose two Bayesian nonparametric approaches. The first approach is based on mixture models and is illustrated through simulation examples. The second approach is based on random basis expansions. We study the theoretical performance of the second approach by quantifying the rate of contraction of the posterior distribution around the true quantum state in the L-2 metric.

International audience; Nuclear instruments such as alpha/beta contamination meter are frequently used in a compensated mode where the contribution of gamma radiation background is compensated by a guard detector. The signal of interest is then the subtraction of counting from both channels. In practice, the noise signal measured by the guard detector is not strictly equal to the noise contribution into the first detector due to anisotropic biases.The random error (under Poisson assumption) is taken into account to build a hypothesis test. The system is also designed to minimize the systematic error but in some cases, this bias could not be completely removed. The measurement system then shows different behavior when the surrounding environment changes exhibiting inopportune false alarms.A method allowing the false alarms to be suppressed is addressed in this study for compensated measurement. An improvement in terms of reliability has been proven.

International audience; Small diamond detectors are useful for the dosimetry of high-energy proton beams. However, linear energy transfer (LET) dependence has been observed in the literature with such solid state detectors. A novel synthetic diamond detector has recently become commercially available from the manufacturer PTW-Freiburg (PTW microDiamond type 60019). This study was designed to thoroughly characterize four microDiamond detectors in clinical proton beams, in order to investigate their response and their reproducibility in high LET regions. Very good dosimetric characteristics were observed for two of them, with good stability of their response (deviation less than 0.4% after a pre-irradiation dose of approximately 12 Gy), good repeatability (coefficient of variation of 0.06%) and a sensitivity of approximately 0.85 nC Gy(-1). A negligible dose rate dependence was also observed for these two microDiamonds with a deviation of the sensitivity less than 0.7% with respect to the one measured at the reference dose rate of 2.17 Gy min(-1), in the investigated dose rate range from 1.01 Gy min(-1) to 5.52 Gy min(-1). Lateral dose profile measurements showed the high spatial resolution of the microDiamond oriented with its stem perpendicular to the beam axis and with its small sensitive thickness of about 1 mu m in the scanning profile direction. Finally, no significant LET dependence was found with these two diamond dosimeters in comparison to a reference ionization chamber (model IBA PPC05). These good results were in accordance to the literature. However, this study showed also a non reproducibility between the devices in terms of stability, sensitivity and LET dependence, since the two other microDiamonds characterized in this work showed different dosimetric characteristics making them not suitable for proton beam dosimetry with a maximum difference of the peak-to-plateau ratio of 6.7% relative to the reference ionization chamber in a clinical 138 MeV proton beam.

International audience; This work aims at developing a generic virtual source model (VSM) preserving all existing correlations between variables stored in a Monte Carlo pre-computed phase space (PS) file, for dose calculation and high-resolution portal image prediction. The reference PS file was calculated using the PENELOPE code, after the flattening filter (FF) of an Elekta Synergy 6 MV photon beam. Each particle was represented in a mobile coordinate system by its radial position (r(s)) in the PS plane, its energy (E), and its polar and azimuthal angles (phi(d) and theta(d)), describing the particle deviation compared to its initial direction after bremsstrahlung, and the deviation orientation. Three sub-sources were created by sorting out particles according to their last interaction location (target, primary collimator or FF). For each sub-source, 4D correlated-histograms were built by storing E, r(s), phi(d) and theta(d) values. Five different adaptive binning schemes were studied to construct 4D histograms of the VSMs, to ensure histogram efficient handling as well as an accurate reproduction of E, r(s), phi(d) and theta(d) distribution details. The five resulting VSMs were then implemented in PENELOPE. Their accuracy was first assessed in the PS plane, by comparing E, r(s), phi(d) and theta(d) distributions with those obtained from the reference PS file. Second, dose distributions computed in water, using the VSMs and the reference PS file located below the FF, and also after collimation in both water and heterogeneous phantom, were compared using a 1.5%-0 mm and a 2%-0 mm global gamma index, respectively. Finally, portal images were calculated without and with phantoms in the beam. The model was then evaluated using a 1%-0 mm global gamma index. Performance of a mono-source VSM was also investigated and led, as with the multi-source model, to excellent results when combined with an adaptive binning scheme.

