Prostate-specific membrane antigen (PSMA) uptake in the scrotum and epididymis on PET-CT: when is it pathological?

Prostate cancer is the most common solid organ tumor in men and has been reported to metastasize to unusual sites such as the epididymis. The clinical standard for detecting recurrent disease is through positive emission tomography/computed tomography with the radiotracer ¹⁸F-DCFPyL binding prostate-specific membrane antigen (PSMA) expressed by cancerous cells. Although PSMA can also be expressed physiologically, metastases are more likely to be intensely PSMA expressing and in a typical distribution depending on the extent of disease burden in the individual patient. A MEDLINE search revealed only three other case reports of isolated epididymal metastases from prostate cancer diagnosed with prostate-specific membrane antigen positron emission tomography-computed tomography. This case series comprising both metastatic and physiological PSMA expression in the epididymis provides a useful framework for the interpreting physician when the possibility of this rare but important finding is encountered in prostate cancer imaging. (author)