Preoperative Cytological Diagnosis of Intrathyroidal Thymic Carcinoma: A Case Report With Review of the Literature

Abstract

Intrathyroidal thymic carcinoma (ITTC) is a rare malignant thyroid tumor that is similar to thymus epithelium-related tumors. Herein, we report a case of ITTC diagnosed by fine-needle aspiration cytology (FNA). A 55-year-old male presented with a slowly increasing cervical mass. Cervical ultrasonography revealed an irregular mass measuring 3.7 × 2.5 × 2.1 cm. FNA was performed. Cytological findings showed that the tumor was highly cellular and tumor cells were arranged in islet-like clusters accompanied by lymphocytes. The tumor cells had a high nuclear-to-cytoplasmic ratio. The nuclei were round to ovoid or spindle-shaped. The fusiform tumor cells exhibited a regular streaming arrangement, similar to squamous cell carcinoma. Immunocytochemically, the tumor cells were positive for CD5 and c-KIT. Pathological examination of the resected thyroid specimen revealed confluent cell nests of epithelioid and spindle cells with significant lymphocyte and plasma cell infiltration throughout the tumor. Immunohistochemically, the tumor cells were positive for cytokeratin, CD5, c-KIT, and p63, and negative for TTF-1 and thyroglobulin. Therefore, the tumor was diagnosed as ITTC. Although ITTC is rare, it should be considered in the differential diagnosis of a mass in the lower pole of the thyroid gland that exhibits large cohesive islet-like clusters with numerous lymphocytes and features resembling those of squamous cell carcinoma.