Primary thymic adenocarcinoma with an aggressive clinical course: An autopsy case showing signet ring cell‐like features

Thymic adenocarcinoma is an extremely rare neoplasm, and little is known about its pathogenesis and clinical characteristics. A 52‐year‐old man presented to our clinic with severe dyspnea. At initial presentation, massive carcinomatous pleuritis and pericarditis were observed, and a lobulated mass in the anterior mediastinum was found on computed tomography. Cytological examination revealed adenocarcinoma accompanied by signet ring cells; however, his tumor showed aggressive growth without any possibility of treatment, and he died as a result of cancer progression within one month of admission. An autopsy confirmed thymic adenocarcinoma showing various histological features including mucinous, signet ring cell‐like, and trabecular features. Immunohistochemically, the tumor cells were positive for cytokeratin (CK) (AE1/AE3) but negative for TTF‐1. In addition, some tumor cells were positive for CD5 and KIT. Further examination revealed that tumor cells of the nonmucinous type were positive for CK7, and negative for CK20 and caudal‐type homeobox 2 (CDX2). The tumor cells with mucinous and signet ring‐like features were positive for CK20 and CDX2 and negative for CK7, indicating enteric differentiation. In particular, tumor cells with signet ring cell‐like features indicated widespread lymphangitic carcinomatosis and pulmonary tumor thrombotic microangiopathy (PTTM). The presence of signet ring cell‐like features with enteric differentiation is suggestive of a fulminant clinical course due to widespread lymphangiosis carcinomatosa and PTTM in patients with thymic adenocarcinoma. KEY POINTS: Thymic adenocarcinoma is an extremely rare neoplasm. Histological features of thymic adenocarcinoma include mucinous, signet ring cell‐like, and trabecular features. Tumor cells with signet ring cell‐like features indicate widespread lymphangitic carcinomatosis and pulmonary tumor thrombotic microangiopathy. The presence of signet ring cell‐like features with enteric differentiation is associated with a fulminant clinical course.