Biologically effective dose and definitive radiation treatment for localized prostate cancer

Background and purpose. It is not clear if prolongation of definitive external radiation therapy for prostate cancer has an effect on biochemical failure. The aim of this work was to evaluate whether the biologically effective dose (BED), and in particular the duration of radiotherapy, intended as overall treatment time, has an effect on biochemical failure rates and to develop a nomogram useful to predict the 6-year probability of biochemical failure. Patients and methods. A total of 670 patients with T1-3 N0 prostate cancer were treated with external beam definitive radiotherapy, to a total dose of 72-79.2 Gy in 4044 fractions. The computed BED values were treated with restricted cubic splines. Variables were checked for colinearity using Spearman's test. The Kaplan-Meier method was used to calculate freedom from biochemical relapse (FFBR) rates. The Cox regression analysis was used to identify prognostic factors of biochemical relapse in the final most performing model and to create a nomogram. Concordance probability estimate and calibration methods were used to validate the nomogram. Results. Neoadjuvant and concomitant androgen deprivation was administered to 475 patients (70 %). The median follow-up was 80 months (range 20-129 months). Overall, the 6-year FFBR rate was 88.3 %. BED values were associated with higher biochemical failure risk. Age, iPSA, risk category, and days of radiotherapy treatment were independent variables of biochemical failure. Conclusion. A prolongation of RT (lower BED values) is associated with an increased risk of biochemical failure. The nomogram may be helpful in decision making for the individual patient.