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The purpose of this investigation was the verification of both the measured data and quality of the implementation of the add-on ModuLeaf miniature multileaf collimator (ML mMLC) into the clinical treatment planning system for conformal stereotactic radiosurgery treatment. To this end the treatment head with ML mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The 6 MV photon beams used in the setup were first benchmarked with a set of measurements. A total ML mMLC transmission of 1.13% of the 10 x 10 cm2 open field dose was measured and reproduced with the BEAMnrc/DOSXYZnrc code. Correspondence between calculated and measured output factors (OFs) was within 2%. Correspondence between MC and measured profiles was within 2% dose and 2 mm distance, only for the smallest 0.5 x 0.5 cm2 field the results were within 3% dose. In the next step, the MC model was compared with Gafchromic film measurements and Pinnacle(3) 7.4 f (convolution superposition algorithm) calculated dose distributions, using a gamma evaluation comparison, for a multi-beam patient setup delivered to a Lucytrade mark phantom. The gamma evaluation of the MC versus Gafchromic film resulted in 3.4% of points not fulfilling gamma

Two Monte Carlo dose engines for radiotherapy treatment planning, namely a beta release of Peregrine and MCDE (Monte Carlo dose engine), were compared with Helax-TMS (collapsed cone superposition convolution) for a head and neck patient for the Elekta SLi plus linear accelerator. Deviations between the beta release of Peregrine and MCDE up to 10% were obtained in the dose volume histogram of the optical chiasm. It was illustrated that the differences are not caused by the particle transport in the patient, but by the modelling of the Elekta SLi plus accelerator head and more specifically the multileaf collimator (MLC). In MCDE two MLC modules (MLCQ and MLCE) were introduced to study the influence of the tongue-and-groove geometry, leaf bank tilt and leakage on the actual dose volume histograms. Differences in integral dose in the optical chiasm up to 3% between the two modules have been obtained. For single small offset beams though the FWHM of lateral profiles obtained with MLCE can differ by more than 1.5 mm from profiles obtained with MLCQ. Therefore, and because the recent version of MLCE is as fast as MLCQ, we advise to use MLCE for modelling the Elekta MLC. Nevertheless there still remains a large difference (up to 10%) between Peregrine and MCDE. By studying small offset beams we have shown that the profiles obtained with Peregrine are shifted, too wide and too flat compared with MCDE and phantom measurements. The overestimated integral doses for small beam segments explain the deviations observed in the dose volume histograms. The Helax-TMS results are in better agreement with MCDE, although deviations exceeding 5% have been observed in the optical chiasm. Monte Carlo dose deviations of more than 10% as found with Peregrine are unacceptable as an influence on the clinical outcome is possible and as the purpose of Monte Carlo treatment planning is to obtain an accuracy of 2%. We would like to emphasize that only the Elekta MLC has been tested in this work, so it is certainly possible that alpha releases of Peregrine provide more accurate results for other accelerators.

Polymer gel dosimetry was used to verify the clinical use of IMAT. There was a satisfactory correlation between calculated and measured dose, thus validating the planning procedure, the calculation algorithm and the delivery. Although still elaborate and costly, polymer gel dosimetry has some unique features (3D absolute dosimetry) for verification of complex irradiation techniques.

The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.

Resume En radiotherapie conformationnelle a l'aide de modulation d'intensite, la connaissance de la dose est indispensable dans le volume cible aussi bien que dans les organes a risque et, par consequent, dans les regions de fort gradient de dose. La planification, qui guide comme reference pendant toute l'optimisation d'un traitement de modulation d'intensite, doit etre validee profondement et specifiquement. A cet effet, on a distingue trois niveaux de dosimetrie experimentale: l'etude dosimetrique des petits segments uniformes ou faisceaux elementaires, la dosimetrie sur fantome d'eau des faisceaux d'intensite modulee et la verification preclinique sur fantome anthropomorphe de la planification de modulation d'intensite segmentaire basee sur un concept geometrique. L'etude revele que les caracteristiques dosimetriques des petits champs sont gravement influencees par l'ajustement geometrique du dispositif de collimation. L'analyse demontre la necessite de caracteriser rigoureusement la penombre des segments. La verification signale un aspect delicat d'une balistique de faisceaux de petite separation angulaire. En modulation d'intensite, la verification dosimetrique est pour le moment aussi indispensable que la planimetrie et la technologie.

