We describe the first case of primary thymic adenocarcinoma with signet ring cell features. The patient was a 39-yearold Filipino male who presented with a huge anterior mediastinal mass extending to the left supraclavicular fossa, which underwent an incisional biopsy. Extensive clinicoradiological work-up showed no evidence of any primary tumor in other organs, and radiological imaging confirmed a primary tumor location in the thymic area. He later developed bilateral pleural and pericardial effusions and eventually died of his tumor. The biopsy contained a neoplastic growth of solid nests made of cells with prominent signet ring features in sclerotic stroma. Immunohistochemically, the tumor cells were reactive for cytokeratin 7, carcinoembryonic antigen, and CD5 and negative for cytokeratin 20, TTF1, napsin A, αfetoprotein, PAX-8, CD-117, CA19-9, CA-125, CDX2, p63, and CD99. No genetic alterations of ALK, RET, and ROS1 were found, nor was any ALK or ROS1 immunostaining detected, as known to occur in a fraction of primary pulmonary adenocarcinomas. Morphologically, this thymic tumor resembled signet ring cell adenocarcinomas of other locations, including, stomach, pancreas, and lung, but CD5 immunoreactivity strongly supported the clinical and radiological evidence of a primary thymic origin. In the English literature, only 58 cases of primary thymic adenocarcinoma are on record and this is the first report of a signet ring cell variant, which further broadens the morphological spectrum of thymic adenocarcinoma subtypes.
