External Validation of Two Nomograms Developed for ⁶⁸Ga-PSMA-11 Applied to the Prostate-specific Membrane Antigen Tracer ¹⁸F-DCFPyl: Is Prediction of the Optimal Timing of Salvage Therapy Feasible?

Two nomograms have been developed to predict the outcome of positron emission tomography (PET)/computed tomography (CT) imaging with⁶⁸Ga-labeled ligands for prostate-specific membrane antigen (⁶⁸Ga-PSMA) for patients with rising prostate-specific antigen after radical prostatectomy (RP). These nomograms quantify the ability of PSMA PET/CT to detect prostate cancer recurrences, and therefore provide critical information in determining the optimal timing for PSMA PET/CT in guiding salvage therapies. We validated the ability of these nomograms to accurately predict PET/CT outcome using another ligand tracer, ¹⁸F-DCFPyL. The external validation cohort consisted of 157 men from the Prostate Cancer Network Netherlands who underwent ¹⁸F-DCFPyL PET/CT to guide salvage therapies after RP. The nomogram of Rauscher et al (predicting a positive scan) showed accurate prediction of 50–80% (discrimination 0.68, 95% confidence interval [CI] 0.59–0.76). The nomogram of Luiting et al (predicting recurrence outside the prostatic fossa) showed accurate prediction for predicted probability values between 15% and 65%, with a small degree of overestimation for predicted probability values between 30% and 50% (discrimination 0.74, 95% CI 0.28–1.24). According to calibration curves, discrimination results, and decision curve analysis, we conclude that clinicians can use these ⁶⁸Ga-PSMA–based nomograms to predict ¹⁸F-DCFPyL PET/CT outcome. These nomograms improve shared decision-making in determining the optimal time to initiate PSMA PET/CT–guided salvage therapies. Patient summary: Prediction tools developed for prostate scans (positron emission tomography, PET) using one type of radioactive tracer (chemicals labeled with gallium-68) are also accurate in predicting scan findings with another tracer (a chemical labeled with fluorine-18). Our study confirms that these tools can be used to guide decisions on the timing of treatments for prostate cancer recurrence.