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.