Hepatoid thymic carcinoma: a case report of a rare subtype of thymic carcinoma

Hepatoid thymic carcinoma is an extremely rare subtype of primary thymus tumor resembling "pure" hepatoid adenocarcinomas with hepatocyte paraffin 1 (Hep-Par-1) expression. A 53-year-old man presented with voice change and a neck mass. Multiple masses involving the thyroid, cervical and mediastinal lymph nodes, and lung were detected on computed tomography. Papillary thyroid carcinoma was confirmed by biopsy, and the patient underwent neoadjuvant chemoradiation therapy. However, the anterior mediastinal mass was enlarged after the treatment whereas the multiple masses in the thyroid and neck decreased in size. Microscopically, polygonal tumor cells formed solid sheets or trabeculae resembling hepatocytes and infiltrated remnant thymus. The tumor cells showed immunopositivity for cytokeratin 7, cytokeratin 19, and Hep-Par-1 and negativity for α-fetoprotein. Possibilities of germ cell tumor, squamous cell carcinoma, and metastasis of thyroid papillary carcinoma were excluded by immunohistochemistry. This report on the new subtype of thymic carcinoma is the third in English literature thus far.