Gastric Cancer Metastasis to the Prostate Detected on 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography (18F-FDG PET/CT): A Report of a Rare Case.

We report an exceptionally rare case of prostate metastasis in a 70-year-old man with a history of gastric cancer. Following distal gastrectomy (pT4aN0M0) and one year of adjuvant chemotherapy with S-1, he remained recurrence-free until a gradual rise in serum carbohydrate antigen 19-9 was observed, reaching 1878.97 U/mL. Contrast-enhanced computed tomography and esophagogastroduodenoscopy failed to identify a site of recurrence. However, 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) detected abnormal uptake at the apex of the prostate, with a maximum standardized uptake value of 11. Magnetic resonance imaging was non-contributory, and serum prostate-specific antigen (PSA) was mildly elevated at 6.04 ng/mL. A transrectal ultrasound-guided prostate biopsy revealed poorly differentiated adenocarcinoma. Immunohistochemistry was negative for PSA and NKX3.1, but positive for cytokeratin 7, and histological comparison with the prior gastric specimen confirmed metastatic gastric cancer to the prostate. This case underscores the diagnostic challenge posed by rare metastatic patterns and highlights the complementary value of 18F-FDG PET/CT in detecting occult lesions when conventional imaging fails. In patients with a history of gastric cancer and rising tumor markers, early consideration of PET/CT followed by histological confirmation may facilitate timely and accurate diagnosis.