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Introduction The implementation of brain stereotactic treatments with the help of VMAT irradiation were configured in the way to be close to the initial practice based on cones. To optimize the dose distribution, four table rotation positions were fixed in the dose planning process. Even if the dose distributions were not comparable, VMAT offered the benefit of accessing the entire sphere around the cranium and new advantages appeared covering multiple targets with the MC. However, the four table position brought to an important session duration. The purpose was to consider removing one table position and validate that the dose distributions remained acceptable. Methods The brain targets are irradiated with the use of VMAT on four table rotations: 0°, 45°, 270° and 315°. The stereotactic brain plans of 15 patients were re-planned, removing the 270° table position and using the same inverse planning parameters. The prescribed total dose was between 24 Gy and 33 Gy. The number of targets were 1–3 with volumes between 2 and 41 cc. The collimator rotations were adapted to optimize the MLC effect on the modulation. The comparisons were established with the use of indexes: Homogeneity, Conformity, healthy tissue coverage and target coverage. Other parameters were collected: MU amount, CTV Dmax, PTV Dmax, Brainstem Dmax, Braim Dmean and V10 Gy. The session duration was evaluated for both configurations. Results The dose distributions were compared and found with similar clinical acceptability. This first analyze was confirmed with the different indexes and parameters (Table 1). HI, CI, HCO and TCO showed a low effect as the other parameters listed in Table 1. The session duration initially between 40 and 50 mn decreased until 30mn. Removal of the 270° rotation decreases the spread of low isodoses in the head-feet direction to create a redistributed stretch in the volume of the arc paths of the other 3 angulations. In the case of several target volumes, according to their positions in space, the dose distribution between them varies. This effect depends on the isocenter position, the arc path and the collimator rotations. Unfavorable arcs are those whose image of the BEV includes two target volumes. Table 1. Dosimetric parameters calculated for stereotactic brain plans of 15 patients. Conclusions An ambitious planning configuration for brain stereotaxy has been modified to reduce the session duration. Removing the 270° rotation had no significant dosimetric effect. The new configuration based on three table positions is acceptable. The indexes and parameters used show a low effect. The future will be to use new inverse planning software which take into account the arcs length and the table positions with variable combinations.

Introduction Inverse planning is an interactive and iterative process that depends on the user experience. This process is time consuming and requires compromises between organ at risk (OAR) sparing and target volume coverage. However, the use of a knowledge based planning system could allow harmonization of practices. Methods RapidPlan™ (Varian) is a mathematical model using existing treatment plans to estimate OARs dose volume histograms and optimization objectives of targets for new plans. The database of the expert system is made with the target volumes and OARs structures, dose prescription and beams arrangement of treatment plan compliant to dosimetric criteria. One year after the introduction of a thesaurus allowing a homogenization in planning practices, a first model composed with 36 patients treated for prostate cancer and seminal vesicles (P + VS) with a dose of 80 Gy in 40 fractions was created. For 125 patients, treatment plans performed with and without the RapidPlan™ system were compared. A second model, extended to 116 patients, was then created on the basis of the first one, including treatment plans with OARs corresponding to a wider range in terms of volumes and also cases with hip prostheses. These prostheses are considered and delineated as normal femoral heads. Optimization objectives and constraints have also been refined in this model. Results With the 36 patient model, 60% of the treatment plans obtained met the current criteria for P + VS localization as early as the first optimization. 45 patients were treated using this model. With the model extended to 116 patients, 119 patients were treated and among them 9 had single or double hip prostheses. 83.2% of treatment plans were acceptable with only one inverse optimization. Among the 20 cases requiring at least one additional optimization, 7 were due to the presence of hip prostheses (2 single and 5 double, or 5.9%), 4 to poor coverage of CTV Prostate (3.4%), 4 to an excess dose delivered to 2% of the rectum (3.4%), 3 to a low bladder filling (2.5%), 1 to an excess dose delivered to 50% of the rectum (0.8%), and 1 to the presence of a femoral cyst (0.8%). Conclusions The RapidPlan™ knowledge based planning system allows homogenization of practices in inverse planning. It increased efficiency with direct access to the result and improved treatment plan quality. RapidPlan™ also opens up an opportunity for knowledge transfer through model transfer.