To compare acute gastrointestinal and genitourinary toxicity for patients positioned with an electronic portal imaging device (EPID) and patients positioned with kilovoltage cone beam computed tomography (CBCT) during postoperative prostate radiotherapy.Between 1999 and April 2010, 196 prostate cancer patients were referred for postoperative salvage radiotherapy. Patient position was corrected using EPID (1999 to December 2006, n=116) or CBCT (January 2007 to present, n=80). The treatment technique, number of beams, dose prescription, dose computation algorithm and planning target volume margins were not altered over time. Grade 1-3 acute gastrointestinal and genitourinary toxicity were compared between the EPID group and the CBCT group.The incidence of grade 1 and 2 genitourinary toxicity was significantly reduced by 17 and 14%, respectively, in the CBCT group compared with the EPID group (P0.05). This was mainly attributed to a decrease in the following grade 1 symptoms: frequency (P0.05), nocturia (P=0.06) and urgency (P=0.07). Grade 2 incontinence (P=0.06) and frequency (P=0.06) were lower in the CBCT group. Grade 3 genitourinary toxicity was comparably low (EPID 3% versus CBCT 1%). There was no significant difference in gastrointestinal grade 1-2 toxicity between both groups. No grade 3 gastrointestinal toxicity was observed.Patient positioning with CBCT significantly reduces acute genitourinary toxicity compared with positioning with EPID.

A new accurate Monte Carlo code for IMRT dose computations, MCDE (Monte Carlo dose engine), is introduced. MCDE is based on BEAMnrc/DOSXYZnrc and consequently the accurate EGSnrc electron transport. DOSXYZnrc is reprogrammed as a component module for BEAMnrc. In this way both codes are interconnected elegantly, while maintaining the BEAM structure and only minimal changes to BEAMnrc.mortran are necessary. The treatment head of the Elekta SLiplus linear accelerator is modelled in detail. CT grids consisting of up to 200 slices of 512 x 512 voxels can be introduced and up to 100 beams can be handled simultaneously. The beams and CT data are imported from the treatment planning system GRATIS via a DICOM interface. To enable the handling of up to 50 x 10(6) voxels the system was programmed in Fortran95 to enable dynamic memory management. All region-dependent arrays (dose, statistics, transport arrays) were redefined. A scoring grid was introduced and superimposed on the geometry grid, to be able to limit the number of scoring voxels. The whole system uses approximately 200 MB of RAM and runs on a PC cluster consisting of 38 1.0 GHz processors. A set of in-house made scripts handle the parallellization and the centralization of the Monte Carlo calculations on a server. As an illustration of MCDE, a clinical example is discussed and compared with collapsed cone convolution calculations. At present, the system is still rather slow and is intended to be a tool for reliable verification of IMRT treatment planning in the case of the presence of tissue inhomogeneities such as air cavities.

