Prostate specific membrane antigen (PSMA) PET is effective in identifying prostate cancer metastases. However, PSMA PET uptake has also been described in other lesions, including non-prostate malignancies and benign processes.
Purpose
To identify causes of unexpected radiopharmaceutical accumulation on PSMA PET.
Methods
2,054 study reports representing 1,873 unique patients who had underwent 68Ga-PSMA-11 PET/CT scans at a single large academic center from December 2015—April 2022 were retrospectively reviewed for the mention of unexpected PSMA uptake not initially thought to represent metastatic prostate cancer. Scans with pathologic outcome were reviewed by two blinded readers for scan indication and lesion location and quantitative parameters.
Results
In 48 patients, the PSMA-ligand avid incidental lesions revealed 19 cases of second malignancies, 17 cases of prostate cancer, and 13 cases of benign lesions. The most common lesion locations were lung (14), thyroid (14), lymph nodes (8), and bowel (4). Benign lesions exhibited lower molecular imaging PSMA (miPSMA) scores (median: 1 IQR: 1.00–1.25, p = 0.017) than metastatic prostate lesions (median: 2, IQR: 1–3). Second malignancies were larger (median: 34 mm, IQR: 27–39 mm) than metastatic prostate cancer (median: 14 mm, IQR: 12–19 mm, p = 0.001) and benign lesions (median: 19 mm, IQR: 13.00—31.00 mm, p = 0.03). PSMA-ligand avid lesions in scans performed in the initial staging for prostate cancer were more commonly associated with a diagnosis of a secondary malignancy than with metastatic prostate cancer (0 vs 8 lesions, p = 0.008). Higher SUVmax was observed for metastatic prostate cancer and second malignancy when compared to benign outcome.
Conclusions
Features that influence the probability of an incidental lesion representing a malignancy include lesion location, reason for the PSMA PET/CT study, and associated imaging features (size, SUVmax, and miPSMA score).
