Feasibility of online adaptive SBRT boost with 1.5-T MR-Linac for cervical cancer patients unsuitable for brachytherapy: a planning study.

This study aimed to evaluate the effectiveness of online adaptive treatment (ADT) planning for stereotactic body radiotherapy (SBRT) boost on a 1.5-T MR-Linac in cervical cancer patients ineligible for brachytherapy, compared with the conventional position-alignment (PA) assumed dose distributions. This retrospective analysis included 15 treatment plans from five patients, each receiving 21.0 Gy in three fractions. Daily magnetic resonance imaging (MRIs) were used to compare target and organ-at-risk (OAR) doses between ADT and PA dose distributions. The planning target volume (PTV) was defined as the high-risk clinical target volume (HR-CTV) plus a 3 mm margin. OARs included the bladder, bowel bag, and planning organ-at-risk volume (PRV) for rectum and sigmoid. Reference plans prioritized OAR constraints while maximizing target coverage; the intended PTV D90% range was 18.9-21.6 Gy. PA doses were reconstructed by dose warping based on alignment of the anterior rectal wall between daily and pretreatment MRIs, whereas ADT plans were reoptimized on the daily MRIs. HR-CTV decreased by 1.6%-41.5% between the planning MRI and the first treatment fraction. ADT plans improved PTV D90%, achieved better target coverage and met all OAR constraints, whereas PA plans showed dose constraint exceedance across the three fractions, with mean excess doses of 6.89 ± 4.39 Gy to PRV-sigmoid and 5.76 ± 3.33 Gy to PRV-rectum. In conclusion, 1.5-T MR-Linac based SBRT boost with daily online adaptation enhanced tumor coverage and reduced OAR doses, supporting MR-guided online adaptive radiotherapy as a promising option for patients ineligible for brachytherapy.