External in-house validation of a prediction nomogram estimating pre-test ⁶⁸ Ga-PSMA-ligand PET/CT positivity in early biochemical recurrent prostate cancer after radical prostatectomy

Ziel/Aim ⁶⁸Ga-PSMA-ligand PET is increasingly used in early biochemical recurrence (BCR). Recently, we proposed a prediction nomogram for ⁶⁸Ga-PSMA-ligand positron-emission tomography (PET) positivity. The aim of this study was to validate this recently established prediction nomogram with a subsequent patient series.

Methodik/Methods 292 consecutive patients with BCR after radical prostatectomy (PSA-values 0.2–1 ng/ml) who underwent ⁶⁸Ga-PSMA-ligand PET/CT were retrospectively included. The cohort was divided into very low PSA-value (0.2–0.5 ng/ml, n = 151) and low PSA-value (> 0.5–1 ng/ml, n = 141) patients. Pre-test positivity probabilities for each patient were calculated according to the previously published compact and comprehensive prediction model using specific clinical variables (PSA-value, Gleason score, T-and N-stage, androgen deprivation therapy, radiation therapy). Calculated pre-test probabilities were correlated with the results from subsequent ⁶⁸Ga-PSMA-ligand PET/CT analysis.

Ergebnisse/Results In the very low PSA-value subgroup the mean pre-test probability for positive findings in ⁶⁸Ga-PSMA-ligand was 57 % (95 % CI: 55 %-60 %) in the compact model and 59 % (95 % CI:56-61 %) in the comprehensive model. 59 % (89/151) of patients in this group had positive imaging findings. In the low PSA-value subgroup the mean pre-test probability was 72 % (95 % CI: 70 %-74 %) in the compact model and 74 % (95 % CI:72 %-76 %) in the comprehensive model. 79 % (112/141) of these patients had positive findings in the ⁶⁸Ga-PSMA-ligand PET/CT. The nomogram performed best in patients with low to moderate predicted positivity while overestimating the actual positivity rate in cases with a high predicted positivity rate.

Schlussfolgerungen/Conclusions Validation of the recently proposed prediction nomograms showed a high concordance of the calculated pre-test probabilities and ⁶⁸Ga-PSMA PET/CT findings confirming the nomogram’s ability to determine the presence of a positive lesion at ⁶⁸Ga-PSMA-ligand PET. These results provide further evidence supporting the potential role of the proposed nomograms in patient selection. In a next step multicentre validation is mandatory.