Ultrahypofractionated radiotherapy for localised prostate cancer: The impact of daily MRI-guided adaptive radiotherapy on delivered dose

Introduction: Magnetic resonance image-guided adaptive radiotherapy (MRIgART) reduces uncertainties by correcting for day-to-day target and organ-at-risk deformation and motion. This is the first study to examine the dosimetric impact of MRIgART for ultrahypofractionated prostate cancer treatment, compared to standard-of-care image-guided non-adapted radiotherapy. Methods: Twenty patients with localised prostate cancer, who received ultrahypofractionated MRIgART on the Unity MR linac (Elekta, Sweden) were retrospectively analysed. Online daily MRI was acquired for replanning (MRIsession) and a second for position verification before treatment (MRIverification). To compare delivered dose with and without adaptation, three plans were generated offline per fraction; a session plan (reference plan adapted to MRIsession anatomy), a verification plan (session plan recalculated on MRIverfication anatomy), and a non-adapted plan (reference plan recalculated on MRIverfication anatomy). Target and organ-at-risk doses were calculated, and dose difference evaluated. Secondary analysis, using deformable dose accumulation, estimated verification and non-adapted dose to primary target (CTVpsv) substructures; prostate, gross tumour volume (GTV) and proximal 1 cm of seminal vesicles (1cmSV). Impact of prostate, rectum and bladder volume changes on dose were evaluated. Results: Median dose to 95 % of the CTVpsv was significantly higher with adaptation; 40.3, 40.0 and 38.2 Gy for session, verification, and non-adapted plans. Adaptation achieved a lower median urethra V42Gy dose but bladder V37Gy dose was lower when not adapting. Rectum V36Gy dose was similar for adapted and non-adapted plans. CTVpsv substructure dose difference was greatest for 1cmSV; 40.0 versus 37.5 Gy for verification/non-adapted plans. Adaptation achieved significantly higher prostate only, but not GTV doses. Prostate and rectal volume changes had a negative impact on non-adapted dose only. Conclusion: MRIgART, offers significant dosimetric benefit for ultrahypofractionated prostate cancer compared to non-adapted strategies. Greatest benefit is expected for those with SV or high-risk of SV involvement, persistent rectal gas, prostate swelling and for the application of novel dose strategies including GTV dose escalation and non-involved prostate dose de-escalation.