International audience; Purpose or Objective: The use of Cone-Beam computed Tomography (CBCT) is progressively increasing in radiotherapy treatments, but additional doses induced are not well quantified and could amplify the risk for patients to develop a second cancer. There is a need, expressed by the medical physics community, to develop tools to estimate, report and potentially help reducing CBCT doses. We hence developed a Monte Carlo (MC) model for the XVI kV-CBCT system. The dosimetric and geometric accuracy of the simulated beams was evaluated by comparisons withmeasurements in a water tank, and x-ray energy spectra acquisitions. Before clinical use, the model requires an evaluation in anthropomorphic phantoms in which were inserted OSL NanoDots (Landauer). The purpose of the present study is to develop an accurate dosimetric protocoltaking into account for OSL energy dependence in keV energy range.Material and Methods: A MC model of the XVI was developed using the PENELOPE code. The dosimetric and geometrical evaluation of the beam MC models was performed by comparing simulations with lateral and depth-dose profiles measured using a PTW Farmer-type chamber, and on-axis energy spectra measured with a CdTe detector. These comparisons were performed at 120, 100 and 80 kVp, and for different filtration/collimation couples. For OSL measurements, the first step was to perform, in different beam qualities, in-air cross-calibrations with a PTW Farmer type chamber. At this energy range, OSL exhibit strong energy dependence, so the signal needs to be corrected for the spectral variations between calibration and measurement conditions. Thus, to ensure accurate dose measurements, a correction method was developed using calculated spectra. The dosimetric protocol was validated by performing dose profiles with OSL inserted in a PMMA tube submerged in water. Preliminary comparisons with XVI model were made with acquisitions in a home-made heterogeneous phantom consisting of a water tank equipped with PMMA, bone and lung equivalent inserts.Results: Experimental and simulated lateral and depth-dose profiles, and energy spectra, are in excellent agreement (Fig 1A).These results validate that the MC model accurately reproduces the dosimetric and geometric properties of the XVI beams. The uncorrected OSL profiles in the PMMA tube over-estimate by 15 % the calculated doses. However, energy corrected measurements are matching the simulations and the differences not exceed 7.5 % (Fig 1B and 1C). Table 1 presents doses measured at different points in the heterogeneous phantom and discrepancies not exceed 11.3 %.Conclusion: The dosimetric protocol developed for OSL allows accurate measurements of imaging doses, and will be then used to validate the dose calculation tool in pre-clinical conditions. Preliminary results obtained in the home-made phantom highlight the accuracy of XVI MC model. Further validations are on-going in anthropomorphic phantoms.

International audience; Positron range impairs resolution in PET imaging, especially for high-energy emitters and for small-animal PET. De-blurring in image reconstruction is possible if the blurring distribution is known. Furthermore, the percentage of annihilation events within a given distance from the point of positron emission is relevant for assessing statistical noise. This paper aims to determine the positron range distribution relevant for blurring for seven medically relevant PET isotopes, $^{18}$F, $^{11}$C, $^{13}$N, $^{15}$O, $^{68}$Ga, $^{62}$Cu and $^{82}$Rb, and derive empirical formulas for the distributions. This paper focuses on allowed-decay isotopes. It is argued that blurring at the detection level should not be described by the positron range $r$, but instead the 2D projected distance $\delta$ (equal to the closest distance between decay and line of response). To determine these 2D distributions, results from a dedicated positron track-structure Monte Carlo code, Electron and POsitron TRANsport (EPOTRAN), were used. Materials other than water were studied with PENELOPE. The radial cumulative probability distribution G$_{2D}$($\delta$) and the radial probability density distribution $g_{2D}$($\delta$) were determined. G$_{2D}$($\delta$) could be approximated by the empirical function 1 - exp(-$A \delta^2$ - $B \delta$), where A = 0.0266 ($E_{mean}$)$^{-1.716}$ and B = 0.1119 ($E_{mean}$)$^{-1.934}$, with $E_{mean}$ being the mean positron energy in MeV and $\delta$ in mm. The radial density distribution $g_{2D}$($\delta$) could be approximated by differentiation of G$_{2D}$($\delta$). Distributions in other media were very similar to water. The positron range is important for improved resolution in PET imaging. Relevant distributions for the positron range have been derived for seven isotopes. Distributions for other allowed-decay isotopes may be estimated with the above formulas.

