Impact of Anatomical Changes and PTV Margin Dimensions on Delivered Dose for Prostate SBRT Patients Treated on an MR-Linac
Abstract
Purpose
Accurate delivery of MR-guided adaptive radiotherapy (MRgART) for prostate SBRT is influenced by inter- and intra-fraction motion, planning target volume (PTV) margin selection, and verification adaptation. Our study quantified motion magnitude and dosimetric impact using accumulated delivered dose on a 1.5 T MR-Linac and evaluated PTV margin reduction.
Methods
Twenty-five prostate SBRT patients received 30 Gy in 5 fractions on an1.5T MR-Linac, followed by a 15 Gy brachytherapy boost. We characterized organ motion and delivered dose impact using deformable dose accumulation. Inter-fraction differences were assessed between reference and adapt MR images, while intrafraction differences were assessed using adapt, verification, and beam-on MR images. DVH metrics were compared using paired t-tests with Bonferroni correction. In a 10-patient subset, retrospective simulations evaluated 4mm APSI and 3mm LR (344) and 3mm APSI and 2mm LR (233) PTV margins relative to the 5 mm isotropic clinical margin. ATP-of-ATS was used when the CTV approached 1mm from the PTV margin.
Results
Inter-fraction motion exhibited large variability but small mean displacements for all structures. Mean intra-fraction CTV motion was 0.1±1.0mm (Right), 1.9±2.7mm (Posterior), and 1.2±2.0mm (Inferior). Rectum and bladder exhibited larger intrafraction motion than CTV and variability across fractions. Accumulated CTV D98 exceeded the 2850cGy goals for all patients, with no significant difference between adapted-plan and delivered doses. Relative to adapted, delivered dose showed reduced rectum D1cc (2406 vs. 2548cGy, p < 0.005) and increased bladder D5cc (2905 vs. 2757cGy, p < 0.001). With margin reduction (50 fractions), ATP-of-ATS frequency increased as margins decreased: 8/50 (5mm), 14/50 (344), and 26/50 (233). ATP-of-ATS restored target coverage for reduced margins.
Conclusion
For MR-guided prostate SBRT, intra-fraction motion has minimal impact on accumulated target dose with a 5mm PTV. Reducing margins improved rectal sparing, but as PTV margins neared intrafraction motion levels, ATP-of-ATS reliance increased, affecting workflow efficiency.