The impact of rectal spacers in MR-guided adaptive radiotherapy.

Background And Purpose: The use of stereotactic ablative radiotherapy (SABR) for prostate cancer has increased significantly. However, SABR can elevate the risk of moderate gastrointestinal (GI) side effects. Rectal spacers mitigate this risk by reducing the rectal dose. This study evaluates the impact of rectal spacers in MR-guided adaptive radiotherapy (MRgART) for prostate SABR.

Materials And Methods: A retrospective analysis was conducted on twenty patients with localised prostate cancer treated on the Unity MR-Linac at a single centre. Half of the cohort (n = 10) had rectal spacers placed before treatment. The adapt-to-shape strategy was used for online MRgART, and non-adapted plans were later generated offline for comparison. Dosimetric assessments were made between spacer and no-spacer cohorts, and between online adapted and non-adapted plans. Clinician-reported outcomes for genitourinary (GU) and GI toxicity were assessed at 3-, 6-, and 12-months post-treatment using Common Terminology Criteria for Adverse Events v.5.0.

Results: No grade 2 or higher toxicity was observed in either cohort. Overall, the dosimetric analysis showed comparable results between the cohorts for target volumes, with D95% of 36.3 Gy in the spacer cohort and 36.0 Gy in the no-spacer cohort (p = 0.08). The spacer cohort demonstrated significant benefits in all rectal dose objectives (p < 0.0001) and in some bladder objectives (V40, p = 0.03; V36, p = 0.03). Failure rates for achieving planning objectives were similar between spacer and no-spacer groups for online adapted plans, with most rates ranging from 0 % to 4 % in both groups.

Conclusion: The findings from this cohort suggest that MRgART is safe and effective for prostate SABR, with comparable toxicity rates in both spacer and no-spacer cohorts. While rectal spacers offer dosimetric advantages, the adaptive nature of MRgART can mitigate some dosimetric disparities, potentially reducing the need for invasive spacer placement. However, further studies with larger patient populations are needed to confirm these results.