In Pressurized Water Reactor (PWR), zirconium-based alloy cladding tubes are immersed in high pressure water containing boron (1000 wt. boron) and lithium (2 wt. ppm) at high temperature (320 °C). The corrosion induced by this environment is mainly due to the oxidation of the zirconium which transforms in zirconia. This phenomenon is one of the limiting factors of the in-pile fuel rod lifetime (~ 5 years). Therefore, it is important to predict the corrosion process of zirconium based alloys in PWR conditions. Zirconium-based alloys oxidation is sub-parabolic inlike the Wagner theory which predicts a parabolic kinetics. Two finite element models were developed to simulate this phenomenon : the diffuse interface model and the sharp interface model. Both simulate parabolic oxidation kinetics. The growth stress effects on oxygen diffusion were studied to explain the gap between theory and experience. Taking into account the influence of the hydrostatic stress and its gradient into the oxygen flux expression, sub-parabolic oxidation kinetics were simulated. The sub-parabolic behavior of the oxidation kinetics can be explained by a non-uniform compressive stress level into the oxide layer. Simulations of oxygen diffusion throught polycristalline layer of zirconia were performed. Zirconia grains are modelled by Voronoï tesselation and a space between grains is meshed to model grain boundaries. These numerical samples are used to study the effect of zirconia microstructure and microtexture on oxygen diffusion. Experimental data from thin foils of zirconia formed on Zircaloy-4 and zirconium hydrure are used in the simulations.; Dans les réacteurs à Eau Pressurisée (REP), les pastilles d’uranium sont isolées de l’eau du circuit primaire par des gaines en alliages de zirconium (Zy-4, M5, ZIRLO). Ces gaines jouent un rôle crucial en termes de sureté car elles sont la première barrière de confinement des produits de fission. En conditions nominales d’utilisation, la corrosion des gaines induite par l’environnement du circuit primaire (320 °C, 155 bars, présence de lithium et de bore) se traduit par l’oxydation du zirconium et la formation de phases fragiles d’hydrures de zirconium sous l’interface oxyde/métal. Ces phénomènes couplés affectent la tenue mécanique de la gaine et, in fine, limitent la durée de vie des assemblages combustibles en réacteur (~ 5 ans). Ce travail vise à mieux comprendre le phénomène de corrosion des alliages de zirconium.Dans un premier temps, une modélisation par éléments finis de la diffusion-corrosion du Zircaloy-4 a été réalisée pour simuler la cinétique d’oxydation observée expérimentalement. Le modèle analytique de Wagner prédit une évolution de l’épaisseur d’oxyde proportionnelle à la racine carrée du temps (régime parabolique). En pratique, la cinétique d’oxydation du Zircaloy-4 en autoclave s’écarte de la loi parabolique et est quasiment cubique. Plusieurs phénomènes sont susceptibles d’expliquer cette différence entre la cinétique expérimentale et le modèle analytique, notamment la présence de fortes contraintes de compression au sein de la couche d’oxyde. La prise en compte dans la modélisation de l’effet des contraintes sur la diffusion de l’oxygène permet de simuler et d’expliquer cet écart à la loi parabolique.Dans un second temps, nous avons simulé par éléments finis la diffusion de l’oxygène à travers une couche polycristalline de zircone. Les grains de zircone sont modélisés par un agrégat de polyèdres de Voronoï. Un espace entre les polyèdres est également maillé pour modéliser les joints de grains. Ces échantillons numériques ont été utilisés pour étudier l’effet de la microstructure et de la microtexture des couches de zircone sur la diffusion de l'oxygène. Les simulations sont nourries par les données expérimentales obtenues sur des lames minces de zircone formées sur Zircaloy-4 et hydrure de zirconium.

In Pressurized Water Reactor (PWR), zirconium-based alloy cladding tubes are immersed in high pressure water containing boron (1000 wt. boron) and lithium (2 wt. ppm) at high temperature (320 °C). The corrosion induced by this environment is mainly due to the oxidation of the zirconium which transforms in zirconia. This phenomenon is one of the limiting factors of the in-pile fuel rod lifetime (~ 5 years). Therefore, it is important to predict the corrosion process of zirconium based alloys in PWR conditions. Zirconium-based alloys oxidation is sub-parabolic inlike the Wagner theory which predicts a parabolic kinetics. Two finite element models were developed to simulate this phenomenon : the diffuse interface model and the sharp interface model. Both simulate parabolic oxidation kinetics. The growth stress effects on oxygen diffusion were studied to explain the gap between theory and experience. Taking into account the influence of the hydrostatic stress and its gradient into the oxygen flux expression, sub-parabolic oxidation kinetics were simulated. The sub-parabolic behavior of the oxidation kinetics can be explained by a non-uniform compressive stress level into the oxide layer. Simulations of oxygen diffusion throught polycristalline layer of zirconia were performed. Zirconia grains are modelled by Voronoï tesselation and a space between grains is meshed to model grain boundaries. These numerical samples are used to study the effect of zirconia microstructure and microtexture on oxygen diffusion. Experimental data from thin foils of zirconia formed on Zircaloy-4 and zirconium hydrure are used in the simulations.; Dans les réacteurs à Eau Pressurisée (REP), les pastilles d’uranium sont isolées de l’eau du circuit primaire par des gaines en alliages de zirconium (Zy-4, M5, ZIRLO). Ces gaines jouent un rôle crucial en termes de sureté car elles sont la première barrière de confinement des produits de fission. En conditions nominales d’utilisation, la corrosion des gaines induite par l’environnement du circuit primaire (320 °C, 155 bars, présence de lithium et de bore) se traduit par l’oxydation du zirconium et la formation de phases fragiles d’hydrures de zirconium sous l’interface oxyde/métal. Ces phénomènes couplés affectent la tenue mécanique de la gaine et, in fine, limitent la durée de vie des assemblages combustibles en réacteur (~ 5 ans). Ce travail vise à mieux comprendre le phénomène de corrosion des alliages de zirconium.Dans un premier temps, une modélisation par éléments finis de la diffusion-corrosion du Zircaloy-4 a été réalisée pour simuler la cinétique d’oxydation observée expérimentalement. Le modèle analytique de Wagner prédit une évolution de l’épaisseur d’oxyde proportionnelle à la racine carrée du temps (régime parabolique). En pratique, la cinétique d’oxydation du Zircaloy-4 en autoclave s’écarte de la loi parabolique et est quasiment cubique. Plusieurs phénomènes sont susceptibles d’expliquer cette différence entre la cinétique expérimentale et le modèle analytique, notamment la présence de fortes contraintes de compression au sein de la couche d’oxyde. La prise en compte dans la modélisation de l’effet des contraintes sur la diffusion de l’oxygène permet de simuler et d’expliquer cet écart à la loi parabolique.Dans un second temps, nous avons simulé par éléments finis la diffusion de l’oxygène à travers une couche polycristalline de zircone. Les grains de zircone sont modélisés par un agrégat de polyèdres de Voronoï. Un espace entre les polyèdres est également maillé pour modéliser les joints de grains. Ces échantillons numériques ont été utilisés pour étudier l’effet de la microstructure et de la microtexture des couches de zircone sur la diffusion de l'oxygène. Les simulations sont nourries par les données expérimentales obtenues sur des lames minces de zircone formées sur Zircaloy-4 et hydrure de zirconium.

