Outcomes and PSA kinetics after Magnetic Resonance Image-Guided Stereotactic Body Radiotherapy (MRgSBRT) for prostate cancer.

[PURPOSE] Magnetic resonance-guided stereotactic body radiotherapy (MRgSBRT) combines high-precision SBRT with superior soft tissue visualization and daily adaptive planning. While prospective studies suggest reduced toxicity compared to CT-based SBRT, data on oncologic outcomes and PSA kinetics in the MR-guided setting remain limited.

[METHODS] We retrospectively reviewed 150 prostate cancer patients treated with MRgSBRT (ViewRay MRIdian) between September 2018 and April 2024. Patients received 36.25 Gy radiotherapy in 5 fractions with or without androgen deprivation therapy (ADT). Outcomes included biochemical recurrence free survival (bRFS), local progression free survival (LPFS), regional recurrence free survival (RRFS), distant metastases free survival (DMFS), event-free survival (EFS), PSA kinetics, and toxicity (CTCAE v5.0).

[RESULTS] The median follow-up was 27.2 months (range: 4-73 months). The estimated 5‑year bRFS, LPFS, RRFS, DMFS and EFS rates were 81.8%, 91.4%, 99.2%, 98.3% and 77.3% respectively. All patients were alive at the time of analysis. The estimated EFS was lowest for the very high-risk group (2-year EFS: 55.6%). The median time to nadir PSA (nPSA) was 12 months (range: 3-54 months), with a median value of 0.46 ng/mL, for all cohort. PSA bounce occurred in 15.3% of patients and was associated with numerically higher 5‑year EFS (95% vs. 73.8%). No acute or late grade ≥ 3 GU or GI toxicities were observed.

[CONCLUSION] MRgSBRT for localized prostate cancer provides favorable tumor control with minimal toxicity. Although not statistically significant, PSA bounce was associated with improved outcomes, warranting further investigation as a potential prognostic marker.