Abstract Book

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ESTRO 37

low-as-reasonably-achievable principle.

work was to adapt a clinical TPS in order to take into account the constraints and requirements of pre-clinical irradiations and to benefit from all the features. Material and Methods µ-RayStation (µ-RS) was derived from RayStation v5. A model of the XRAD225Cx was created based on measurements, allowing arc and static beams for 7 cylindrical collimators from 20mm to 1mm of diameter. Dose distributions are calculated with a Monte Carlo algorithm (VMC++) (1,2) . Calculations were compared with EBT3 measurements in water for all static beams and with GATE in heterogeneous media (a 5mm static beam in layers of water/bone/lung/water) and a mouse CT for 5mm static and arc beams. Results Minimal dose calculation grid size was reduced from 1x1x1mm 3 to 0.1x0.1x0.1mm 3 . The size of contouring tools was reduced and the contour resolution was increased, both by a factor of 10, to fit the voxel size of the small animal images (about 0.2x0.2x0.2mm 3 ). In water, µ-RS calculations and EBT3 measurements agreed within 3% and maximal distance-to-agreement (DTA) was 0.2mm at 50% of the central dose. However, due to the focal spot fluence heterogeneity, the two smallest beams (2.5 and 1mm diameter) presented irregular shape not considered in the model. For a 5mm static beam in heterogeneous media, the mean absolute error between µ-RS and GATE calculations was below 1.2% in each medium and DTA was 0.01mm at interfaces. For calculations on a mouse CT, µ-RS and GATE calculations for static and arc beams agreed. Conclusion µ-RayStation is a complete TPS, adapted and fully validated for pre-clinical irradiations. A large set of relevant clinical tools available in RayStation v5 can be applied for pre-clinical studies in µ-RS: contouring tools, rigid and deformable registrations, planning facilities, plan evaluation tools, dose deformation and summation, etc. We expect that this new TPS will expand the possibilities of mimicking patient radiotherapy in preclinical studies. 1. Kawrakow I et al . The Use of Computers in Radiation Therapy, 126-128, Springer, 2000. Terribilini D et al . Medical physics, 37 (10), 5218-5227, 2010. EP-1920 Can delivered dose explain local recurrence in patients with prostate radiotherapy? Y. Sayous 1 , G. Delpon 1 , V. Libois 1 , S. Supiot 1 , S. Chiavassa 1 1 Institut de cancerologie de L'Ouest, René Gauducheau, Saint-Herblain, France Purpose or Objective Following radical radiotherapy for prostate cancer, up to 15% of patients might relapse locally. Whether local relapse following radiotherapy is due to intrinsic tumour radioresistance or due to prostate motion-related target miss remains an open question. To evaluate dose distribution during the course of radiotherapy, modern tools of treatment planning systems, such as Deformable Image Registration (DIR) and dose accumulation, allow investigating the impact of the irradiation on these recurrences. In this study, we retrospectively evaluated the planned and the delivered dose to the recurrence area using RayStation 6 (RS6). Material and Methods In RS6, DIR obtained from two images can be applied to deform contour or dose distribution in order to put all data on the same referential, in our case, the planning CT (pCT). RS6 provides 3 DIR methods: (1) “Hybrid method” (ANACONDA) using intensity and moving organs as controlling contours, (2)“Intensity method” using ANACONDA without controlling contours and (3) MORFEUS without intensity consideration. These 3 approaches were 2.

EP-1919 µ-RayStation: an adaptation of RayStation 5 for small animal radiotherapy S. Chiavassa 1,2 , R. Nilsson 3 , K. Clement-Colmou 1,2 , V. Potiron 1,2 , G. Delpon 1,2 1 Institut de cancerologie de L'Ouest, René Gauducheau, Saint-Herblain, France 2 CRCINA, UMR1232, Nantes, France 3 RaySearch Laboratories, Stockholm, Sweden Purpose or Objective Modern pre-clinical radiotherapy allows to mimic 3D image-guided clinical radiotherapy but has to be adapted to small animal and target size constraints: beam size, targeting accuracy and image resolution are scaled-down; and beam energy is reduced from MV to kV. In our institution, the XRAD225Cx µ-irradiator is used for pre- clinical studies and a Monte Carlo (MC) model (GATEv7) was previously created and validated for dose calculation in small animals. However, typical MC environments do not provide the same tools that are available in a clinical treatment planning system (TPS) to manage patient workflow and irradiation. Moreover, these tools are not adapted for pre-clinical requirements. The goal of this