Simulation of the head of an accelerator: Calculation optimization and ...

Abstracts/Physica Medica 31 (2015) e23–e54

point has no significant influence on the dose at this point. Thickness of tissue treated with flat and surface applicators is only a few millimetres, depending on the applicator’s size. Conclusions: the INTRABEAM® system with surface and flat applicators is a reliable way of treating superficial cutaneous malignancies as long as there is a good contact between the applicator and the skin. References [1] Schneider F, Clausen S, Thölking J. A novel approach for superficial intraoperative radiotherapy (IORT) using a 50 kV X-ray source: a technical and case report. J Appl Clin Med Phys 2014;15(1):4502. [2] Goubert M, Parent L. Dosimetric characterization of INTRABEAM® miniature accelerator flat and surface applicators for dermatologic applications. Phys Med 2015;31(3):224–32. http://dx.doi.org/10.1016/j.ejmp.2015.10.054 53 EVALUATION OF GEOMETRIC UNCERTAINTIES AND PLANNING TARGET VOLUME (PTV) MARGIN IN HYPO-FRACTIONATED RADIOSURGERY USING GAMMA KNIFE EXTEND SYSTEM J. Champoudry, A. Dorenlot. Service de Neurochirurgie, Hôpital La Timone, Marseille, France Introduction: This study aims to assess the uncertainty of Extend system repositioning and to determine planning target margins in hypo-fractionated Gamma Knife radiosurgery. Methods: The study includes a cohort of 30 patients. A dose of 24 Gy to the prescription isodose line was delivered in 5 sessions. The Extend system was used for daily patient repositioning and all measurements acquired using the repositioning tool box (RTB) were recorded. Every day a new planning was simulated taking into account these displacements by moving shots coordinates in the treatment planning system (Leksell Gamma Plan v10.1.1) according to the daily displacements. The initial planning was then compared with the sum of daily planning (SDP) and gamma index evaluation (3%/0 mm, 3%/0.5 mm, 3%/0.8 mm and 3%/1 mm with local dose criteria) in three dimensions were performed. Selectivity, conformity, coverage and gradient indexes were also calculated to quantify the impact of patient repositioning on the dose distribution. Results: The mean value of displacement in each of the three directions x, y and z are equal to 0.09 mm, 0.04 mm and 0.02 mm respectively. The mean absolute dose difference between the initial planning and SDP is equal to 4.2 Gy (SD: 2.2 Gy) with a maximum dose deviation up to 12.15 Gy. The mean value of gamma index assessment in the following configurations (3%/0 mm, 3%/ 0.5 mm, 3%/0.8 mm and 3%/1 mm) are respectively 56% (SD: 23.6%), 96.8% (SD: 6.4%), 99.6% (SD 1.1%) and 99.9% (SD: 0.2%). In the meantime the acceptability criteria (at least 95% of evaluation points respect gamma index criteria) are observed in 0%, 90%, 100% and 100% of cases with these gamma index configurations. We do not notice any degradation of the gradient index from the initial planning to the SDP but we observe a reduction of around 3%, 6% and 7% of the conformity, selectivity and coverage indexes. Conclusions: In the case of this study and taking into account the factors influencing the accuracy of patient repositioning (quality of dental imprint, team experience, learning curve, etc.) a margin of 0.8 mm between the clinical and planning target volume seems sufficient for our practice. However, in the case of a general recommendation a margin of 1 mm seems appropriate for this type of treatment.