Introduction Verification of day-to-day dose delivery is possible with 3D-in room imaging modalities as Cone-Beam CT (CBCT). CBCT technique is impacted by artifacts related to the acquisition and reconstruction physical principles, responsible of Hounsfield Units (HU) variability. The purpose of this work is to evaluate CBCT dose calculation performance and precision with HU correction using deformable registration algorithm (Velocity® 3.2, VMS, USA) between CT and CBCT. Methods This study is divided in two axes: (a) an image quality evaluation and HU variability among CTREF (Aquilion LB®, CMS, Holland), CBCTOBI (OBI® v1.6, VMS, USA), CBCTTB (TrueBeam® v2.5, VMS, USA) and corrected CBCT (CBCTDIR,OBI et CBCTDIR,TB) obtained with Velocity® (VMS, USA) using Catphan® 504 (Phantom Laboratory, USA). The analysis is performed with Artiscan® (Aquilab, France). (b) Using CIRS® Pelvis phantom (CIRS, USA), 3DCRT bladder and hip treatment plans and VMAT prostate plan are calculated (AAA v13.5®, Aria®, VMS, USA) on CTREF (120 kV), then calculated with fixed MU’s, with same electronic density-HU curve, on CBCTTB (TrueBeam® v2.5, 125 kV) and CBCTDIR,TB obtained via Velocity®. A comparison of dosimetric parameters (D98%, D50%, D2%) is performed as well as a CT-CBCT dose matrices comparison calculated for each modality using the gamma index obtained with 3DSlicer® (v.4.6). Results Uniformity evaluation on Artiscan® with Catphan® 504 using predefined regions of interest (ROI) is compared. The average deviations of absolute uniformity for ROI between reference (CTREF) and evaluated CBCT’s are in average 24 for CBCTTB, 30 for CBCTOBI, and less than 20 for CBCTDIR TB, OBI. Pixel size is identical among CT and CBCTDIR TB, OBI and distances restitution is infra-millimetric for all modalities evaluated. Attenuation is compared for each insert. HU average differences for all inserts, between CTREF and CBCTOBI,TB/ CBCTDIR OBI,TB modalities, are 1HU for CBCTDIR TB,OBI (min.=0 HU; max.=3HU) and greater than 13HU for CBCTTB,OBI (min.=2 UH; max.=132UH). Largest differences are observed for low and high density regions. Comparison of dosimetric parameters, respectively D98%, D50%, D2%, between CBCT and CT shows deviations of more than 3% in HU regions greater than 200HU and less than 0.5% for water-equivalent or around regions. Among CBCTDIR,TB and CT, average deviations observed are systematically lower than 1% for evaluated organs and percentages of points with a gamma value less than 1 (1%/ 1 mm) are on average 98% for the three locations studied. Conclusions This HU correction method proposed by Velocity 3.2® (VMS, USA) is relevant to account physical uncertainties related to CBCT. This tool is promising and will be evaluated on patient images to confirm these results obtained on anthropomorphic phantom.

Introduction Intra cranial hypo-fractionated radiotherapy concerns small targets which need a complex delivery to optimize the dose distribution. Current techniques combine multiple arcs and intensity modulated beams. Such very precise irradiations need an efficient dosimetric validation. Many detectors enable the comparison between the calculation and the irradiation. They are variably efficient to get a real proof of a good match. We investigated dosimetric validations from 2D detector to 3D anthropomorphic configurations. Material and methods A patient head Dicom CT image files were sent to Rt-Safe company. They 3D-printed the head with a plastic material and filled the brain cavity with an MRI dosimetric gel. The head phantom was placed in the same patient position using the original immobilization devices. The patient plan was imported in the head phantom CT-data to be calculated and then to deliver the irradiation. An MRI acquisition of the head phantom was obtained with a specific configuration. The MRI Dicom file was used to provide a 3D measured dose distribution. The same plan was used with the portal dosimetry PDIP/Varian and the Delta4/Scandidos phantom. The three dosimetric systems provided profiles comparisons and gamma index evaluations. Results The measurements and analyses concern a very small target which PTV diameter is 1cm. The profiles comparisons show a perfect matching with the Delta4 system. The PDIP measurement is slightly compromised with the continuous detector position face to the beam. The gel dosimetry contains uncertainties linked to the complex process and particularly to the MRI acquisition. Delta4 and MRI-gel display respectively a percentage of gamma index passing 3%/3 mm tolerances, of 100% and 99.2%. Table 1 displays the results with different parameters in %, mm for a global Gamma index. Download : Download high-res image (79KB) Download : Download full-size image Conclusion The way to validate and secure an irradiation depends on its complexity. The PDIP is well adapted to a routine check. The delta 4 gives a high level of validation but cannot be irradiated with non-coplanar beams or arcs. Concerning the highest dose per session treatments it is relevant to rely on an end-to-end QA process based on an anthropomorphic phantom close to the real treatment conditions. The 3D-printed head and the gel dosimetry reproduce the exact patient conditions and give a fully satisfactory validation.

