Validation and comparison of the two Kattan nomograms in patients with prostate cancer treated with ¹²⁵iodine brachytherapy

OBJECTIVE To validate and compare the preoperative and postoperative Kattan prediction nonograms for prostate cancer recurrence after brachytherapy. PATIENTS AND METHODS Patients (n= 747) treated with ¹²⁵I- brachytherapy were evaluated. Both nomograms were used to calculate the prediction of 5-year biochemical-freedom from failure (BFFF) based on clinical stage, Gleason score, prostate-specific antigen (PSA) level, receipt of androgen deprivation therapy and the post-implant dosimetry variable D90 (values of the minimal dose received by 90% of the prostate volume). The predicted values using the Kattan nomograms and the observed values were compared. Predictive accuracy was determined using the concordance index. RESULTS The 5-year BFFF probability was 94% (95% confidence interval [CI], 92-96%) for the modified American Society for Radiation Oncology (ASTRO) definition and 97% (95% CI, 95-98%) for the Phoenix definition using Kaplan-Meier analysis. The predicted values of BFFF using both Kattan nomograms were lower than the observed rates in our cohort. The concordance index values were 0.51 and 0.52 for preoperative and postoperative nomograms, respectively. Concordance correlation coefficient between the two nomograms was 0.15. CONCLUSIONS In our population, the 5-year BFFF outcomes rates were superior to nomogram predictions. Neither nomogram predicted outcomes after ¹²⁵I-brachytherapy in this non-US cohort. The postoperative nomogram was also a poor predictor, although it included D90 dosimetry values, as a variable of treatment quality. Strict inclusion criteria, perhaps more favourable than the ones on which the Kattan nomograms were based, could be the explanation for these discrepancies.