ESTRO 2025 - Abstract Book

S3353

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Material/Methods: In the proposed dose-reconstruction method (TRF-PC), each region of interest (ROI) is represented by a point cloud (PC) each of which is moved according to its positions during treatment to accumulate the received dose from each organ’s frame of reference. 3D ROI positions are calculated from the cine-MR sequence which acquires 2D slices centred on the target in alternating orthogonal directions at 4.8Hz. To calculate the time-dependent delivered dose, MLC-positions and linac states of the Elekta Unity MRL are extracted from the treatment log files (TRFs). The patient mass-density map is accessed through the Research Monaco TPS interface (v6.09.01) and dose calculations are performed with the GPUMCD dose engine (both Elekta AB, Stockholm, Sweden). TRF entries and cine-MR images are synchronised by their timestamps. Figure 1 shows the workflow. The dose was reconstructed for a prostate patient from the DESTINATION trial with substantial intra-fraction motion (primary endpoint D90%>42Gy for the dominant intra-prostatic lesion including a 4mm margin (GTV+4mm)). For comparison, the treatment was also replayed with zero-motion (TRF-zero) and evaluated on the contours delineated on the post-treatment MR image (TRF-post).

Results: Motion traces of the target and rectum are shown at the top of Figure 2. Substantial differential motion between target and rectum is present in the IEC-z direction. DVHs for TRF-zero, TRF-post, and TRF-PC are shown at the bottom. Considering motion in the reconstruction caused a drop in the D90%(GTV+4mm)/D90%(GTV) from 45.8Gy/47.9Gy to 24.2Gy/24.9Gy, and 35.3Gy/37.1Gy respectively whereas the rectum received a higher dose.