Despite attempts of treatment intensification, improvement of radio(chemo)therapy outcome in head-and-neck cancer remains marginal. Non-selective dose escalation in large planning volumes has not increased the rates of disease control and survival but severe debilitating treatment-induced toxicity. The use of biological imaging in radiotherapy allows detecting other than anatomical imaging-defined targets and enables tailoring dose prescription to tumor biology, .i.e. dose painting [1]. Dose painting can be employed in treatment intensification by non-uniformly escalating dose at no or minimal theoretical increase in the rates of treatment-induced toxicity relative to conventional or 3D-conformal radiotherapy [2]. Because of changes in tumor and non-tumor anatomy and biology throughout treatment, radiotherapy should not be based solely on pretreatment anatomical and biological imaging but adapted to occurring changes detected by pertreatment imaging. Combination of treatment adaptation with dose painting presents a new paradigm of treatment individualization with the promise of substantial improvement of treatment outcome. In Ghent University Hospital we tested dose painting in 41 head-and-neck cancer patients during 2003-2005 demonstrating feasibility of homogenous dose escalation up to 77.5 Gy in 18[F]FDG-PET-based subvolumes within the gross tumor volumes [3]. We introduced adaptive strategy in dose painting (18[F]FDG voxel intensity-based IMRT) in 2007 firstly in a clinical phase I trial with one per-treatment adaptation (21 patients) [4,5] and then in a clinical phase I trial with two per-treatment adaptations (10 patients) [6]. Significant non-homogenous dose escalation in the former trial (Normalized isoeffective dose equivalent to dose delivered in 2.0-Gy fractions [NID2Gy] of 91 Gy at the first level of dose escalation and 102 Gy at the second level of dose escalation) and adaptive procedures using deformable image co-registration in the latter trial were technically feasible and clinically tolerable. Update of clinical results of the trials will be presented at the meeting. The encouraging results of the only clinical trial on combined biologicalimage guided and adaptive IMRT [4,5] supported initiation of a clinical phase II randomized trial currently running in Ghent University Hospital [NCT01341535]. The trial compares adaptive dose painting against standard IMRT with increased loco-regional control and diminished treatment-induced toxicity as primary and secondary objectives, respectively.

The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.

Intensity modulated radiotherapy (IMRT) is used in our department for treatment of paranasal sinuses. We describe the methodology that was developed together with the clinical implementation, illustrated by a case report.Patient history, treatment and short follow-up are described. An IMRT, obtained by superposition of static beam segments was implemented. Electronic portal images, compared to digitally reconstructed radiographs (DRR) were used to evaluate and adjust patient positioning.IMRT is an appropriate and feasible treatment technique for head and neck cancer in anatomical regions that are difficult to treat. A high tumour dose can be combined with a good sparing of the surrounding organs at risk (OAR's).

In head and neck cancer, intensity modulated radiotherapy (IMRT) allows to generate concave dose distributions to match the shape of the planning target volume (PTV). In our department, IMRT is delivered by multiple static convergent beams, that consist of superposition of unmodulated segments. The segment shapes were obtained from the beam’s eye view (BEV) projection of PTV(s) and organs at risk (OARs). By making the shape of segments also a function of the patient’s outlines (skin surface), a substantial refinement was obtained.

The case in favour of IMRT is made as IMRT allows to shape the dose distribution in such a way that at least three challenges in clinical radiotherapy can be addressed. These are i) conforming dose to concave PTV shapes (i.e. head and neck (HN ii) reducing the volume of surrounding tissues irradiated to intermediate dose levels by using beam combinations with restricted hinge angles (lung, mediastinal tumours) and iii) generating controlled 3D-inhomogeneous dose distributions (i.e. prostate, paranasal sinus tumours). The two largest groups of patients that we treat with IMRT are H&N and prostate cancer patients.

Background and purpose : Concave dose distributions generated by intensity modulated radiotherapy (IMRT) were applied to re-irradiate three patients with pharyngeal cancer. Patients, materials and methods : Conventional radiotherapy for oropharyngeal (patients 1 and 3) or nasopharyngeal (patient 2) cancers was followed by relapsing or new tumors in the nasopharynx (patients 1 and 2) and hypopharynx (patient 3). Six non-opposed coplanar intensity modulated beams were generated by combining non-modulated beamparts with intensities (weights) obtained by minimizing a biophysical objective function. Beamparts were delivered by a dynamic MLC (Elekta Oncology Systems, Crawley, UK) forced in step and shoot mode. Results and conclusions : Median PTV-doses (and ranges) for the three patients were 73 (65–78), 67 (59–72) and 63 (48–68) Gy. Maximum point doses to brain stem and spinal cord were, respectively, 67 Gy (60% of volume below 30 Gy) and 32 Gy (97% below 10 Gy) for patient 1; 60 Gy (69% below 30 Gy) and 34 Gy (92% below 10 Gy) for patient 2 and 21 Gy (96% below 10 Gy) at spinal cord for patient 3. Maximum point doses to the mandible were 69 Gy for patient 1 and 64 Gy for patient 2 with, respectively, 66 and 92% of the volume below 20 Gy. A treatment session, using the dynamic MLC, was finished within a 15-min time slot.

