ESTRO 2025 - Abstract Book

S3079

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

accuracy of dose calculations on CBCT images was confirmed by comparing dose calculations on CBCTs of homogeneous and heterogeneous phantoms to dose calculations completed on the planning CT. A retrospective study of 20 prostate oART fractions from 4 patients (prescription 60Gy/20 fractions) was conducted. Reference treatment plans were created on Ethos treatment management using local clinical protocols. For all fractions studied, patients were treated with the daily adaptive plans. Daily CBCT, sCT and adapted plans were exported to Eclipse (V15.6), where dose calculations were performed on both CBCT and sCT data sets using AcurosXB. Dmean, D98, D0.2cc, and V98 for the PTV, along with the bladder V48 Gy and rectum/bowel V40 Gy were compared between CBCT and sCT datasets. Results: HyperSight HU curves closely matched the clinical curve with a maximum deviation of 115HU at 1.53g/cc. Measurements on phantoms showed agreement within 0.2Gy for all DVH parameters. Figure 1 shows DVH results for all 20 fractions. Maximum PTV dose variations were 0.77Gy (Dmean), 5.1Gy (D98), and 6.5Gy (D0.2cc), with CBCT distributions generally higher than sCT, likely due to variation in abdominal gas during CBCT acquisition.

A fraction from a patient’s treatment with this variation is shown in Figure 2. For two patients, target metrics agreed within 1.1 Gy for all fractions studied.