Due to the significant rise of computed tomography (CT) exams in the past few years and the increase of the collective dose due to medical exams, dose estimation in CT imaging has become a major public health issue. However dose optimization cannot be considered without taking into account the image quality which has to be good enough for radiologists. In clinical practice, optimization is obtained through empirical index and image quality using measurements performed on specific phantoms like the CATPHAN®. Based on this kind of information, it is thus difficult to correctly optimize protocols regarding organ doses and radiologist criteria. Therefore our goal is to develop a tool allowing the optimization of the patient dose while preserving the image quality needed for diagnosis. The work is divided into two main parts: (i) the development of a Monte Carlo dose simulator based on the PENELOPE code, and (ii) the assessment of an objective image quality criterion. For that purpose, the GE Lightspeed VCT 64 CT tube was modelled with information provided by the manufacturer technical note and by adapting the method proposed by Turner et al (Med. Phys. 36: 2154-2164). The axial and helical movements of the X-ray tube were then implemented into the MC tool. To improve the efficiency of the simulation, two variance reduction techniques were used: a circular and a translational splitting. The splitting algorithms allow a uniform particle distribution along the gantry path to simulate the continuous gantry motion in a discrete way. Validations were performed in homogeneous conditions using a home-made phantom and the well-known CTDI phantoms. Then, dose values were measured in CIRS ATOM anthropomorphic phantom using both optically stimulated luminescence dosimeters for point doses and XR-QA Gafchromic® films for relative dose maps. Comparisons between measured and simulated values enabled us to validate the MC tool used for dosimetric purposes. Finally, organ doses for several acquisition parameters into the ICRP 110 numerical female phantoms were simulated in order to build a dosimetric data base which could be used in clinical practice. In parallel to this work, image quality was first studied using the CATPHAN® 600. From the CTP 404 inserts, the signal-to-noise ratio (SNR) was then computed by using the classical Rose model (J. Opt. Soc. Am. A 16:633-645). An extensive number of images, linked to several acquisitions setups, were analyzed and SNR variations studied. Acquisitions with a SNR closed to the Rose criterion were selected. New acquisitions, based on those selected, were performed with a pre-clinical phantom containing suspect structures in PMMA. These images were presented to two senior radiologists. Both of them reviewed all images and indicated if they were able to locate the structures or not using a 5 confidence levels scale. Two ROC curves were plotted to compare the detection ability if the bead was detectable (SNR > 5) or not. Results revealed a significant difference between the two types of image and thus demonstrated the Rose criterion potential for image quality quantification in CT. Ultimately, organ dose estimations were linked to SNR values through acquisition parameters. Preliminary results proved that an optimization can be performed using the Rose criterion and organ dose estimation, leading to a dose reduction by a factor up to 6.; Ces dernières années, la multiplication du nombre d’actes d’imagerie scanographique a eu pour conséquence l’augmentation de la dose collective due aux examens d’imagerie médicale. La dose au patient en imagerie scanographique est donc devenue un enjeu de santé publique majeur impliquant l’optimisation des protocoles d’examen, ces derniers devant tenir compte de la qualité image, indispensable aux radiologues pour poser leur diagnostic. En pratique clinique, l’optimisation est réalisée à partir d’indicateurs empiriques ne donnant pas accès à la dose aux organes et la qualité image est mesurée sur des fantômes spécifiques, tel que le fantôme CATPHAN®. Sans aucune information sur la dose aux organes et aucun outil pour prendre en compte l’avis du praticien, il est difficile d’optimiser correctement les protocoles. Le but de ce travail de thèse est de développer un outil qui permettra l’optimisation de la dose au patient tout en préservant la qualité image nécessaire au diagnostic. Ce travail est scindé en deux parties : (i) le développement d’un simulateur de dose Monte Carlo (MC) à partir du code PENELOPE, et (ii) l’estimation d’un critère de qualité image objectif. Dans ce but, le scanner GE VCT Lightspeed 64 a été modélisé à partir des informations fournies dans la note technique du constructeur et en adaptant la méthode proposée par Turner et al (Med. Phys. 36:2154-2164). Les mouvements axial et hélicoïdal du tube ont été implémentés dans l’outil MC. Pour améliorer l’efficacité de la simulation, les techniques de réduction de variance dites de splitting circulaire et longitudinal ont été utilisées. Ces deux réductions de variances permettent de reproduire le mouvement uniforme du tube le long de l’axe du scanner de manière discrète. La validation expérimentale de l’outil MC a été réalisée dans un premier temps en conditions homogènes avec un fantôme fabriqué au laboratoire et le fantôme CTDI, habituellement utilisé en routine clinique pour les contrôles qualité. Puis, la distribution de la dose absorbée dans le fantôme anthropomorphe CIRS ATOM, a été mesurée avec des détecteurs OSL et des films Gafchromic® XR-QA2. Ensuite, la dose aux organes a été simulée pour différentes acquisitions dans le fantôme femme de la CIPR 110 afin de créer une base de données utilisable en clinique. En parallèle, la qualité image a été étudiée en utilisant le fantôme CATPHAN® 600. A partir du module CTP 404, le rapport signal sur bruit (SNR pour signal to noise ratio) a été calculé en utilisant le modèle développé par Rose (J. Opt. Soc. Am. A 16:633-645). Un grand nombre d’images, correspondant à différents paramètres d’acquisition et de reconstruction, ont été analysées afin d’étudier les variations du SNR. Les acquisitions avec un SNR proche du critère de Rose ont été sélectionnées pour permettre des nouvelles acquisitions avec un fantôme préclinique contenant des petites structures suspectes en PMMA de différents diamètres. Ces images ont été analysées par deux radiologues expérimentés. Sur chaque image, ils devaient déterminer si une anomalie était présente ou non et indiquer leur niveau de confiance sur leur choix. Deux courbes ROC ont ainsi été obtenues : une pour les anomalies dites « détectables » par le critère de Rose (SNR > 5), et une pour les anomalies dites « non-détectables ». L’analyse des courbes montre que les deux radiologues détectent plus facilement les lésions suspectes lorsque que le critère de Rose est satisfait, démontrant le potentiel du modèle de Rose dans l’évaluation de la qualité image pour les tâches cliniques de détection. En conclusion, à partir des paramètres d’acquisition, la dose aux organes a été corrélée à des valeurs de SNR. Les premiers résultats prouvent qu’il est possible d’optimiser les protocoles en utilisant la dose aux organes et le critère de Rose, avec une réduction de la dose pouvant aller jusqu’à un facteur 6.

