[A case of prostate cancer presenting as extensive abdominal metastasis]

A 73-year-old man was admitted to our hospital with a chief complaint of hematemesis. Two years before admission, adenocarcinoma of unknown origin was diagnosed. Since then, the patient had been taking TS-1 (pramoxine hydrochloride) medication, which caused gastroduodenal mucosal damage. A large abdominal tumor and elevated prostate specific antigen (PSA) level of 13,190 ng/ml, caused by this damage, were detected. Extensive abdominal metastasis of prostate cancer was diagnosed and combined androgen blockade was initiated. After 3 months, the PSA level decreased to 4.4 ng/ml and the abdominal tumor shrunk significantly. Physicians should keep in mind prostate cancer in the differential diagnosis of unexplained adenocarcinoma.