Most adenocarcinomas of the mediastinum are metastatic lesions. Primary thymic adenocarcinomas are extremely rare neoplasms. We could find only 12 cases reported in the literature; of these 12, only 4 were of the mucinous subtype.
We report 2 additional cases of the mucinous subtype, including a previously unreported mucinous variant with numerous psammoma bodies.
The first case in a 61-year-old woman resembled a mucinous (colloid) carcinoma of other organs such as the breast and colon. It consisted of islands and strips of tumor cells floating in large pools of extracellular mucin. A unique feature of this tumor was the presence of numerous psammoma bodies. Immunohistochemically, the tumor cells were positive for cytokeratin (CK) 7 and negative for CD5. The second case in an 82-year-old woman was a mucinous adenocarcinoma arising from a thymic cyst with areas of transition from benign to dysplastic epithelium. The tumor cells formed dilated glands, cords, and small nests that infiltrated the thymic cyst wall and exhibited evidence of mucin production. Immunohistochemically, the tumor cells were positive for CK 7 and focally positive for both CD5 and CK 5/6.
Mucinous adenocarcinoma, with or without, psammoma bodies, may be of primary thymic origin and should be considered in the differential diagnosis of malignant mediastinal tumors. These 2 cases provide further documentation of the rare occurrence of primary mucinous adenocarcinomas of the thymic gland.