Atypical gynecomastia mimicking breast malignancy on mammography, sonography, and 18F-fluciclovine PET/CT in the setting of biochemical recurrence of prostate cancer

A 60-year-old man with metastatic prostate cancer presented with breast swelling for one year. Suspicious breast masses were identified in both breasts with mammography and ultrasound. Biopsy of both masses showed florid gynecomastia without malignancy. Sixteen months later, the patient underwent 18F-fluciclovine PET/CT for biochemical recurrence of prostate cancer; this showed focal radiotracer uptake in both breasts. Repeat mammogram and ultrasound showed these areas to correspond with the previously biopsied masses, which were stable. To our knowledge, this is the first reported case of gynecomastia mimicking malignancy on 18F-fluciclovine PET/CT.