Stereotactic Body Radiation Therapy to the Prostate Bed: Results of a Phase 1 Dose-Escalation Trial.

PURPOSE: The primary objectives of this study were to evaluate toxicity of escalating doses of prostate bed stereotactic body radiation therapy and to provide dose recommendations for a phase 2 study.

METHODS AND MATERIALS: Patients with organ-confined, node-negative prostate cancer who had biochemical failure (prostate-specific antigen [PSA] less than 2.0) after prostatectomy were eligible for this phase 1 dose-escalation trial. Doses delivered were 35 Gy, 40 Gy, and 45 Gy in 5 fractions, given every other day. Dose-limiting toxicity (DLT) was defined as Common Terminology Criteria for Adverse Events (version 4.0) grade 3 or higher gastrointestinal or genitourinary (GU) toxicity within 90 days of treatment. Maximum tolerated dose was the highest dose to be tested where fewer than 2 of the patients experienced DLT. Patients completed quality-of-life questionnaires at regular time intervals.

RESULTS

Twenty-six patients completed treatment between October 2013 and December 2017. Three patients received 35 Gy, 8 patients received 40 Gy, and 15 patients received 45 Gy. The median follow-up was 60 months for 35 Gy, 48 months for 40 Gy, and 33 months for 45 Gy. No acute DLT events were observed. Late grade ≥2 and ≥3 gastrointestinal toxicity occurred in 11% and 0%, respectively, and late grade ≥2 and ≥3 GU toxicity occurred in 38% and 15%, respectively. No difference was observed in late GU toxicity between 40 Gy and 45 Gy. Sexual function scores were significantly lower in the patients receiving androgen deprivation therapy (P < .01). In all patients, the crude rate of PSA control (<0.2 ng/mL) was 11 out of 26 (42%).

CONCLUSIONS

Dose escalation to 45 Gy did not result in acute DLT events, had similar rates of late grade 3 toxicity, and did not demonstrate higher rates of PSA control, compared with 40 Gy. While allowing for higher plan heterogeneity, the recommended dose for phase 2 study will be 40 Gy in 5 fractions.