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

## TL;DR

MRI-guided adaptive radiotherapy improves prostate cancer treatment by adjusting doses based on daily imaging, especially benefiting patients with specific conditions.

## Contribution

This study is the first to show dosimetric benefits of MRI-guided adaptive radiotherapy in ultrahypofractionated prostate cancer treatment.

## Key findings

- MRI-guided adaptation significantly increased prostate dose and decreased urethra dose compared to non-adapted strategies.
- Prostate and rectal volume changes negatively impacted non-adapted dose delivery.
- Adaptive radiotherapy showed the greatest benefit for patients with seminal vesicle involvement or prostate swelling.

## Abstract

•MRI-guided adaptation offers significant dosimetric benefit for ultrahypofractionated prostate cancer RT.•Dosimetric benefits of MRIgART, compared to non-adapted strategies, are most apparent for target and urethra doses.•MRIgART achieved significantly higher prostate and significantly lower urethra doses.•Greatest benefit expected for individuals with; SV/high-risk of SV involvement, persistent rectal gas, prostate swelling and for the application of novel dose strategies.•Prostate and rectal volume changes had a negative impact on non-adapted dose only.

MRI-guided adaptation offers significant dosimetric benefit for ultrahypofractionated prostate cancer RT.

Dosimetric benefits of MRIgART, compared to non-adapted strategies, are most apparent for target and urethra doses.

MRIgART achieved significantly higher prostate and significantly lower urethra doses.

Greatest benefit expected for individuals with; SV/high-risk of SV involvement, persistent rectal gas, prostate swelling and for the application of novel dose strategies.

Prostate and rectal volume changes had a negative impact on non-adapted dose only.

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.

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.

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.

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.