International audience; The goal of the present study was to evaluate and compare the radiosensitizing properties of gadolinium nanoparticles (NPs) with the gadolinium contrast agent (GdCA) Magnevist® in order to better understand the mechanisms by which they act as radiation sensitizers. This was determined following either low energy synchrotron irradiation or high energy gamma irradiation of F98 rat glioma cells exposed to ultrasmall gadolinium NPs (GdNPs, hydrodynamic diameter of 3 nm) or GdCA. Clonogenic assays were used to quantify cell survival after irradiation in the presence of Gd using monochromatic x-rays with energies in the 25 keV–80 keV range from a synchrotron and 1.25 MeV gamma photons from a cobalt-60 source. Radiosensitization was demonstrated with both agents in combination with X-irradiation. At the same concentration (2.1 mg mL−1), GdNPS had a greater effect than GdCA. The maximum sensitization-enhancement ratio at 4 Gy (SER4Gy) was observed at an energy of 65 keV for both the nanoparticles and the contrast agent (2.44 ± 0.33 and 1.50 ± 0.20, for GdNPs and GdCA, respectively). At a higher energy (1.25 MeV), radiosensitization only was observed with GdNPs (1.66 ± 0.17 and 1.01 ± 0.11, for GdNPs and GdCA, respectively). The radiation dose enhancements were highly 'energy dependent' for both agents. Secondary-electron-emission generated after photoelectric events appeared to be the primary mechanism by which Gd contrast agents functioned as radiosensitizers. On the other hand, other biological mechanisms, such as alterations in the cell cycle may explain the enhanced radiosensitizing properties of GdNPs.

International audience; Purpose/Objective: Cone-Beam Computed Tomography (CBCT) is progressively more used in clinical practice to ensure precision in target localization and patient positioning required by new radiotherapy treatment techniques. However, intensive use of image guidance procedures could add a significant extra dose to normal tissues and potentially amplify the risk for patients to develop radiation-induced cancer. Therefore, there is an increasing interest to evaluate the dose delivered by CBCT scans. The most accurate tool to evaluate this imaging dose is the Monte Carlo (MC) method. To this end, this study aims at developing a MC-based dose calculation tool to compute the imaging dose, and to validateit in pre-clinical conditions against dosimetric measurements.

