ESTRO 37 Abstract book

S1045

ESTRO 37

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. 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 first evaluated for 15 CT/CBCT pairs, qualitatively and 2. Terribilini D et al . Medical physics, 37 (10), 5218-5227, 2010.

quantitatively using Dice coefficient (DC) for rectum, bladder and hip joints. We selected 3 patients with a local relapse (prostate only 2 pts; prostate + seminal vesicles -SV- 1 pt). For each one, [18F]-choline-PET/CT recurrence images were delineated using the “automatic adaptive segmentation” method in PLANETdose (Dosisoft®).Recurrence contours were then copied on the pCT in RS6 using DIR. In the same way, dose-of-the-day from daily CBCT were deformed and summed to obtain delivered dose on the pCT. Results The hybrid method (1) provided better quantitative results with a mean DC of 0.99±0.01 comparing with 0.87±0.07 and 0.95±0.03 for Intensity method (2) and MORFEUS (3) respectively. Moreover, MORFEUS was qualitatively less effective. The hybrid method (1) was then used for the 3 patients of the study, requiring manual delineations for each image. For recurrence area within the prostate, planned and delivered dose in 99% of the recurrence areas (D99p and D99d) were higher than prescribed dose. In one patient, local relapse was located not only in the prostate but also at the tip of both SV which were not included in the PTV. D99p and D99d of the SV relapses were significantly lower than the prescribed dose of 60Gy (D99p=44.3±0.3Gy and D99d=47.5±1.1Gy). Conclusion The delivered dose to intraprostatic local relapse regions is not lower than the prescribed dose. In other words, local relapses are likely to be due to intrinsic radioresistance rather than geographic miss. Reduced dose to the seminal vesicles might explain local relapse to this region, if not included in the PTV. This study shows that RS6 tools allow to investigate the planning causes of treatment failure. However, for pelvic area, the methodology remains time consuming requiring manual delineations for each image. EP-1921 Dosimetric study of neck skin dose for early- stage nasopharyngeal carcinoma X.F. Liao 1 , J. Li 1 , L.C. Orlandini 1 , P. Xu 1 , C. Li 1 , P. Wang 1 , Y. Chen 1 , J. Xu 1 , T. Tan 1 , Y. Zhang 1 1 Sichuan Cancer Hospital & Institute, Radiotherapy, ChengDu, China Purpose or Objective Acute skin toxicity observed in radiotherapy treatment of head-and-neck cancer is a big concern. The purpose of this work was to investigate the feasibility to reduce skin toxicity using a specific delivery technology or parameters for planning optimization. Neck skin dose obtained for dIMRT, VMAT and helical tomotherapy (TOMO) of early-stage nasopharyngeal carcinoma was investigated. Moreover, the treatments planning optimized with and without constraints on the skin dose were compared. Material and Methods 16 patients admitted and treated in our hospital for early-stage nasopharyngeal carcinoma were included in this study. Six treatments planning were performed for every patient, firstly, three traditional plans (TP) for each modality (dIMRT; VMAT, TOMO) ; secondly, three new plans (NP) taking into consideration also the skin as a sensitive structure during inverse planning optimization. A 6 MV photons beam produced by a Trilogy linear accelerator (Varian , CA, USA) and by Tomotherapy (Accuray, V4.2 USA) were used for dIMRT/VMAT and TOMO treatments planning respectively. The targets (CTV1, CTV2, CTV3) and regions of interest (ROIs) were defined by an experienced radiation oncologist while the neck skin tissue included 3 mm from the skin surface and was generated automatically with planning contour tools. The doses prescribed of 70Gy, 66Gy and 60Gy (for CTV1, CTV2 and CTV3 respectively) were delivered in 33 fractions. Dose volume histogram cut-off points D 95%, D 98% ,