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 (MRI 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.