Conference of 4th International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications, ANIMMA 2015 ; Conference Code:121554; International audience; With the growth of industrial risks and the multiplication of CBRNe (Chemical Biological Radiological and explosive) attacks through toxic chemicals, biological or radiological threats, public services and military authorities face with increasingly critical situations, whose management is strongly conditioned by fast and reliable establishment of an informative diagnostic. Right after an attack, the five first minutes are crucial to define the various scenarii and the most dangerous for a human intervention. Therefore the use of robots is considered essential by all stakeholders of security. In this context, the SISPEO project (Système d'Intervention Sapeurs Pompiers Robotisé) aims to create/build/design a robust response through a robotic platform for early intervention services such as civil and military security in hostile environments. CEA LIST has proposed an adapted solution to detect and characterize nuclear and radiological risks online and in motion, using a miniature embedded CdZnTe (CZT) crystal Gamma-ray spectrometer. This paper presents experimental results for this miniature embedded CZT spectrometer and its associated mathematical method to detect and characterize radiological threats online and in motion.

Conference of 4th International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications, ANIMMA 2015 ; Conference Code:121554; International audience; Radionuclide identification and quantification is a serious concern for many applications as safety or security of nuclear power plant or fuel cycle facility, CBRN risk identification, environmental radioprotection and waste measurements. High resolution gamma-ray spectrometry based on HPGe detectors is a performing solution for all these topics. During last decades, a great number of software has been developed to improve gamma spectra analysis. However, some difficulties remain in the analysis when full energy peaks are folded together with a high ratio between their amplitudes, when the Compton background is much larger compared to the signal of a single peak and when spectra are composed of a great number of peaks. This study deals with the comparison between a conventional analysis method and an innovative approach, called SINBAD ("Spectrométrie par Inférence Non paramétrique BAyesienne Déconvolutive"), for radionuclide identification and quantification. For many years, SINBAD has been developed by the CEA LIST for unfolding complex spectra from HPGe detectors. Contrary to the conventional method using fitting procedures, SINBAD uses a probabilistic approach with nonparametric Bayesian inference to process spectrum data. The conventional fitting method founded for instance in Genie 2000 is compared with the nonparametric SINBAD approach regarding some key figures of merit as the peak centroid estimation (identification step) and net peak area determination (quantification step). Complex cases are studied for nuclide detection with closed gamma-rays energies and high full energy peak intensity differences. Tests are performed with spectra from the International Atomic Energy Agency (IAEA) for gamma spectra analysis software benchmark and with spectra acquired in our laboratory. It appears that SINBAD results are better than GENIE 2000 ones in most of the cases even if hard deconvolutions can be achieved thanks to GENIE 2000 at the cost of expert parameters fine tuning which has to be compared with the user-friendly SINBAD operating.

Article téléchargeable sur Pubmed https://www.ncbi.nlm.nih.gov/pubmed/25386999; International audience; In addition to conventional short-lived radionuclides, longer-lived isotopes are becoming increasingly important to positron emission tomography (PET). The longer half-life both allows for circumvention of the in-house production of radionuclides, and expands the spectrum of physiological processes amenable to PET imaging, including processes with prohibitively slow kinetics for investigation with short-lived radiotracers. However, many of these radionuclides emit 'high-energy' positrons and gamma rays which affect the spatial resolution and quantitative accuracy of PET images. The objective of the present work is to investigate the positron range distribution for some of these long-lived isotopes. Based on existing Monte Carlo simulations of positron interactions in water, the probability distribution of the line of response displacement have been empirically described by means of analytic displacement functions. Relevant distributions have been derived for the isotopes (22)Na, (52)Mn, (89)Zr, (45)Ti, (51)Mn, (94 m)Tc, (52 m)Mn, (38)K, (64)Cu, (86)Y, (124)I, and (120)I. It was found that the distribution functions previously found for a series of conventional isotopes (Jødal et al 2012 Phys. Med. Bio. 57 3931-43), were also applicable to these non-conventional isotopes, except that for (120)I, (124)I, (89)Zr, (52)Mn, and (64)Cu, parameters in the formulae were less well predicted by mean positron energy alone. Both conventional and non-conventional range distributions can be described by relatively simple analytic expressions. The results will be applicable to image-reconstruction software to improve the resolution.

International audience; This paper considers a problem stemming from the analysis of spectrometric data. When performing experiments on highly radioactive matter, electrical pulses recorded by the spectrometer tend to overlap, thus yielding severe distortions when computing the histogram of the pulses' energies. In this paper, we propose a fast recursive algorithm which estimates efficiently this histogram from measurements of the duration and energies of overlapping pulses. Its good performances are shown both on simulations and real data. Furthermore, its lower algorithmic complexity makes it more fitting for real-time implementation.

Conference of 2014 24th IEEE International Workshop on Machine Learning for Signal Processing, MLSP 2014 ; Conference Date: 21 September 2014 Through 24 September 2014; Conference Code:109223; International audience; We propose a Bayesian nonparametrics method, including algorithm for posterior computation, for the sparse regression problem. Our method applies in a general setting, when there are direct or indirect noisy observations of the signal. We try to make a wide focus on smoothness properties and sparsity of the approximate. As an example, we consider the ill-posed inverse problem of Quantum Homodyne Tomography.

