Mismatched Lesions on ¹⁸F-FDG PET and ¹⁸F-Fluciclovine PET Images in a Patient with Metastatic Prostate Small Cell Carcinoma

A 65-year-old man with fluciclovine-avid metastatic prostate small cell carcinoma with prostate-specific antigen (PSA) 19.4 ng/mL at diagnosis underwent system therapy and subsequent surgery and achieved hormonal response with PSA <0.1 ng/mL. An 18F-fluciclovine PET/CT scan 3 months after surgery was negative for disease. Although PSA remained <0.1 ng/mL, the rising carcinoembryonic antigen prompted an 18F-FDG PET/CT 6 weeks later. It showed multiple hypermetabolic lesions in the prostatectomy bed, liver, and right iliac bone, suggestive of malignant disease. The FDG-avid prostatectomy lesions were further confirmed on MRI. This case demonstrates that FDG PET/CT has a role in suspected metastatic prostate small cell carcinoma with negative fluciclovine PET examination.