Introduction Adaptive Radiotherapy (ART) is a technique which minimizes the dosimetric impact of anatomical changes that may occur during treatment. One of the milestones of ART for dose warping is deformable image registration (DIR) between images acquired during treatment and original planning images. The aim of this study is to propose a method to evaluate the performances and characterize the use of a b-spline DIR algorithm implemented in Velocity® v.3.2.0 (Varian Medical Systems, USA). Methods Two types of dataset were used to perform the evaluation, numerical phantoms and physical phantoms (called “Phydeform”). IMSIMQA™ software (OSL, UK) provides a numerical phantoms library and the opportunity to generate deformation. For algorithm characterization, deformation scenarios on simple volumes were created and two sets of images were systematically obtained for each induced deformation: initial CT and deformed CT . The evaluation related to the comparison of deformation vectors field via error histograms and contours with metrics like DICE and MDC (Mean Distance Conformity). The contour comparison was done between distorted contours obtained by Velocity® and expert contours on deformed CT . Voxel’s size influence with DIR was also evaluated. Physical phantom “Phydeform” with cubic shape of a homogenous material was used to evaluate the spatial deformation with markers and dose deformation with detectors like TLD’s (IRSN, France) and MOSFET’s (Best Medical, Canada). Two images series were compared: no-deformed CT and deformed CT obtained following the application of uniform compression on phantom. Marker’s position was evaluated between non deformed and deformed reference. Three fixed beams were delivered. The conditions were selected for highlighting dose variation between reference and deformed phantom state. Dose “deformed” value obtained after DIR application was compared with detectors measurement. Results For numerical phantoms, average Dice values are 0.83 (±2σ = 0.1) and 1.8 mm for MDC for tested deformation scenarios. For vector fields comparison, the criterion 95% voxels not exceeding 2 mm is respected. A reduced voxel size improved results. The markers position comparison between reference frame “no deformed – deformed” and “no deformed – deformed after DIR” gives us an uncertainty associated with voxels mapping of 10.3% on average. In terms of dose, uncertainty associated with voxels mapping is 4.3% for MOSFET’s and 10.4% for TLD’s. Conclusions ART software commissioning requires images deformation and dose warping validation. Results with numerical and physical phantoms make it possible to consider implementation of the first clinical applications using Velocity®.

Introduction Since 2005, the Conformal segmented mono-isocentric radiotherapy (3D-CRT) remains the standard treatment for all breast cancers in our center. However, volumetric modulated arctherapy (VMAT) may have an interest in irradiation of complex thoracic conformations. Material and methods For pectus excavatum, 3D-CRT and VMAT treatment plans [1] were calculated on CT scanner managed with and without deep inspiration breath hold (DIBH). The prescribed dose to the breast volume was 50 Gy in 25 fractions. Plan normalization was such that 95% of the target volume received 95% of the prescribed dose. Eclipse™ treatment planning system (Varian Medical Systems) and Anisotropic Analytical Algorithm (AAA) were used in this study. The field arrangement was based on two segmented opposed tangential fields in 3D-CRT, 3 arcs in VMAT and 5 arcs in DIBH VMAT. The mono-isocentric technique required direct optimization by a “Field in Field” method useful to replace the enhanced dynamic wedge unusable in quarter beams. Unlike the 3D-CRT technique, where the outer opening of the tangential fields takes into account the respiratory movements and anatomical changes (edema), the body contour on CT scanner used for inverse VMAT planning had to be artificially expanded. The optimized plan was then pasted on the initial CT and calculated keeping the monitor units obtained on the extended scanner calculation [2] . Results The main organs at risk (OAR) for this pathology were both healthy lungs (Healthy L), the healthy ipsilateral lung (healthy ILL), the heart and the contralateral breast (CL breast). For this OAR, the characteristic parameters such as the volume receiving 20 Gy (V20 Gy) or 30 Gy (V30 Gy), the mean dose (D mean ), or the dose at 2% of the volume (D2%), were recorded. They were analyzed according to the criteria noted in the table below. VMAT planning meets all the dosimetric validation criteria and allows better lung sparing. In contrary, even if the criteria are acceptable, the heart is less protected by this technique. The major benefit provided by the VMAT technique remains the contralateral breast protection. Conclusion This dosimetric comparison shows that the volumetric modulated arctherapy is benefic for treating specific conformations like funnel chest, with a better OAR sparing. This comparison should be made for the treatment of left side breast with lymph node, and for thin and concave thoracic walls.