International audience; Purpose/Objective: Historically designed as a control system for patient positioning for radiotherapy treatment, Electronic Portal Imaging Devices (EPIDs) are nowadays widely used for quality assurance and dosimetric verifications in new irradiation techniques. One of the main advantages of the EPID is its high resolution which can detect small details. The objective of this study is to compare the EPID image acquired during the treatment with a predicted high resolution portal image computed by Monte Carlo (MC) simulation. A new method for prediction of high resolution EPID images is tested for in vivo treatment verification. Materials and Methods: Experiments were carried out on a Siemens ARTISTETM, equipped with a 160-MLCTM, and its Siemens OptivueTM 1000 EPID. This EPID has an active detection area of 41 x 41 cm2 and a matrix of 1024 x 1024 pixels. A model of this linac and the EPID was developed with the MC code Penelope, and commissioned. We focus on a breast treatment conformational beam (6 MV) on the CIRS adult female phantom. The CT-scan of the phantom was used as input, and Hounsfield numbers were converted in density and atomic composition, so as to obtain a voxelized geometry used in the Penelope code. Particles exiting the phantom and impinging on the EPID are simulated up to the EPID in order to compute the predicted portal image by scoring the energy deposited in the phosphor layer on a 1024 x 1024 virtual grid. The simulated image was then smoothed using a denoising algorithm in order to keep the high resolution advantage. Several denoising algorithms were tested, among them IRON, LASG and a recently developed one called DPGLM. For now, we use the gamma-index technique to evaluate the accuracy of the simulated image against the experimental one. Results: Figure 1 shows the acquired image and the simulated one. The gamma-index is satisfied for 94.4 % of the pixels for 3.5 % and 3.5 mm criterion. The DPGLM gives the best result toward accuracy and computed time. Indeed, the denoising of 1024 x 1024 images takes about 1h30 mn, 2h and 5 mn using DPGLM, IRON, and LASG, respectively. The LASG algorithm is really fast but the result is too smoothed for the high resolution purpose. Conclusions: This work is the first step in the aim of in vivo dosimetry by comparing experimental portal images with high resolution predicted images obtained using MC simulations in a voxelized geometry. First results obtained on a breast treatment are encouraging, and we can expect to detect treatment errors.

Purpose/Objective: Electronic Portal Imaging Devices (EPIDs) are widely used for quality assurance in radiotherapy, and also for dosimetric verifications. For this latter application, the image obtained during the treatment session can be compared to a pre-calculated reference image in order to highlight dose delivery errors. A Monte Carlo(MC)-based method to predict high-resolution reference images was recently developed. However, the use of this method in clinical routine is still hampered by the need to store huge phase space (PS) files. This study aims at developing a new virtual source model (VSM) which retains all the existing correlations between particle characteristics stored in the PS file, and hence to provide a more compact way to model the irradiation beam, while keeping the accuracy of using a PS file. Materials and Methods: A model of the studied linac (Elekta Synergy) was first developed using both the MC codes EGSnrc and PENELOPE, and commissioned. Using this model, a reference PSF (PSFr) was calculated after the flattening filter using EGSnrc, where the axial symmetries assumed, and particles were sorted out in three sub-sources according to the position of their last interaction (target, primary collimator or flattening filter). Each particle is described by its radial position (r) in the PS plane,its energy (E), and its polar and azimuthal angles (phi, theta), representing the particle deviation compared to its direction after bremsstrahlung, and the orientation of this deviation. For each subsource, a 4D histogram was built by storing the particle distributions accordingto their r, E, phi, theta values. Our VSM hence contains all correlations existing between these four variables contrary to other existing VSMs. This new VSM is now being implemented in the PENELOPE code, and its accuracy is being tested by comparing first the PSF build using our VSM with the PSFr, and then dose distributions in water and portal images calculated with the MSV and PSFr. Results: Figure 1 shows for each sub-source the correlated 2D histogram depicting particle energy against their radial position. Some characteristics are shared by all sources. Few particles are beyond the collimator aperture (4.5 cm from the linac axis). The annihilation photons at 511 keV are always visible. However, the content of a histogram depends widely of the sub-source it represents: the target produces the most energetic particles. The flattening filter and the primary collimator sub-sources generate particles beyond 4.5 cm of lower energy, because of their large deflection angle. Conclusions: A new VSM taking into account for all correlations between particle characteristics was developed and implemented in the PENELOPE code. It will be used to calculate accurate reference portal images for quality assurance in radiotherapy.

National audience; En radiothérapie, la maîtrise de la dose délivrée lors d'un traitement est cruciale pour assurer la réussite de celui-ci. Grâce à la méthode de Monte Carlo (MC), une précision de l'ordre de 2-3% sur le calcul de dose lors de la planification du traitement est possible, à condition que le modèle MC de l'accélérateur linéaire (linac) reproduise fidèlement les caractéristiques physiques du faisceau d'irradiation. Pour cela, les paramètres décrivant les distributions spatiale et énergétique du faisceau primaire d'électrons doivent être réglés finement. La détermination de ces paramètres s'effectue généralement par essai/erreur jusqu'à reproduire au mieux les mesures, ce qui peut s'avérer fastidieux et chronophage, et ne garantit pas que les paramètres optimaux ont été retenus. Nous présentons ici une nouvelle méthode de détermination automatique des paramètres du faisceau primaire d'électrons.

