ESTRO 2020 Abstract book

S848 ESTRO 2020

not significantly different between the two planning strategies

The IMRT plans generated by the AI-driven TPS were better at fulfilling the clinical goals than the generated VMAT plans for all the treatment sites. Both, the coverage of the PTV and the sparing of OAR were very similar between the best AI-driven IMRT plans and the reference plans from the standard TPS (Table 1). The CI was similar between the two TPS for IMRT plans and better for VMAT reference plan compared to the AI driven plan. The HI was closer to 0 for the reference plan for both IMRT and VMAT plan. The modulation complexity was higher for the AI-driven VMAT plans compared to the reference plan and comparable for the IMRT treatment plans generated by the two TPS. The dose to the normal tissue was equivalent for all techniques investigated (Table 2). Conclusion At treatment planning level, the study demonstrated that the use of an AI-driven TPS are competitive with the current IMRT/VMAT plans created with a standard TPS and can improve the sparing of healthy tissues while maintaining a full coverage of the PTV. Further studies are required to confirm these early results. PO-1484 MRI only based planning to decrease toxicity in prostate cancer IMRT/IGRT: a dosimetric study K. Stephane 1 , A. Houssayni 2 , U. Cassard 2 , A. Barateau 2 , L. Duvergé 1 , A. Largent 2 , O. Acosta 2 , J. Nunes 2 , T. Messai 1 , A. Simon 2 , J. Dowling 3 , N. Perichon 1 , R. De Crevoisier 1 1 Centre Eugene Marquis, Radiotherapy, Rennes, France ; 2 University Rennes, Inserm LTSI-UMR 1099, Rennes, France ; 3 CSIRO, e-Health Research Centre, Herston, Australia Purpose or Objective Unlike CT-scans, MRI allows improved visualization of the prostate and the anatomical structures involved in erectile function. The aim of this study was to evaluate the dosimetric benefit of MRI-only based radiotherapy planning to spare the organs-at-risk (OARs) in IMRT/IGRT for prostate cancer, compared to a standard radiotherapy planning from CT. Material and Methods A total of 15 patients were considered in this study. Each patient had a CT and an MRI scan. Pseudo-CTs (pCT) were generated from MRI scans using a generative adversarial network deep learning method. “Standard” volumes of interest such as prostate, seminal vesicles (SV), rectum, bladder and femoral heads were manually delineated on pCT (to mimic contours on CT-scan) and MR images. However, penile bulb (PB), internal pudendal arteries (IPA) and corpus cavernosum (CC) were only delineated on MRI. Then, two planning strategies were computed on the pCT for each patient, with the Pinnacle (Philips) treatment planning system using a VMAT technique (6 MV, 1 arc per phase). One strategy used the contours from the pCT and the other one the contours from the MRI, both delivering 78 Gy in the prostate and 50 Gy in the SV. The CT-based planning considered dose constraints for the “standard” OARs, respecting the GETUG/RTOG recommendations. The MRI-based planning considered dose constraints in the “standard” OARs and additional constraints to spare the PB, the IPA and the CC. The prostate volumes, DVHs, monitor units (MU), conformity and homogeneity indexes were compared between the two planning strategies using the Wilcoxon test. Results The prostate volume was significantly smaller using the contours from MRI (median value: 49.9 cc (range, 16.1- 121.6) than on CT (64.9 cc (32.4-124.2)) (p<0.01). The prostate coverage (V95%) was not significantly different between the two plans (98.6% versus 98.4%; p=0.10). Table 1 displays the dose in the OARs, by planning strategy. The doses were significantly reduced in all the OARs (except for the femoral heads) using the MRI-based planning. The number of MUs, conformity and homogeneity indexes were

Conclusion Compared to a standard CT-based planning approach, MRI- only based planning significantly decreased the dose in all the OARs, in particular in the structures involved in the erectile function. This dosimetric study justifies a clinical trial demonstrating the clinical benefit (toxicity reduction) of this new planning strategy. PO-1485 Comparison of electron beam shaping efficiency with metal and 3D-printed plastic collimators S. Stuchebrov 1 , A. Bulavskaya 1 , Y. Cherepennikov 2 , B. Gavrikov 3 , I. Miloichikova 2,4 1 Tomsk Polytechnic University, Research School of High- Energy Physics, Tomsk, Russian Federation ; 2 Tomsk Polytechnic University, School of Nuclear Science & Engineering, Tomsk, Russian Federation ; 3 Moscow City Oncological Hospital No. 62, the 1st Radiological Department, Moscow, Russian Federation ; 4 Cancer Research Institute of Tomsk National Research Medical Center of the Russian Academy of Sciences, Radiotherapy Department, Tomsk, Russian Federation Purpose or Objective Electron beam therapy is highly effective for shallow tumors treatment. Modern radiotherapy techniques allow delivering a high single-time dose directly to the target volume. Consequently, there are strict requirements for the spatial profiles of therapeutic beams in order to ensure treatment accuracy. The latter causes a necessity to develop new patient-specific approaches to the shaping of electron beam radiation fields. This report proposes an alternative approach to electron beam shaping with plastic produced by fused deposition modeling instead of standard manufactured by metal cutting or melting. The purpose of this study is to compare the radiation fields of a 6 MeV electron therapeutic beam shaped by the plastic collimator and a standard metal one. Material and Methods