ESTRO 2020 Abstract book

S1085 ESTRO 2020

Conclusion Although significant differences were found in almost OARs, it could not be used to predict that which portion caused exceeded dose of OARs, especially the removed teeth contained restoration materials. The location and volume of primary tumor and PTV54 could not be used to assess the applicability of original plan. The results alert us the need of overriding removed teeth densities to estimate the dose changes and consider CT images acquisition and re-planning again. PO‐1853 Dosimetric Studies of VMAT for Prostate Cancer: Comparison of Single and Mixed Energy 1 The University of Hong Kong, Department of Diagnostic Radiology and Clinical Oncology- Li Ka Shing Faculty of Medicine, Hong Kong, Hong Kong SAR China Purpose or Objective Utilization of high energy photons with an optimal weight using a mixed energy technique is considered as a practical way to spare organs-at risk (OARs) while maintaining adequate target coverage in radiotherapy. The present study compared the dosimetric performance of single energy VMAT plans (SEP) using 6MV photon and 10MV photon, and a mixed energy VMAT plan (MEP) using both 6MV-10MV photon, in the treatment of stage IIIB prostate cancer. Material and Methods A cohort of 20 patients with stage IIIB prostate cancer were recruited in this retrospective study. All patients have undergone computed tomography simulation in supine position. SEP using 6MV photon and 10MV photon, and MEP using both 6MV-10MV photon, were generated for each patient. All VMAT plans comprised of 2 full arcs (181°-179° and 179°-181°). Collimator angle pair of 30° and 330° were used to reduce tongue-and-groove effect in multi- leaf collimators. Location of isocentre and field size at each angle were adjusted and defined according to the PTV in each case by the VMAT optimizer. Plan metrics from 3 techniques were compared using one-way repeated measures ANOVA. The magnitude and statistical significance of the observed dosimetric differences were quantified. The optimization objectives were not modified Techniques A.H.L. Yuen 1