Purpose: To perform an external validation of a recently published nomogram aimed to predict positive 68Ga-PSMA-11 PET/CT in patients with biochemical recurrence (BCR) after radical prostatectomy (RP) by Rauscher et al. (Eur Urol 73(5):656-661, 2018).
Methods: Overall, 413 PCa patients with BCR after RP (two consecutive PSA ≥ 0.2 ng/ml) and PSA value between 0.2 and 1 ng/ml were included. A multivariable logistic regression model was produced to assess the predictors of positive 68Ga-PSMA-11 PET/CT results. The performance characteristics of the model were assessed by quantifying the predictive accuracy, according to model calibration. Yuden's index was used to find the best nomogram's cut-off. Finally, decision curve analysis (DCA) was implemented to quantify the nomogram's clinical value.
Results: In the external cohort, the overall detection rate of 68Ga-PSMA-11 PET/CT was 44% vs. 64.7% in the original population. At multivariate analysis, PSA at 68Ga-PSMA-11 PET/CT (OR: 7.06, p < 0.001) and ongoing ADT at time of 68Ga-PSMA-11 PET/CT (OR: 2.07, p = 0.03) were the only independent predictors of PET/CT positivity. The predictive accuracy of nomogram was suboptimal and comparable to that reported in the original model (64% vs. 67%, respectively). The calibration plot indicated suboptimal concordance. The best nomogram's cut-off to predict positive 68Ga-PSMA-11 PET/CT was 35% (AUC = 0.61). In DCA, the nomogram revealed clinical net benefit when the threshold probabilities of positive 68Ga-PSMA-11 PET/CT is > 35%.
Conclusion: We assessed similar suboptimal predictive accuracies in the external cohort compared to the original one. PSA and ongoing ADT were confirmed as positive predictors, and the most informative nomogram cut-off resulted 35%.
Keywords: Biochemical recurrence; Nomogram; PSMA; PSMA PET; PSMA nomogram; Salvage therapy.