Introduction Adaptive Radiotherapy (ART) is a technique which minimizes the dosimetric impact of anatomical changes that may occur during treatment. One of the milestones of ART is deformable image registration (DIR) between original planning images and images acquired during treatment. The aim of this study is to propose a method to evaluate the performances and validate the clinical use of a “Demons” intensity-based DIR algorithm implemented in SmartAdapt® v.13.5 (Varian Medical Systems, USA). Methods Two types of dataset were used to perform the evaluation: numerical phantoms and CT-CBCT images of 5 patients with head and neck (HN max = 0.92) and 0.73 (min = 0.65; max = 0.9) for registrations CTD0-CTre-CT and CTD0-CBCTD16 respectively. The dispersion of the values is mainly due to organs shape and algorithm choice. No significant variation is obtained between DIR for single-modality (CT–CT) and multi-modality (CT–CBCT). Moreover, contours propagation reduces significantly physicians required time for contouring patient images. Conclusions Given the results obtained with numerical phantoms and CT-patients, a clinical use appears possible. SmartAdapt® enables significant time-saving compared to manual re-contouring for re-planning strategies. However, a careful review by a specialist for DIR-generated contours is essential for clinical cases.

Introduction A daily repositioning Cone Beam Computed Tomography image (CBCT) for prostate radiotherapy is realized using exposure templates (mAs, kV) which affect image quality and imaging dose. Settings should be optimized to minimize patient exposure while maintaining sufficient image quality to register the initial planning CT with CBCT using soft tissue matching. Methods 20 prostate patients (without hip prosthesis) with daily CBCT (40 fractions) acquired on a TrueBeam™ (Varian Medical Systems) machine were selected. After the first fraction using the standard pelvis template (125 kV 1080 mAs CTDIw 14 mGy), the therapists manually applied, day after day, a low mAs reduction and assessed if the CBCT image quality was good enough for patient repositioning. The iterative process stopped when image quality was assessed too bad and the last proper mAs were selected. The link between the mAs reduction and corpulence (patient volume inside CBCT FOV) was studied. For one example patient 23 therapists registered CBCT images with CT for 3 fractions: the first fraction (S 0% ), a fraction with 50% mAs reduction (S −50% ) and the fraction with maximum mAs reduction (S −71% ). Fisher’s test was applied to every direction, to compare the variance between S 0% /S −50% and S 0% /S −71% . Results The median mAs reduction for all patients was 64% (13–85%). Patient corpulence was not correlated to the mAs reduction achieved (Spearman’s correlation r s = 0.465). Variance analysis, for every direction, shows no significant difference ( p 0% /S −50% and S 0% /S −71% . Conclusion mAs reductions recorded across the 20 patients are highly variable, due to the subjective assessment of CBCT image quality, but a median reduction of 64% indicates a great potential for reducing imaging dose. For one patient it has been demonstrated that image quality deterioration has no impact on interobserver variability. A 50% mAs reduction for the Pelvis CBCT template is therefore considered.

Introduction The quality of the dosimetric calculation is directly related to computerized calculation model and its operating parameters of the irradiation. The toughest conditions affect areas containing air cavities such as sinuses. Outside the Monte Carlo method, no other model correctly reproduces the dose distribution. The film dosimetry combined with the use of an anthropomorphic phantom help to assess the exact isodoses positions Material and methods A head phantom, dedicated to this study is voluntary cut in coronal planes passing through the air area cavities that were hollowed out. The film is constructed with EBT3 sensitive layer sandwiched between two plastic films. The shapes of the cavities are laser cut so as to accurately reproduce the contours. Laser cutting allows, at the same time to weld the two edges of the two plastic films and thus to maintain the sensitive layer in normal.The evaluation method was to compare dose distributions without cutting and with cutting, respect to the calculation, for a fixed rectangular irradiation (dimension 10 × 12 cm2) and a modulated irradiation arc therapy Results Laser cutting edge creates a film of constant quality. Soldering is uniform and effective. Comparison of dose distributions using the gamma index does not provide a useful analysis because of the very important differences. The comparison with profiles provides an information on the actual dose in the first few millimeters at the edge of the cavities. The uncut part of the film contributes to change the dose measurement in the first millimeters. So, cutting optimizes the measurement conditions. Comparison with the calculations brings provisionally useful information for potential clinical judgment. Conclusion The method of laser cutting the EBT3 film is effective. The measurement is accurate and reflects the dose distribution at the edge of the cavities. Perspectives: Measurements of cut films will be compared to a Monte Carlo calculation.

