Carcinoma of possible thymic origin presenting as a thyroid mass: a new subgroup of squamous cell carcinoma of the thyroid.

Intrathyroidal thymoma is a rare tumor that occurs in the thyroid gland and has been reported as a low-grade malignant tumor. The present report describes a 59-year-old male patient with this tumor, who was treated with subtotal thyroidectomy followed by radiation and chemotherapy. The tumor was located in the lower pole of the left lobe and the isthmus of the thyroid, and it had invaded the thyroid parenchyma, thyroid capsule, adjacent connective tissue, and neck muscles. Continuity between the cervical thymus and the tumor was noted. Subsequently, the patient developed widespread metastases in the liver, lungs, bones, and lymph nodes without local recurrence. Histologic examination disclosed a solid growth of epithelial cells with squamous cell differentiation and keratinization. The tumor cells had an ill-defined cell border and large nuclei with large nucleoli. Moderate mitoses and slight necrosis of the tumor were seen. Immunohistochemical examination showed that the tumor cells were stained positively for keratin but not for calcitonin or thyroglobulin. There were infiltrations of lymphocytes in the tumor and fibrous stroma, and most of them proved to be T cells.