Relating dose outside the prostate with freedom from failure in the Dutch trial 68 Gy vs. 78 Gy

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.

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.

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 < 0.01). Univariate hazard ratios per 10 Gy for selected dose points were 0.83 (p = 0.01, obturatorial) and 0.72 (p = 0.002, presacral). These hazard ratios were stable under multivariate analysis correcting for established prognostic factors, hospital, and dose to the prostate.

Patients without failure have received on average a higher dose to regions where regional cancer spread could be expected.