Biologically effective dose (BED) correlation with biochemical control after low-dose rate prostate brachytherapy for clinically low-risk prostate cancer.

ABSTRACT

PURPOSE: To assess the correlation of postimplant dosimetric quantifiers with biochemical control of prostate cancer after low-dose rate brachytherapy. METHODS AND MATERIALS The biologically effective dose (BED), dose in Gray (Gy) to 90% of prostate (D(90)), and percent volume of the prostate receiving 100% of the prescription dose (V(100)) were calculated from the postimplant dose-volume histogram for 140 patients undergoing low-dose rate prostate brachytherapy from 1997 to 2003 at Durham Regional Hospital and the Durham VA Medical Center (Durham, NC). RESULTS: The median follow-up was 50 months. There was a 7% biochemical failure rate (10 of 140), and 91% of patients (127 of 140) were alive at last clinical follow-up. The median BED was 148 Gy (range, 46-218 Gy). The median D(90) was 139 Gy (range, 45-203 Gy). The median V(100) was 85% (range, 44-100%). The overall 5-year biochemical relapse-free survival (bRFS) rate was 90.1%. On univariate Cox proportional hazards modeling, no pretreatment characteristic (Gleason score sum, age, baseline prostate-specific antigen, or clinical stage) was predictive of bRFS. The BED, D(90), and V(100) were all highly correlated (Pearson coefficients >92%), and all were strongly correlated with bRFS. Using the Youden method, we identified the following cut points for predicting freedom from biochemical failure D(90) >or= 110 Gy, V(100) >or= 74%, and BED >or= 115 Gy. None of the covariates significantly predicted overall survival. CONCLUSIONS: We observed significant correlation between BED, D(90), and V(100) with bRFS. The BED is at least as predictive of bRFS as D(90) or V(100). Dosimetric quantifiers that account for heterogeneity in tumor location and dose distribution, tumor repopulation, and survival probability of tumor clonogens should be investigated.