ESTRO 2021 Abstract Book

S1294

ESTRO 2021

PO-1570 Anatomical changes in prostate SBRT: dosimetric effects and CTV to PTV margins verification I. Flores-Cacho 1 , A. Prado Barragán 2 , M.Á. De la Casa 2 , D. Collado Álvarez 2 , C. Gómez Agudo 2 , R. Martín Álvarez 2 , Á. Montero Luis 3 , P. Fernández Letón 2 1 Hospital Central de la Defensa "Gómez Ulla", Servicio de Protección Radiológica y Radiofísica, Madrid, Spain; 2 Hospital Universitario HM Sanchinarro, Radiofísica, Madrid, Spain; 3 Hospital Universitario HM Sanchinarro, Oncología Radioterápica, Madrid, Spain Purpose or Objective Prostate SBRT requires consistent bladder volume throughout the treatment. To that effect, patients are given strict regiments to follow. However, there are still cases with significant inter-fraction bladder changes which can alter the dose distributions on targets and OARs. In this retrospective study we quantify these effects and assess the suitability of the CTV-to-PTV margins. Materials and Methods We selected a sample of 8 high-risk prostate SBRT patients (40Gy in 5 fractions) treated in a Versa LINAC (Elekta Medical) with daily CBCT. To reduce prostate motion and guarantee reproducibility, a rectal balloon filled with 100cc of air was used. Once all treatments were completed, we exported the CBCTs to the TPS (RayStation, RaySearch Labs) where rectum and bladder were contoured. We applied rigid CT- CBCT registrations matching those applied for each fraction and patient and deformable registrations where the bladder was used as a controlling ROI. Using the Dose Tracking module, we computed the dose distribution delivered in each fraction and over the whole treatment. Results Table 1 shows bladder volume for each patient, at simulation and averaged during treatment, and a few dosimetric indices for bladder wall, PTV and CTV. We observe a large variability in bladder volume both inter-fraction and with respect to the simulation (see top panel of Fig. 1). We find volume deviations from simulation larger than 30% in 57.5% of the CBCTs. On average, these large volume variations do not significantly alter the dosimetric indices of the bladder wall (V24Gy and maximum dose) from plan to delivery even if the DVHs do change (as shown in the bottom panel of Fig. 1 for two of the patients). In terms of target volumes, we found that the dosimetric effects of the bladder changes are very patient specific. This is due to the fact that each patient's anatomy determines how the bladder pushes and/or