ESTRO 2025 - Abstract Book

S3325

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

ESTRO 2025

3735

Digital Poster Online CBCT-guided Adaptive Radiotherapy for prostate + pelvic lymph nodes, results on margin reduction Suzan Gerrets-van Noord, Nienke Losekoot, Paula Bos, Carmen Seller Oria, Marloes Frantzen-Steneker, Wouter Vogel, Jan-Jakob Sonke, Peter Remeijer, Zeno Gouw Department of Radiation Oncology, the Netherlands Cancer Institute, Amsterdam, Netherlands Purpose/Objective: Online adaptive radiotherapy (OART) guided by Cone Beam Computed Tomography (CBCT) can reduce inter-fraction changes while prolonging fraction times, possibly leading to larger intra-fraction motion. Here, we report on a prospective trial, comparing residual misalignments between Image Guided Radiotherapy (IGRT) and OART and their impact on PTV margins. Material/Methods: 19 N+ prostate cancer patients from a prospective OART trial (NCT06691776) received 25 fractions to the prostate (6875cGy), pathologic nodes (5750cGy), and elective lymph node areas (4750cGy) on an Elekta Versa HD. Standard IGRT rigidly translates the RT field, prioritizing the prostate. OART corrected for rotations and differential motion between prostate and lymph node areas [1]. In short, a local-rigid dual registration was performed on 1) prostate(+seminal vesicles) and 2) bony anatomy (lymph node surrogate). Subsequently, a synthetic CT and corresponding delineations were generated by applying a deformation vector field that rigidly propagates both targets and interpolates in between (Figure 1). Then, the treatment plan was adapted in Monaco. Before treatment delivery, a verification-CBCT was acquired and registered against the synthetic CT used for a final table shift for the prostate, compensating intra-fraction motion during the adaptation time. A post-CBCT followed RT once per week. Residual systematic (∑) and random (σ) misalignments were collected for pre- and verification CBCTs after the prostate-guided table shift. Residual rotations for both prostate and lymph nodes were converted to an estimated maximum translation error on the PTV edge. Errors quadratically accumulated with other uncertainties determined the margin reduction according the linear van Herk recipe[2].