[Mediastinal signet-ring cell carcinoma of unknown primary site; report of a case].

The question whether the tumor is primary or metastatic sometimes arises in patients with intrathoracic tumor. Especially, adenocarcinoma presents more difficulties in distinguishing primary tumors from secondary or deciding its origin. We reported a case of mediastinal signet-ring cell carcinoma. A 51-year-old female was admitted because of an abnormal shadow during mass screening. Chest X-ray showed a semicircular, well-demarcated shadow on right cardiophrenic angle. Chest computed tomography (CT) revealed a homogeneous mass, approximately 4 cm in diameter. The investigations of whole body failed to reveal any other tumor. At operation, the tumor was adherent to the esophagus but easily resected. Histologically, the tumor was mainly composed of signet-ring cells. Immunohistochemically, surfactant apoprotein (SPA) was strongly demonstrated in tumor cells. SPA is specific to the lung and the tumor was diagnosed as the metastasis in mediastinal lymph nodes. The primary site was considered to exist in lung. But the primary site was not found over 46 months in spite of repeated further examinations.