ESTRO 2025 - Abstract Book

S4111

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

Purpose/Objective: Online Adaptive radiotherapy (OART) offers the potential to enhance treatment precision by reducing margins and minimizing toxicity. However, the traditional OART process often requires the presence of physicians and physicists, imposing a significant operational burden. In response, our fully RTT-led OART workflow utilizes established image guidance techniques. Here, we report on a prospective feasibility trial of this innovative workflow for lymph node positive prostate cancer. Material/Methods: N+ prostate cancer patients from a prospective OART trial (NCT06691776) received 25 fractions CBCT-OART to the prostate (6875cGy), pathologic nodes (5750cGy) and elective lymph node areas (4750cGy) on an Elekta Versa HD. RTTs performed individual local-rigid registrations of both the lymph node regions and the prostate. Subsequently, a deformation vector field (DVF) was composed that rigidly propagates both targets and interpolates in between them [1]. Using in-house developed software, the DVF applied to the initial planning CT and delineations created a synthetic planning CT (with rigid target areas). The treatment plan was adapted in Monaco 6.2.1. Feasibility criteria were: for 18 of 20 patients completion of >90% of fractions according to protocol in under 1 hour, and the absence of unexpected grade 4 or 5 adaptation-related acute toxicities. Results: Between 9-2023 and 11-2024, 20 patients were included of which 2 patients are still undergoing treatment. Out of 480 adaptations, 12 failed, and then the patient was treated with the reference plan. The most common adaptation failures were technical issues such as software issues generating the synthetic CT (n=2) and data transfer between software systems (n=4), as well as patients inability in maintaining a full bladder (n=3) (Figure 1).

For the first 3 fractions of patient 1 to 5, a physician and physicist were present for safety, afterwards RTTs operated the entire workflow independently. The median fraction time was 45 minutes of which about 30 minutes for the adaptation (Figure 2).