Cancer of unknown primary inside the gastric wall identified by endoscopic submucosal dissection.

We report the rare and interesting case of cancer of unknown primary (CUP) detected by endoscopic submucosal dissection (ESD). A 67-year-old man with a gastric adenoma was referred to our hospital for endoscopic treatment. Esophagogastroduodenoscopy revealed a 15-mm submucosal tumor (SMT) at the lesser curvature of the lower gastric body, near the gastric adenoma. Both lesions were resected by ESD. Pathological examination showed that the SMT was a poorly differentiated adenocarcinoma with lymphatic tissue. Additional surgical resection was performed, and the lymph nodes were found to have the same pathological findings as the SMT. These lesions were diagnosed as CUP because the obvious primary site was not detected with additional examination. The patient has been followed up for 24 months without recurrence.