The effect of isotope selection on the prostate-specific antigen response in patients treated with permanent prostate brachytherapy

Purpose: To assess the difference in prostate-specific antigen (PSA) response kinetics in patients undergoing either 125I or 103Pd permanent prostate brachytherapy (PPB). Methods and Materials: Between 1997 and 1999, 333 patients underwent PPB as monotherapy. Forty-eight patients received a 125I implant, and 285 received a 103Pd implant. Biochemical relapse-free survival was defined by the Kattan modification of the American Society for Therapeutic Radiology and Oncology consensus, based on three PSA increases. In addition, the time to reach a PSA threshold of ≤1.0 ng/ml was noted. Log-rank testing was performed, and multivariate analysis was used to evaluate those variables associated with biochemical freedom from recurrence. Results: With a mean 36-month follow-up, the actuarial biochemical relapse-free survival at 4 years was 86.8%. No significant difference in biochemical relapse-free survival was noted between patients treated with 125I and 103Pd (p = 0.417). Multivariate analysis failed to identify isotope as an independent variable to predict for biochemical relapse-free survival. The mean time for patients treated with 103Pd to reach the threshold PSA value was 10.2 weeks, whereas it was 22 weeks for 125I (p = 0.014). When the median time to reach the PSA threshold of ≤1.0 ng/ml was used to calculate the percentage of delivered dose for each isotope relative to the prescribed dose, there was no significant difference noted between 125I (84%) and 103Pd (94%) (p = 0.86). Conclusions: Isotope selection does not appear to influence biochemical relapse-free survival in patients treated with monotherapy PPB. There was a significant difference (p = 0.014) in time to reach a PSA threshold of ≤1.0 ng/ml noted between 125I and 103Pd. However, the percentage of delivered dose relative to the time to reach the threshold was the same between 125I and 103Pd. This information is important for during patients their post-PPB period.