68Ga-PSMA-11 PET/CT interobserver agreement for prostate cancer assessments: an international multicenter prospective study

Background: The interobserver agreement for (68)Ga-PSMA-11 Positron Emission Tomography/Computed Tomography (PET/CT) study interpretations in patients with prostate cancer is unknown. Methods:(68)Ga-PSMA-11 PET/CT was performed in 50 patients with prostate cancer for biochemical recurrence (n = 25), primary diagnosis (n = 10), biochemical persistence after primary therapy (n = 5) or staging of known metastatic disease (n = 10). Images were reviewed by 16 observers who used a standardized approach for interpretation of local (T), nodal (N), bone (Mb), or visceral (Mc) involvement. Observers were classified as having low (\textless30 prior (68)Ga-PSMA-11 PET/CT studies; n = 5), intermediate (30 to 300 studies; n = 5), or high level of experience (\textgreater300 studies; n = 6). Histopathology (n = 25, 50%), post-external beam radiation therapy prostate-specific antigen (PSA) response (n = 15, 30%), or follow-up PET/CT (n = 10, 20%) served as standard of reference (SOR). Observer groups were compared by overall agreement (% patients matching the SOR) and Fleiss' \textgreekk with mean and corresponding 95% confidence interval (CI). Results: Agreement among all observers was substantial for T (\textgreekk=0.62, 95%CI 0.59-0.64) and N (\textgreekk=0.74, 95%CI 0.71-0.76) staging and almost perfect for Mb (\textgreekk=0.88, 95%CI 0.86-0.91) staging. Level of experience positively correlated with agreement for T (\textgreekk=0.73/0.66/0.50 for high/intermediate/low experience, respectively), N (\textgreekk=0.80/0.76/0.64), and Mc staging (\textgreekk=0.61/0.46/0.36). Interobserver agreement for Mb was almost perfect irrespective of prior experience (\textgreekk=0.87/0.91/0.88). Observers with low experience, when compared to intermediate and high experience, demonstrated significantly lower median overall agreement (54% versus 66% and 76%, P = 0.041) and specificity for T staging (73% versus 88% and 93%, P = 0.032). Conclusion: The interpretation of (68)Ga-PSMA-11 PET/CT for prostate cancer staging is highly consistent among observers with high levels of experience, especially for nodal and bone assessments. Initial training on at least 30 patient cases is recommended to ensure acceptable performance.