Evaluation of Practical Interpretation Hurdles in 68Ga-PSMA PET/CT in 55 Patients: Physiological Tracer Distribution and Incidental Tracer Uptake.

Purpose: To investigate the physiologic Ga-PSMA distribution and evaluate focal or diffuse radiotracer uptake in nonprostate cancer malignancies and in incidental findings.

Methods: Ga-PSMA PET/CT scans in 55 men performed for prostate cancer (49) or renal cell carcinoma (6) staging were analyzed retrospectively. Two radiologists evaluated the datasets in 2 reading sessions. First, physiological Ga-PSMA uptake was evaluated. Second, scans were analyzed for incidental uptake. SUVmax and SUVmean were recorded. Other imaging modalities, histopathology, or clinical follow-up served as standard of reference.

Results: Homogenous Ga-PSMA uptake of the lacrimal glands (SUVmax, 15.7 ± 7.2), parotid glands (SUVmax, 24.4 ± 8.1), submandibular glands (SUVmax, 26.7 ± 7.1), vocal cords (SUVmax, 8.4 ± 3), Waldeyer ring (SUVmax, 10.4 ± 4.3), liver (SUVmax, 8.2 ± 2.5), spleen (SUVmax, 10.9 ± 3.9), kidneys (SUVmax, 66.4 ± 25.4), and pars descendens duodeni (SUVmax, 17.6 ± 8.9) was observed in all patients. In 65% and 36%, respectively, homogenous Ga-PSMA uptake of the colon descendens (SUVmax, 10.6 ± 9.2) and the rectum (SUVmax, 3.7 ± 1.1) was found. Approximately 22% exhibited a Ga-PSMA uptake of the thyroid (SUVmax, 4.5 ± 1.2), and 21% exhibited a Ga-PSMA uptake of the knee's synovia (SUVmax, 2.9 ± 0.2). Furthermore, Ga-PSMA uptake was found in 1 patient because of fibrous dysplasia of the right os ilium (SUVmax, 7.7).

Conclusions: Physiologic distribution of Ga-PSMA comprises uptake in lacrimal and salivary glands, vocal cords, Waldeyer ring, liver, spleen, and kidneys as well as various parts of the intestine. Moreover, nonspecific tracer uptake is regularly found in the thyroid and the synovia of the knee. Incidental Ga-PSMA uptake can occasionally reveal nonprostate cancer-associated remodeling processes, such as fibrous dysplasia.