This thesis deals with the radiotherapy treatments planning issue which need a fast and reliable treatment planning system (TPS). The TPS is composed of a dose calculation algorithm and an optimization method. The objective is to design a plan to deliver the dose to the tumor while preserving the surrounding healthy and sensitive tissues.The treatment planning aims to determine the best suited radiation parameters for each patient’s treatment. In this thesis, the parameters of treatment with IMRT (Intensity modulated radiation therapy) are the beam angle and the beam intensity. The objective function is multicritiria with linear constraints.The main objective of this thesis is to demonstrate the feasibility of a treatment planning optimization method based on a fast dose-calculation technique developed by (Blanpain, 2009). This technique proposes to compute the dose by segmenting the patient’s phantom into homogeneous meshes. The dose computation is divided into two steps. The first step impacts the meshes: projections and weights are set according to physical and geometrical criteria. The second step impacts the voxels: the dose is computed by evaluating the functions previously associated to their mesh.A reformulation of this technique makes possible to solve the optimization problem by the gradient descent algorithm. The main advantage of this method is that the beam angle parameters could be optimized continuously in 3 dimensions. The obtained results in this thesis offer many opportunities in the field of radiotherapy treatment planning optimization., Cette thèse s'inscrit dans la perspective des traitements de radiothérapie en insistant sur la nécessité de disposer d’un logiciel de planification de traitement (TPS) rapide et fiable. Le TPS est composé d'un algorithme de calcul de dose et d’une méthode d’optimisation. L'objectif est de planifier le traitement afin de délivrer la dose à la tumeur tout en sauvegardant les tissus sains et sensibles environnant. La planification des traitements consiste à déterminer les paramètres d’irradiation les mieux adaptés au patient. Dans le cadre de cette thèse, les paramètres d'un traitement par RCMI (Radiothérapie Conformationnelle avec Modulation d'Intensité) sont la position de la source, les orientations des faisceaux et, pour chaque faisceau composé de faisceaux élémentaires, la fluence de ces derniers. La fonction objectif est multicritère en associant des contraintes linéaires. L’objectif de la thèse est de démontrer la faisabilité d'une méthode d'optimisation du plan de traitement fondée sur la technique de calcul de dose rapide développée par (Blanpain, 2009). Cette technique s’appuie sur un fantôme segmenté en mailles homogènes. Le calcul de dose s’effectue en deux étapes. La première étape concerne les mailles : les projections et pondérations y sont paramétrées en fonction de critères physiques et géométriques. La seconde étape concerne les voxels: la dose y est calculée en évaluant les fonctions préalablement associées à leur maille.Une reformulation de cette technique permet d’aborder le problème d’optimisation par la méthode de descente de gradient. L’optimisation en continu des paramètres du traitement devient envisageable. Les résultats obtenus dans le cadre de cette thèse ouvrent de nombreuses perspectives dans le domaine de l’optimisation des plans de traitement en radiothérapie.

International audience; Modeling the radio-induced effects in biological medium still requires accurate physics models to describe the interactions induced by all the charged particles present in the irradiated medium in detail. These interactions include inelastic as well as elastic processes. To check the accuracy of the very low energy models recently implemented into the GEANT4 toolkit for modeling the electron slowing-down in liquid water, the simulation of electron dose point kernels remains the preferential test. In this context, we here report normalized radial dose profiles, for mono-energetic point sources, computed in liquid water by using the very low energy "GEANT4-DNA" physics processes available in the GEANT4 toolkit. In the present study, we report an extensive intra-comparison of profiles obtained by a large selection of existing and well-documented Monte-Carlo codes, namely, EGSnrc, PENELOPE, CPA100, FLUKA and MCNPX.

Monte Carlo simulations of S-values have been carried out with the Geant4-DNA extension of the Geant4 toolkit. The S-values have been simulated for monoenergetic electrons with energies ranging from 0.1 keV up to 20 keV, in liquid water spheres (for four radii, chosen between 10 nm and 1 μm), and for electrons emitted by five isotopes of iodine (131, 132, 133, 134 and 135), in liquid water spheres of varying radius (from 15 μm up to 250 μm). The results have been compared to those obtained from other Monte Carlo codes and from other published data. The use of the Kolmogorov–Smirnov test has allowed confirming the statistical compatibility of all simulation results.

International audience; This article presents the synthesis and the blend of bismuth complexes in polystyrene based plastic scintillators. A specific design has enabled the fabrication of a scintillator loaded with up to 17 wt% of bismuth. Tri-carboxylate and triaryl bismuth compounds were used to explore and understand the influence of bismuth loading on the two main criteria of plastic scintillation: light yield and detection efficiency of gamma-rays. For gamma radiation with an energy