Prospective validation of stringent dose constraints for prostatic stereotactic radiation monotherapy: results of a single-arm phase II toxicity-oriented trial.

Purpose: There are no safety-focused trials on stereotactic body radiotherapy (SBRT) for localized prostate cancer. This prospective 3‑year phase II trial used binomial law to validate the safety and efficacy of SBRT with stringent organ at risk dose constraints that nevertheless permitted high planning target volume doses.

Methods: All consecutive ≥ 70-year-old patients with localized prostate adenocarcinoma who underwent SBRT between 2014 and 2018 at the National Radiotherapy Center in Luxembourg were included. Patients with low Cancer of Prostate Risk Assessment (CAPRA) scores (0-2) and intermediate scores (3-5) received 36.25 Gy. High-risk (6-10) patients received 37.5 Gy. Radiation was delivered in 5 fractions over 9 days with Cyberknife-M6™ (Accuray, Sunnyvale, CA, USA). Primary study outcome was Common Terminology Criteria for Adverse Events version 4 (CTCAEv4) genitourinary and rectal toxicity scores at last follow-up. Based on binomial law, SRBT was considered safe in this cohort of 110 patients if there were ≤ 2 severe toxicity (CTCAEv4 grade ≥ 3) cases. Secondary outcomes were biochemical progression-free survival (bPFS) and patient quality of life (QOL), as determined by the IPPS and the Urinary Incontinence QOL questionnaire.

Results: The first 110 patients who were accrued in a total cohort of 150 patients were included in this study and had a median follow-up of 36 months. Acute grade ≥ 3 toxicity never occurred. One transient late grade 3 case was observed. Thus, our SBRT program had an estimated severe toxicity rate of < 5% and was safe at the p < 0.05 level. Overall bPFS was 90%. QOL did not change relative to baseline.

Conclusion: The trial validated our SBRT regimen since it was both safe and effective.