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device includes an ionization chambers matrix and provides 2D signal maps for any irradiation field, upstream to the patient. Converting the detector’s signal into dose in the patient is a challenge. To do so, Monte Carlo simulation is a powerful tool, and will give accurate results if correctly parameterized. The work described here focuses on the optimization of the treatment head simulation (especially on the target, the source of X-rays) and the determination of the correct nominal energy and radial distribution of electrons on the target. In order to calibrate the detector, one must reach a perfect agreement between the observables from the simulation and from the beam of our reference accelerator. Methods: The interactions of the electrons in the target are numerous and time consuming to simulate. A study was conducted on both parameters of different particles transport methods and variance reduction in PENELOPE Monte Carlo code, which remains a reference for electron transport for the considered energies [1]. We have optimized a set of parameters to keep a reasonable computation time without biasing the physical observables. The determination of the initial characteristics of the electron beam (nominal energy/radial distribution of electrons on the target) is done by trial and error process. Several simulations are performed at various energies and radial electron distributions. The dose deposited in a simulated water phantom is compared with the depth-dose curves and dose profiles acquired under irradiation. We propose two efficient methods of comparison, the calculation of the Kolmogorov-Smirnov test and an original extension with more sensitivity. Results: The set of optimized parameters provides an overall increase on the simulation efficiency of nearly 300%. The useful secondary particles generation rate (stored in the PSF) was also increased by about 250%. Determining the characteristics of the electron accelerator of our reference beam is ongoing, we currently have simulated three energies around 6 MeV. In the appendix are presented the results of two comparative tests for these three energies. These tests including measurement uncertainties are robust and will accurately lead to the beam nominal energy with only five simulated energy sets. To date, the simulation has been running for six months on the IN2P3 computing center. Conclusions: The developed statistical tests are robust and will allow us to accurately determine the characteristics of the beam by comparing the simulated and the measured depth-dose curves and dose profiles. We will then be able to validate the reference accelerator simulation and calibrate the detector in terms of dose. This will allow us to reconstruct the 3D dose matrix in water from upstream information collected on a TraDeRa simulated model. Appendix

http://dx.doi.org/10.1016/j.ejmp.2015.10.055 54 SIMULATION OF THE HEAD OF AN ACCELERATOR: CALCULATION OPTIMIZATION AND STATISTICAL COMPARISON METHODS R. Fabbro a, R. Delorme a, Y. Arnoud a, J.-F. Adam b, B. Boyer a, L. Gallin-Martel a, M.-L. Gallin-Martel a, O. Rossetto a, I. Fonteille a, J.-Y. Giraud c. a LPSC, Université Joseph Fourier Grenoble 1, CNRS, IN2P3, Grenoble, France; b Grenoble Institut des Neurosciences (GIN), Inserm U836, Grenoble, France; c CHUG, Inserm U836, Grenoble, France Introduction: The group Developments and Application for Medicine of the LPSC, in collaboration with the Public Hospital of Grenoble, is developing the TraDeRa detector (Transparent Detector for Radiotherapy). This

Top left: Simulated (points) and measured (blue solid line) depth-dose curves, normalized to the dose at 100 mm. Top right: Relative differences between simulations and measurement. Bottom left: Distribution of the results of Kolmogorov-Smirnov test. Bottom right: Distribution of the results of Enhanced Kolmogorov test.

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Abstracts/Physica Medica 31 (2015) e23–e54

Reference [1] Faddegon B, et al. The accuracy of EGSnrc, Geant4 and PENELOPE Monte Carlo systems for the simulation of electron scatter in external beam radiotherapy. Phys Med Biol 2009;54(20):6151–63. http://dx.doi.org/10.1016/j.ejmp.2015.10.056

