PURPOSE: For prostate cancer patients at risk for subclinical spread of the disease, we investigated whether incidental dose outside the target was associated with tumor control. METHODS AND MATERIALS: We selected 352 intermediate-risk (mainly T2b-T3a) and high-risk (mainly T3b) patients treated in a randomized trial. Target volume was prostate (68-78 Gy) and seminal vesicles (50-78 Gy). Failure (clinical or biochemical) was evaluated at 4 years. To compare three-dimensional dose distributions, an automated mapping procedure was introduced. Between patients, these maps provide an approximate identification of corresponding anatomical locations. Maps of the dose difference between patients with and without failure were constructed. Univariate and multivariate analyses were performed including the dose in selected points. RESULTS: Dose differences were mainly found in the obturatorial region for the high-risk patients, and in the presacral region for the intermediate risk group (>7 Gy, p