Intrathyroidal thymic carcinoma: a case report.

Carcinoma showing thymus-like differentiation (CASTLE) is a rare malignancy that occurs in the soft tissues of the neck or in the thyroid gland. When it occurs in the thyroid, it is difficult to differentiate from thyroid tumor. Here, we report such a case. A 34-year-old man presented with a mass in the left lower neck in August 1999. Thyroid ultrasonography showed a hypoechoic mass, which replaced most of the left thyroid gland. Fine-needle aspiration cytology showed poorly differentiated carcinoma. He received left lobectomy in February 2001. Grossly, the mass measuring 3.7 x 3.5 x 3.5 cm was located in the lower part of the left lobe of the thyroid gland. The cut surface was yellowish gray. Microscopically, the tumor was separated into lobules by fibrous tissues infiltrated with small lymphocytes. It was composed of poorly differentiated squamoid cells and focal keratin pearls. Thymus-like tissue with Hassall's corpuscles was seen adjacent to the tumor cells. Immunohistochemically, the tumor cells were positive for cytokeratin and CD5, but negative for thyroglobulin. Keratinizing squamous cell carcinoma arising from intrathyroid thymic tissue was diagnosed. Because of a dubious section margin, adjuvant radiotherapy with a total dose of 5000 cGy was given. There was no evidence of recurrence twenty months after surgery. Although intrathyroidal thymic carcinoma is rare, it should be differentiated from anaplastic thyroid carcinoma because these conditions have different prognosis.