Purpose: To evaluate the accuracy of automated contour deformation for head‐and‐neck cancer in adaptive treatment. Methods: Data from 13 head‐and‐neck patients in a phase I trial for adaptive treatment were used. Adaptation was based on [18F]FDG‐PET‐guided dose painting by numbers (DPBN) plans. Each patient had two DPBN plans based on: (i) a pretreatment PET/CT scan and (ii) a during‐treatment PET/CT scan acquired after 8 fractions. Contours manually drawn on the pretreatment CT scan were deformed using commercial deformable image registrationsoftware onto the during‐treatment CT scan. Deformed contours of regions of interest (ROIdef) were visually inspected by an experienced radiation oncologist and, if necessary, adjusted (ROIdef_ad) and both sets of contours were compared to manually redrawn ROIs (ROIm) using Jaccard (JI) and overlap indices (OI). ROI indices and volumes were compared for all contour sets used a paired t‐test and one‐way ANOVA pairwise comparison, respectively. Results: Almost all deformed ROIs in all patients required adjustment after visual inspection. The largest adjustments were made in GTVs when substantial tumor regression occured, e.g., ROIdef=9.2 cm3 vs. ROIdef_ad=2.2 cm3 vs. ROIm=2.1 cm3. The swallowing structures were the most frequently adjusted ROIs. The mandible was the most acurately propagated ROI requiring little or no adaptation: JI=0.7 and OI=0.8. The upper esophageal sphincter was the worst propagated ROI: JI=0.3 and OI=0.3 for the ROIdef, JI=0.5 and OI=0.6 for the ROIdef_ad. Despite the variation in indices, there was no statistically significant difference between ROIdef, ROIdef_ad and ROIm volumes. Generating ROIm took 4–6 hours, generating ROIdef took a few minutes and generating ROIdef_ad took less than 2 hours. Conclusions: Deformable image co‐registration followed by visual inspection does require adjustment of most deformed ROIs. Nevertheless, fast automatic ROI propagation followed by user‐driven adjustments appears to be more efficient than labor intensive de‐novo re‐contouring.

3685 Background: In rectal cancer, radiotherapy decreases local recurrence rates and improves survival. Small bowel (SB) radiation toxicity is strongly influenced by the volume of irradiated SB. In...

Resume L’irradiation en modulation d’intensite est utilisee en routine dans les tumeurs des voies aerodigestives superieures a l’Universite de Gand. La distribution de dose souhaitee est determinee au prealable comme une fourchette de doses acceptables pour chaque volume d’interet. Il est important de specifier separement ces contraintes de dose pour les differentes regions du volume cible previsionnel : en dehors et dans les zones de gradient, dans les zones d’intersections ou non avec les organes a risques. Pour eviter des doses elevees a distance du volume cible previsionnel, la creation d’une « zone d’enveloppe », definie comme le volume couvert par le scanner minore du volume cible previsionnel, est une methode efficace, surtout si des sous-volumes, a l’interieur de la « zone d’enveloppe », ont ete definis a des distances croissantes du volume cible previsionnel. En attribuant des contraintes de dose maximale a ces sous-volumes, decroissantes avec la distance au volume cible previsionnel, la conformation est assuree. La creation de ces volumes cibles previsionnels additionnels et de ces sous-volumes d’organes a risque permet une definition claire et sans ambiguite des limites de doses acceptables et evite d’avoir a utiliser des ponderations de contraintes, utilisateur-dependantes, pour la procedure d’optimisation. L’efficacite de la planification inverse s’en trouve grandement renforcee. Son resultat est previsible, l’evaluation d’un plan de traitement devient objective puisque le plan repond ou non aux contraintes de doses predefinies. La description d’un plan de traitement est facilitee par l’expression des differentes doses a tous les volumes. Le cout de cette procedure est modere et reside principalement : 1. dans la creation des sous-volumes, qui peut-etre semi-automatique grâce aux outils modernes de segmentation des images ; dans l'inclusion des contraintes a tous les sous-volumes dans la fonction objective.