We set out to evaluate the safety and efficacy of homogeneously dosed salvage stereotactic body radiation therapy (SBRT) for intraprostatic recurrences following low dose rate (LDR) brachytherapy.

An institutional prostate SBRT database was interrogated for patients treated between January 2018 and December 2021 with salvage SBRT for intraprostatic recurrences who were previously treated with LDR brachytherapy. Patients received 30 to 34 Gy in 5 fractions to the prostate with a simultaneous integrated boost of 34 to 37.5 Gy to gross disease. The maximum urethral dose allowed was 34 Gy. Toxicities were graded using Common Terminology Criteria for Adverse Events, version 5.0.

Eleven patients met our study's inclusion criteria with a median follow-up time of 37.9 months (range, 24.3-51.8 months). Median time between LDR brachytherapy and salvage SBRT was 7 years (range, 2-11 years) with a median PSA of 3.15 ng/mL (range, 0.90-9.83) at the time of salvage radiation. All 11 patients were alive at the time of last follow-up. Our 3-year Kaplan-Meier progression-free survival rate was 70.1%. Median time to recurrence was 24.1 months (range, 18.7-29.7 months). Late (≥3 months) grade 1, 2, and 3 urinary toxicity rates were 27.3%, 36.4%, and 9.1%, respectively. Late (≥3 months) grade 1, 2, and 3 gastrointestinal toxicity rates were 18.2%, 0%, and 9.1%, respectively.

Homogeneous salvage SBRT to the prostate with urethral dose minimization has a favorable safety and efficacy profile for treating intra-prostatic recurrences following LDR brachytherapy. This may represent an ideal form of salvage SBRT for re-irradiation.