55 EVALUATION OF DETECTORS RESPONSE FOR SMALL FIELD OUTPUT FACTOR MEASUREMENT USING MULTICHANNEL FILM DOSIMETRY G. Rucka, P. Budillon, J.C. Mouttet, N. Asquier. Hopital de la Croix Rouge, Toulon, France Introduction: Most irradiation technics require dose computing from TPS. Calculation accuracy highly depends on the measurements used for beam modeling. Depending on their characteristics, available detectors may be best suited for specific field sizes when measuring Output Factors (OF). Recent studies compare several active with passive detectors and MonteCarlo calculation. The goal of our study is to evaluate the response of several active detectors exposed to 6 MV X-ray beams of different sizes, down to 1 × 1 cm2, while considering EBT3 Gafchromic films as reference. Methods: Eight EBT3 films were irradiated with field sizes ranging from 1 × 1 to 10 × 10 cm2. Measurements were done in a homemade RW3 solid water phantom. Multichannel film dosimetry was used for film opacityto-dose conversion. All films (including background) were irradiated and scanned simultaneously using the efficient protocol described by D. Lewis et al. Among available active detectors, two ionization chambers and two diodes were studied. Measurements were carried out in a water phantom. OF measurements were also done by placing both chambers in the solid water phantom, in the same condition as the films. Results were compared to measurements done in water in order to verify scattering components correspondence for all field sizes. This allows active detectors irradiated in water to be compared to the films in RW3 slabs. Results: OF obtained with the ionization chambers placed in the water and solid water phantom are identical for field sizes smaller than 15 × 15 cm2. As described in P. Andreo publication, active detector response for each field size was normalized with respect to the reference data. Concerning ionization chambers, the influence of partial volume averaging is similar to the published results. The three major effects mentioned for the diodes also appear in our results: the charged particles equilibrium between detector material and water, the over-response of the unshielded diode in broad beams and the partial volume averaging.

56 BREAST PATIENT POSITIONING WITH CATALYST ON TOMOTHERAPY F. Crop a, E. Steux b, J. Bouillon b, A. Gadroy b, J. Doré b, A. Baczkiewic b, Q. Olivier b, L. Bequet b, E. Lartigau b, D. Pasquier b. a Medical Physics,Centre Oscar Lambret, Lille, France; b Radiotherapy Department, Centre Oscar Lambret, Lille, France Introduction: Breast treatments with nodal involvement may require an IMRT technique, combined with IGRT. An important factor is the correct positioning of these breast patients, as incorrect placement of chin or arms can have a deformation of the volumes. We evaluated a surface positioning technique, Catalyst. Due to intrafraction movements, the precision of the IGRT process has to be offset with the time taken for the IGRT process. When taking a lot of time, the patient will have moved more than the “thought” precision. Methods: 620 sessions (43 patients) with catalyst + MVCT positioning were compared with laser based positioning. MVCT fusion inter-user variability was evaluated, as this is the theoretical limit for the auxiliary Catalyst system. A statistically balanced study was conducted: 10 users performed MVCT fusion of 12 cases twice (once starting with automatic MVCT fusion, once without). These 12 cases were composed of 3 patients, each with 2 sessions positioned with Catalyst and 2 sessions with lasers. Differences in variability were evaluated using the robust Brown-Forsythe test. As there is always intra-fraction movement of the patient, the longer the IGRT process takes, the more the patient will have moved. The inter-user MVCT fusion variability is combined with the differences between automatic fusion and the mean results of all users. By applying the model of patient movement [1], a theoretical time limit can be evaluated on the manual fusion process. Using this same standard deviation, a limit can be evaluated when Catalyst will outperform MVCT positioning. Results: The results of the catalyst positioning are compared to the laser positioning in Fig. 1. We clearly see that a gain in X and Z direction is obtained. The Y direction could be improved by repositioning the camera: the FOV of the camera is in its current position is not ideal for the Y direction positioning. Inter user MVCT fusion variability (1 standard deviation) was evaluated as (X, Y, Z, roll) 1.4 mm; 1.8 mm; 1.3 mm; 1°. For Catalyst based positioning, followed by automatic fusion and manual adjustment, the inter user variability of the MVCT fusion process was evaluated as 1 mm; 1.7 mm; 1.1 mm; 0.8° and statistically significant better (except Y direction). This is most likely due to (a) lower MVCT resolution in the Y direction and (b) suboptimal Catalyst camera placement.

Conclusions: Our study confirms that partial volume averaging is not the only undesirable effect for OF measurement. Thus, the detector having the best spatial resolution is not systematically the best suited for small fields OF measurements. http://dx.doi.org/10.1016/j.ejmp.2015.10.057

Figure 1. Cumulative Histograms of patient positioning. Green: Catalyst, Red: Laserbased, black: theoretical limit (inter user variability MVCT fusion).