ESTRO 2025 - Abstract Book

S4368

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Oncology, Zealand University Hospital, Naestved, Denmark

Purpose/Objective: While the introduction of AI has allowed advancement of oART clinically, it can be challenging to obtain comparable AI-contoured target volumes (prostate, elective lymph nodes, seminal vesicles) on CBCT images, for example when the treatment plan utilizes MRI for target delineation. [1] Rigid propagation of the Radiation Oncologist’s (RO) original elective volume contours eliminates the risk of AI slightly altering these volumes, and thus in most cases effectively eliminates the need for RTTs to interpret their correctness. Material/Methods: Our clinic has developed the following process for RTT-led oART of prostate and elective lymph nodes on Ethos: 1. Influencer editing: a. Bladder and rectum are quickly assessed and edited if necessary. (Artifacts can reduce the quality of the AI generated structures, requiring more manual editing). b. Prostate and seminal vesicles are not addressed in this phase. 2. Contour editing: a. Prostate CTV including fiducials is rigidly propagated as a constellation and is placed precisely using 6 degrees of freedom, without any further editing. b. CTV of elective lymph nodes is similarly rigidly propagated and placed, using landmarks of bony structures and muscles, with manual editing allowed but very rarely needed. [2]. c. CTV of seminal vesicles is also rigidly propagated but often edited more extensively. [3] d. The RTT then edits the bladder area adjacent to the prostate as well as the bowel bag. 3. Contours are sent to plan calculation. RTTs assess and approve either the scheduled or adaptive plan. 4. Verification CBCT is acquired. RTTs match images using fiducials, assess target coverage, move couch and initiate treatment. 5. Multidisciplinary support throughout treatment course: a. ROs and Medical Physicists (MP) attend the first fraction. b. Delivered fractions assessed at a biweekly conference including RTTs, ROs and MPs. c. Offline assessment at least once a week by MPs. d. Continuing education of RTTs as necessary. We conducted a retrospective analysis of 119 fractions given to 16 patients during our first year of offering adaptive treatment (2/2023-2/2024).

Results: 1.

98% of fractions were treated with the clinically superior adaptive plan.

2. 3.

91% of fractions required couch corrections after verification CBCT of less than 5 mm. Total time between first and second CBCTs averaged 18 minutes. (Experienced RTTs’ current average

is 14 minutes).

Conclusion: RTT-led oART of prostate and elective lymph nodes benefits from the rigid propagation of RO/MRI delineated target volume contours, ensuring acceptable and timely delivery of dose.

Keywords: RTT-led oART, rigid propagation, oART prostate

References: [1] Gunnlaugsson A, Persson E, et al. Target definition in radiotherapy of prostate cancer using magnetic resonance imaging only workflow. Physics and Imaging in Radiation Oncology 2019; 9:89-91