External validation of the pretreatment nomogram to predict acute urinary retention after (125)I prostate brachytherapy.

PURPOSE

Acute urinary retention (AUR) after (125)I prostate brachytherapy has a negative impact on quality of life. Recently, the authors developed a nomogram to predict the risk of AUR preoperatively. The aim of this study was to assess the external validity of the nomogram.

METHODS AND MATERIALS

The nomogram was initially developed on 714 patients treated with (125)I prostate brachytherapy at the University Medical Center Utrecht, the Netherlands. Predictive factors included in the nomogram were prostate volume, international prostate symptom score, neoadjuvant hormonal treatment, and prostate protrusion. For external validation, the data of 715 consecutive patients treated between January 2003 and July 2008 at the Princess Margaret Hospital, Toronto, were used. The performance of the nomogram was evaluated by discrimination (ability to distinguish between patients who develop AUR yes or no) and calibration (agreement between observed and predicted numbers of AUR).

RESULTS

Of the 715 patients treated at the Princess Margaret Hospital, 67 patients (9.4%) developed AUR compared with 8.0% in the University Medical Center Utrecht cohort. In the validation data set, the discriminatory ability of the nomogram was good (receive operating characteristic area: 0.86; 95% confidence interval: 0.82-0.91), and comparable to the derivation data set (receive operating characteristic area: 0.82; 95% confidence interval: 0.77-0.88). Comparison between the predicted risks and the observed frequencies of AUR showed underestimation of the nomogram in the validation data set for high AUR risks values. Still, the negative predictive value for the risk of AUR, using a cutoff value of 5%, was high (98.1%).

CONCLUSION

External validation of the nomogram shows adequate discrimination of patients with and without AUR. Therefore, the nomogram can aid in individualized treatment decision making.