Impact of Inter-and Intra-Fraction Motion on Delivered Dose for Prostate SBRT Patients Treated on an MRL-Linac Using a Novel Semi-Automated Dose Accumulation Framework

Abstract

Online MR-guided adaptive RT enables generation of an adapted treatment plan at each fraction to account for daily anatomical changes, with both inter- and intrafraction internal motion impacting the accumulated dose. Our objective is to quantify the impact of motion on the accumulated delivered dose for prostate SBRT patients treated on an MRL using a novel semi-automated process for dose accumulation.We included 18 prostate SBRT patients treated on a prospective clinical trial with 30 Gy in 5 fraction SBRT on an integrated 1.5 T MRL with consecutive 15 Gy in 1 brachytherapy boost. We performed dose accumulation across the MRL treatment using a novel semi-automated workflow to import, deformably register, and deform each fraction dose to a reference MR. All deformable image registration (DIR) steps were performed automatically without manual input. We accumulated both the adapted plan dose, as assessed on localization MR image, and the final delivered dose, assessed on beam-on images acquired during treatment delivery. We qualitatively evaluated the DIR for all registrations using a 3-point scale and extracted organ motion from the deformation vector field. To assess the impact of intrafraction motion during online adaptation, we compared target and organ-at-risk dose-volume (DVH) metrics on the total accumulated dose for adapted plans versus delivered dose via paired t-tests with Bonferroni Correction.We qualitatively scored the DIR agreement for CTV, proximal bladder, rectum, and femurs to be within 2 mm agreement for all fractions, however, the distal bladder was unacceptable (> 5 mm differences). Mean absolute intrafraction motion was evaluated over all patients and fractions to be 0.7 ± 0.6 mm LR, 2.0 ± 1.4 mm AP and 1.8 ± 1.3 mm SI for the CTV, and 1.7 ± 0.7 mm LR, 2.5 ± 1.1 mm AP and 2.6 ± 1.0 mm SI for the rectum. The CTV D98 for accumulated delivered dose met the clinical goal of > 2850 cGy for all patients, with no significant difference compared to the adapted plans. The accumulated rectum D1cc DVH metric exhibited a significant decrease for delivered dose (2406 cGy) compared to the adapted plan (2548 cGy, P < 0.005), whereas bladder D5cc exhibited a significant increase for delivered dose (2905 cGy) compared to the adapted plan (2757 cGy, P < 0.001). No differences existed for rectum D20, D50, or femur D5.Here we demonstrated the feasibility of MRL dose accumulation for both online adapted plan distributions and estimated delivered dose for prostate SBRT treatments. In the presence of intrafraction motion between adapted plan generation and beam delivery, our results suggest only a minor impact on the accumulated dose. In the future we plan to use this dose accumulation framework to investigate different treatment approaches, such as adaptive intraprostatic dose escalation.