Cystic Squamous Cell Carcinoma of the Thyroid A Possible New Subgroup of Intrathyroidal Epithelial Thymoma.

In the thyroid gland, both squamous cell carcinoma and intrathyroidal epithelial thymoma are rare tumors with squamous features, but their prognoses differ; the former is usually fatal, and the latter is favorable. We describe a 38-yr-old female patient with a cystic tumor diagnosed cytologically as squamous cell carcinoma, who was treated with subtotal thyroidectomy. The tumor was located intrathyroidally in the middle third of the left lobe, and no invasion to the surrounding tissues was found. Histologic examination disclosed a cyst lined by epithelial cells with squamous cell differentiation. The tumor cells had ill-defined cell borders and large nuclei with prominent nucleoli. Invasion of the tumor capsule, a few mitoses, and infiltration of lymphocytes in the tumor periphery were noted, but neither necrosis nor metastasis to lymph nodes was observed. The patient has been free from recurrence for 3 yr after thyroid lobectomy. We propose that this type of cystic intrathyroidal squamous cell carcinoma may represent a cystic variant of intrathyroidal epithelial thymoma.