Validation of a bone scan positivity risk table in non-metastatic castration-resistant prostate cancer

Objectives To test the external validity of a previously developed risk table, designed to predict the probability of a positive bone scan among men with non-metastatic (M0) castration-resistant prostate cancer (CRPC), in a separate cohort. Patients and Methods We retrospectively analysed 429 bone scans of 281 patients with CRPC, with no known previous metastases, treated at three Veterans Affairs Medical Centers. We assessed the predictors of a positive scan using generalized estimating equations. Area under the curve (AUC), calibration plots and decision-curve analysis were used to assess the performance of our previous model to predict a positive scan in the current data. Results A total of 113 scans (26%) were positive. On multivariable analysis, the only significant predictors of a positive scan were log-transformed prostate-specific antigen (PSA): hazard ratio (HR) 2.13; 95% confidence interval (CI) 1.71-2.66 (P <0.001) and log-transformed PSA doubling time (PSADT): HR 0.53; 95% CI 0.41-0.68 (P <0.001). Among men with a PSA level <5 ng/mL, the rate of positive scans was 5%. The previously developed risk table had an AUC of 0.735 to predict positive bone scan with excellent calibration, and provided additional net benefit in the decision-curve analysis. Conclusion We have validated our previously developed table to predict the risk of a positive bone scan among men with M0/Mx CRPC. Use of this risk table may allow better tailoring of patients' scanning to identify metastases early, while minimizing over-imaging. Regardless of PSADT, positive bone scans were rare in men with a PSA <5 ng/mL.