Intramucosal stomach adenocarcinoma metastasizing as a large intraabdominal mass with focal choriocarcinomatous differentiation.

We report a case of gastric intramucosal adenocarcinoma giving rise to a large metastatic intraabdominal mass with focal choriocarcinomatous differentiation. The main histological picture of the surgically resected abdominal tumor was well differentiated adenocarcinoma with focal choriocarcinomatous differentiation. Serum hCGβ level showed 710mIU/dl postoperatively, and the gastric lesion featured small foci of well differentiated adenocarcinoma in adenoma with no choriocarcinomatous differentiation. The origin of the abdominal tumor was the main point of question. HNF4α is a transcription factor of embryonic liver differentiation, whose distribution is restricted to hepatocytes and certain neoplastic tissue including gastric adenocarcinoma. Hep Par 1 is originally developed for the discrimination of hepatocellular carcinoma, but a part of gastric adenocarcinoma also shows positive staining. Immunostaining with panel of antibodies for CK7, CK20, HNF4α, and Hep-Par1 showed pattern of gastric adenocarcinoma, and a diagnosis of a very rare intramucosal gastric adenocarcinoma metastatic to the abdominal cavity was established. Diagnostic utility of the panels of above antibodies for discrimination of the tumor origin was confirmed, and the relation between the metastatic ability of the gastric adenocarcinoma and its choriocarcinomatous differentiation is discussed.