3240 - Evaluating Reduced PTV Margins Enabled by MR-Linac Comprehensive Motion Management (CMM) in Prostate Stereotactic Radiotherapy (SBRT): Dosimetric and Clinical Implications

Purpose/Objective(s): Hypofractionated prostate radiotherapy is increasingly employed as a favourable regimen due to the low alpha-beta ratio of prostate tissue. Comprehensive Motion Management (CMM) is a recent upgrade in MR-Linac that enables real-time intra-fractional motion monitoring and beam gating. The advancement opens the possibility for PTV margin reduction in hypofractionated prostate radiotherapy. This study hypothesizes that reduced PTV margins under CMM favour dose reductions to rectum and bladder while target coverage is maintained, and it aims to give insights into intra-fractional prostate motion.

Materials/Methods: Thirty prostate cancer patients treated with hypofractionated radiotherapy (40Gy/5 fractions) under MR-Linac were identified retrospectively. Retrospective treatment planning was conducted for each subject, which included 1 conventional margin plan and 3 reduced PTV margins under CMM. Dosimetric analysis comprised (i) comparison of rectal and bladder doses across setup variations (i.e. Barrigel, rectal balloon) using the Kruskal-Wallis test; (ii) within-setup dose comparison with respect to PTV margins through the Kruskal-Wallis test; (iii) multiple linear regression to evaluate the combined effects of Barrigel, rectal balloon and PTV margins on each dosimetric endpoint. For subjects (n=9) who were indicated for CMM, intra-fractional prostate motion was retrieved from log files with translation vectors updated every 200ms and averaged over elapsed treatment duration.

Results: A total of 120 computed plans and 41 fractions of log data were analyzed. Barrigel and rectal balloon significantly reduced rectal V40Gy, V36Gy, V32Gy, and V20Gy (all p=0.001), and bladder V36Gy (p=0.027) and V20Gy (p=0.014). Reduced margin plans yielded statistically significant dose reductions (p<0.05) in rectum (V36Gy, V32Gy, V20Gy) and bladder (V36Gy, V20Gy) in the multiple linear regression analysis, although within-setup comparison did not reach statistical significance due to limited subgroup size. Intra-fractional prostate motion was predominantly anterior–posterior, remained within approximately -1mm to 1mm with rectal balloon, and extended to about -2mm to 1mm without rectal balloon. Intra-fractional motion progressively increased with treatment duration and became pronounced beyond 40 minutes.

Conclusion: MR-Linac CMM facilitates PTV margin reduction that improves rectal and bladder sparing in hypofractionated prostate radiotherapy, in addition to dose reductions by Barrigel and rectal balloon. Intra-fractional prostate motion occurred mainly in the anterior–posterior direction, and was reduced with a rectal balloon immobilisation, supporting the robustness of the reduced margin strategy under CMM. These findings suggested that CMM can safely reduce PTV margins, and emphasized the importance of real-time motion management and rectal stabilisation in a hypofractionation regimen.