ESTRO 2021 Abstract Book

S1420

ESTRO 2021

Conclusion We demonstrated that our proposed model has a good accurate and robust performance in TRUS prostate segmentation. Further improvements are expected when the algorithm will be tested on different kinds of noises and compared with different new methods. PO-1693 4D CT analysis of organs at risk (OARs) in stereotactic radiotherapy V. Nardone 1 , F.M. Giugliano 2 , A. Reginelli 3 , M. Mormile 2 , L. Iadanza 4 , S. Cappabianca 3 , C. Guida 2 1 Unit of Radiation Oncology, Ospedale del Mare, NAPLES, Italy; 2 Unit of Radiation Oncology, Ospedale del Mare, Naples, Italy; 3 Department of Precision Medicine, University of Campania "Luigi Vanvitelli", Naples, Italy; 4 Unit of Radiation Oncology, Rummo General Hospital, Benevento, Italy Purpose or Objective Internal organs at risk volumes (IRV) represent the propagation of organs at risk (OARs) in 4DCT. OARs, like target volumes, can be propagated to obtain an internal organ at risk volume (IRV). To our knowledge no one has tested the IRV in a clinical setting. This work aims at studying the IRVs in a retrospective database of lung SABR. Materials and Methods Sixty consecutive patients that underwent 4DCT for thoracic stereotactic radiotherapy were analyzed and IRVs for heart, trachea, esophagus, bronchial tree, great vessels and spinal cord were calculated. We correlated the volumes of OARs with the volumes of IRVs, with the Wilcoxon sign correlation test. A p-value ≤ 0.05 was considered as statistically significant. We also calculated whether IRVs still respected the dosimetric dose constraints and we calculated equivalent dose 2 Gy (EQD2) in order to visually compare different schedules of SABR. The respect of dose constraints was finally correlated with the lesions’ localization (central vs peripheral), with the Chi-Square test. Results The gated IRVs were significantly bigger than the standard OAR (Wilcoxon analysis, p-value<0,001 for each organ). All the patients respected SABR dose constraints for standard OARs. Conversely, the IRVs not respecting the dose constraints were, respectively, 7/60 (11,7%) for Heart IRV, 7/60 (11,7%) for Esophagus IRV, 11/60 (18,3%) for Trachea IRV, 16/60 (26,6%) for Bronchial Tree IRV and 0/60 (0%) for great vessel IRV and spinal cord IRV. In the subset (n.38) of central targets, the percentage of the IRVs not complying with the dose constraints are 18,4% for Heart IRV, 18,4% for Esopaghus IRV, 28,9% for Trachea IRV and 42% for Bronchial Tree IRV. Conclusion In our analysis, IRVs do not respect dose constraints in a significant percentage of patients, that is even higher in central lesions. The correlation of IRVs with clinical parameters and toxicity deserve future investigations. PO-1694 Target localization uncertainty in Gamma Knife SRS: Comparison of three frame-based workflows E. Pappas 1 , I. Seimenis 1 , P. Kouris 1 , D. Dellios 1 , S. Theocharis 1 , P. Karaiskos 2 1 National and Kapodistrian University of Athens, Medical Physics Laboratory, Medical School, Athens, Greece;