ESTRO 36 Abstract Book

S80 ESTRO 36 _______________________________________________________________________________________________

propagate urethra delineation to the test patients. The n- most similar individuals were selected and final segmentation was obtained by a weighted vote. Leave-one out cross validation of the atlas for urethra segmentation was first performed on the training data set. Mean Centerline Dispersion (MCD) and Hausdorff Distance (HD) were used for accuracy assessment. The method was then applied to a second set of 95 patients having received 78 Gy by IMRT for prostate cancer. Target volume and organs at risks (bladder, prostate) were delineated on computed tomography (CT) slices according to the French GETUG group recommendations. Then, the urethra was segmented using the proposed approach and dose was measured inside the resulting segmentation and compared to the dose to the prostate. Results From the training data set, the number of most similar atlases was optimized to 10 in the leave one out scheme. Average MCD of 2.3 mm and HD of 3.5 mm were thereby obtained. In the testing data base dose received by the segmented urethra were significantly higher than the whole prostate in a range of dose from 74 Gy to 79 Gy (Wilcoxon test p<0.01). Conclusion An accurate atlas based segmentation method was proposed allowing assessment of dose within prostatic urethra. Dose in this organ was significantly higher than the whole prostate, mainly in the highest dose range. Results open the way to further NTCP studies relating urinary toxicity such as obstructive symptoms to the urethra dose.

on gantries for proton therapy that have a different geometry and did not use a bow-tie filter. The performance of an a priori scatter correction algorithm was in this study compared for the first time on CBCT systems for photon vs. proton therapy gantries. Material and Methods The a priori scatter correction algorithm used a plan CT (pCT) and raw CB projections. The projections were acquired with On-Board Imagers of a Varian photon therapy Clinac and of a Varian proton therapy ProBeam system. The projections were initially corrected for beam hardening followed by reconstruction using the RTK back projection Feldkamp-Davis-Kress algorithm (rawCBCT). Manual, rigid and deformable registrations were applied using Plastimatch on the pCT to the rawCBCT. The resulting images were forward projected onto the same angles as the raw CB projections. The two projections sets were then subtracted from each other, Gaussian and median filtered, and then subtracted from the raw projections and finally reconstructed to a scatter corrected CBCT. To evaluate the algorithm, water equivalent path length (WEPL) maps were calculated from anterior to posterior on different reconstructions of the data sets (CB projections and pCT). Initially we evaluated CB projections of an Alderson phantom acquired on the Clinac system before comparing CB projections of the same CatPhan phantom acquired on both the Clinac and the ProBeam systems. Results In the analysis of the Clinac projections of the Alderson phantom, the scatter correction resulted in sub-mm mean WEPL difference from the rigid registration of the pCT, considerably smaller than what was achieved with the regular Varian CBCT reconstruction algorithm (Figure 1). The largest improvement was for the half-fan (below neck) scans. With the Catphan phantom the rawCBCT was very similar to the Varian reconstruction, due to a refitting of beam hardening curve. When comparing reconstructions of photon to proton gantry CB projections (Figure 2) we found that the a priori scatter correction improved the mean WEPL difference while preserving image quality (the number of countable line pairs) for both gantry types. The photon gantry showed less WEPL difference, however used a higher pulse current per acquisition ( 2.00 mAs), compared to the proton gantry (1.4 mAs). The complete scatter correction is performed within three minutes on a desktop with NVidia graphics.

OC-0158 a priori scatter correction of cone-beam CT projections in photon vs. proton therapy gantries A.G. Andersen 1 , Y. Park 2 , O. Casares-Magaz 1 , U. Elstrøm 1 , J. Petersen 1 , B. Winey 2 , L. Dong 3 , L. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus V, Denmark 2 Massachusetts General Hospital, Department of Radiation Oncology, Boston- Massachusetts, USA 3 Scripps Proton Therapy Center, Department of Medical Physics, San Diego- California, USA Purpose or Objective Cone-beam (CB) CT is becoming available on proton therapy gantries, to allow image/dose-guidance and adaptation for protons. To use these techniques clinically, the challenges related to image quality and Hounsfield Unit accuracy need to be solved. Algorithms for scatter correction have been developed, and have been explored for CBCT systems on photon therapy gantries but so far not