Segment-specific PTV margins for rectal cancer radiotherapy on the MR-Linac: characterization of rectal motion and dosimetric validation.

BACKGROUND

Dose escalation in rectal radiotherapy shows promise for improving complete response rates and organ preservation, yet optimal expansion margins for rectal lesions remain unclear. This study aims to utilize high-resolution real-time imaging from MR-guided radiotherapy to characterize rectal motion uncertainties, optimize planning target volume (PTV) margins, and validate dosimetric outcomes, thereby enhancing the precision in rectal cancer dose escalation.

METHODS

A cohort of 22 advanced rectal cancer patients undergoing neoadjuvant radiotherapy on a 1.5T MR-Linac. Pre-treatment, intra-fraction, and post-treatment MR images were acquired for each fraction, with the rectum delineated according to the distance from the anus (upper, middle, lower). Inter- and intrafractional margins were determined by isotropic expansion of the segmented clinical target volume (CTV), with adequacy defined as covering 95% of the CTV volume in 90% of all fractions. Dosimetric validation was performed on 15 patients (10 internal, 5 external), comparing the segmental margins with literature-based uniform margins to assess target coverage and organs at risk (OARs) sparing.

RESULTS

Rectal motion variability was segment-dependent, with the upper rectum requiring the largest margins and the lower rectum the smallest. Interfractional margins for the upper, middle, and lower rectum were 13 mm, 7 mm, and 6.5 mm, respectively. Intrafractional analysis revealed that treatment duration also influenced margin requirements, with 7 mm for the upper rectum and 3 mm for the middle and lower rectum sufficient for durations under 30 min; longer durations necessitated slightly larger margins. Dosimetric analysis confirmed that the segmented margin expansion strategy achieves adequate target coverage while substantially sparing OARs with dose reductions ranging from 7.4% to 44.5%.

CONCLUSIONS

Segment-specific margin expansion suggest a potential improvement for rectal radiotherapy, ensuring target coverage while reducing OAR dose, thereby facilitating organ and function preservation.

CLINICAL TRIAL NUMBER: Not applicable.