Purpose/Objective Cranial stereotactic treatments are planned with Volumetric Arctherapy (VMAT) and delivered with an accelerator TrueBeam (STX-HD)/Varian. Several arcs are defined with three iscocentric treatment table rotations. The images used to control the positioning are obtained only with the kV imager OBI (On Board Imaging). The accelerator room does not contain any additional inroom imagers. Each table rotation is validated with specific planar kV images. Material/methods The patient positioning is assessed with the help of a CB-CT compared to the reference treatment planning CT. An automatic table displacement is performed when set-up errors appear. This position being defined as a reference; two additional anterior and posterior planar images are performed. Three markers are placed on the tabletop (MT), on the mask at the front isocenter projection (MI) and on the patient's skin (MP) through a hole above an orbit. Positions of the 3 markers in the orthonormal coordinate system are recorded (L–R and H–F directions). The markers positions are then compared to those measured at the other treatment table rotations. Results The evaluation involved 1094 measurements of 360 images and 15 different patients. The images comparison is carried out for three markers in the two orthogonal directions. The measurement error, due to the use of a graphic rule, is estimated to be 0.2 mm. The evaluation is performed for each marker (MT, MI, MP). The maximum deviation is, respectively, 0.9, 0.9, 4.2 mm. The average deviation is, respectively, 0.4, 0.3, 0.4 mm. The standard deviation is, respectively, 0.3, 0.2, 0.5 mm. Conclusion Knowing that no additional in-room images are used, comparing the position of every marker between the images obtained at table rotations, relative to the reference position, ensures the traceability of each non-coplanar position. The three markers provide additional information on the three elements (table, patient, mask) related to the target. The results are consistent with the mechanical precision of the TrueBeam accelerator and its treatment table. We finally validate the safety margin PTV set to 3 mm.

Introduction The quality of a treatment performed by intensity modulation is based on the positioning accuracy of the multileaf collimator leaves (MLC, 120 leaves). The dosimetric impact achieved by the introduction of systematic errors on the leaves positioning has been studied to assess the generated effects. Materials and methods The study was performed on 8 patients treated for prostate localization with 80 Gy prescribed dose in dynamic arc therapy. Original treatment plans clinically approved were exported in DICOM RT to be processed using Matlab ® software. Systematic errors of magnitude between 0.5 mm and 5 mm were introduced by opening the MLC leaves, or by closing and shifting. The 128 revised plans were then introduced into the treatment planning system (TPS). They were recalculated keeping the same optimization constraints than the original plan. Geometric and dosimetric index (SFRO and ICRU used index) were calculated using the Artiview ® software, Aquilab. Results Changes introduced to the original plans are significantly impacting dosimetric treatment planning validation index. Among these index, the near maximum dose D_2%(Gy) received by the target volume (PTV) differs from 12%, 15% and 20% to the original treatment plan for maximum openings, shifts and closures respectively. For the same index and amplitude errors of up to 2 mm, an offset MLC error is less penalizing ( 5%) systematic error. Other dosimetric and geometric index complete the analysis as the Dice similarity coefficient (DSC), which is more sensitive to errors introduction in treatment plans. Conclusion Dosimetric validation is based on calculated and measured doses distributions comparison, thanks to tolerances (generally 3%/3 mm). This study allows us to understand the impact of potential MLC positioning errors on planned treatment and to discuss about the validity of quality assurance tolerance values.

Leipzig H-Type applicators are used with a microSelectron HDR (Elekta, Nucletron, Stockholm, Sweden) for treatments of small size cutaneous lesions (up to 2 cm) and shallow depth (3–5 mm). This work presents the checks for the commissioning and the tests for clinical implementation of these devices. Applicators have three different diameters (1, 2, 3 cm). Controls are: – Determination of reference source to indexer distance (DSI): the optimal DSI is determined with EBT-3 gafchromic film. The evaluation criteria are the centering position of irradiation and the orthogonal profiles symmetry. – Comparison with Monte Carlo simulations provided by the manufacturer: • PDD and profiles at different depths (surface, 3 mm, 5 mm, 10 mm) comparison with gafchromic films • Output verification (cGy/h) at 3 mm depth on axis, with a well chamber and specific insert measuring the corresponding factor and compared with the reference values (Perez- Calatayud et al., Med Phys 33(1) 2006), • comparison with small parallel-plate ionization chamber (Markus), – Profile symmetry evaluation with gafchromic films. – checking irradiation time calculated manually with Oncentra TPS (v4.3). The reference distance is 1321 mm. Dose differences are less than 7% for the implemented techniques. The profiles symmetry is between 2% and 5%. Double checking is an efficient technique to prevent human error which may occur during manual calculation of the irradiation time. Testing results validate the Leipzig applicators clinical use for skin lesions treatment.