A framework for planning organ at risk volume margin derivation for the small bowel during magnetic resonance-guided radiotherapy

Background and purpose The small bowel is highly radiosensitive and prone to motion during abdominal and pelvic radiotherapy, risking overdosage. This study extended a previously presented small bowel tracking framework to enable automated planning organ at risk volume (PRV) margin derivation.Materials and methods MR scans were obtained in 18 patients treated for abdominal lesions, with abdominal compression. The mean motion, maximum displacement and position of the small bowel during the acquisition (7.5 min) were determined. The position of the small bowel over time was used to derive PRV margins based on geometrical coverage.Results Displacements >20 mm in the craniocaudal direction were observed, with approximately half of the bowel voxels experiencing positional displacements close to 10 mm. Despite this substantial motion, margin analysis demonstrated that applying a 4.5 mm isotropic margin around the delineated bowel loops was sufficient to cover 95% of the bowel voxels occupied for at least 5% of the time in 90% of the cases. It was shown that shorter acquisitions (2.5 min) already gave stable margin results, supporting the feasibility of using individualized PRV margins to balance motion coverage with treatment planning flexibility.Conclusions While the small bowel can undergo considerable intrafraction displacements during treatment, most of this motion can be effectively encompassed using a moderate intrafraction PRV margin for this specific cohort. This study presents an automated framework to derive patient- and treatment-specific margins, directly from bowel tracking data, enabling tailored coverage-based margins.