PURPOSE: Low-dose-rate brachytherapy using iodine-125 seeds is a highly efficacious treatment for low-risk prostate cancer. We propose a bioeffect model may be used to target an ablative dose to the tumor with a lower, therapeutic dose to low-risk regions to maintain high rates of tumor control with reduced toxicity. We report on the validation of the model and derivation of the optimal cutpoint value of tumor control probability (TCP) that predicts for freedom from biochemical failure. METHODS AND MATERIALS: The TCP was calculated from postimplant dosimetry data for 423 prostate cancer patients from three Australian institutions. To apply the model, the prostate was mathematically divided into 12 subsections with tumor characteristics, including a nonuniform distribution of tumor cell density, derived from the literature. RESULTS: When TCP values were above and below 0.62, the 5-year freedom from biochemical failure were 93.7% (95% confidence interval [CI], 90.4, 96.4%) and 88.8% (95% confidence interval, 81.3, 94.5%), respectively (p = 0.004). CONCLUSIONS: Using postimplant dosimetry data, the TCP model was able to predict for freedom from biochemical failure. The cutpoint value that would be used clinically depends on the selection of the model parameters, but the potential for the model to be used in the optimization of treatment planning is demonstrated.