ESTRO 2025 - Abstract Book

S3138

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Figure 1. Example of a HYPO-RT-PC-boost dose distribution

Material/Methods: Within the trial protocol, four different target volumes are simultaneously treated to individual dose levels in 7 fractions, using a dose-painting technique. All treatments are carried out on a conventional linear accelerator. The trial participants have three intraprostatic fiducial gold markers implanted and a hydrogel spacer is injected, increasing the separation between prostate and rectum. The bowel is emptied with the aid of Microlax before CT and MR imaging as well as before each treatment. A bladder filling protocol is also in place. Initial patient positioning is done using an optical surface scanning system, followed by a CBCT bone match. Perpendicular planar kV images are subsequently used to reposition the patient with respect to the fiducial gold marker/markers closest to the intraprostatic lesion. The maximum allowed shift from the CBCT bone match is 5 mm. A tolerance of 3 mm is applied for online supervision of the prioritized marker/markers using triggered kV imaging. The tolerance for the remaining fiducial marker/markers is 5 mm. The first five patients, out of the planned trial sample size of 76, are included in this analysis. Results: The treatment start was postponed 45 minutes in two instances (6%) due to internal anatomy discrepancies. Repositioning during treatment due to fiducial markers moving out of tolerance occurred in a total of four instances (11%), affecting three patients. The maximum shift between the CBCT bone match and the kV-kV match using the intraprostatic fiducial markers was 4.9 mm (total vector).

Figure 2. Shift from CBCT bone match to kV-kV fiducial marker match. A total of 35 fractions included.