Primary squamous cell carcinoma of the thyroid arising in Hashimoto's thyroiditis in an adolescent.

Squamous cell carcinoma is a rare thyroid neoplasm that has been described exclusively in adults. We report what appears to be the first example of a primary squamous cell carcinoma of the thyroid gland arising in a background of Hashimoto's thyroiditis in an adolescent female. The tumor was well demarcated, confined to the right thyroid lobe, and did not metastasize, although follow up has been limited. The squamous cell carcinoma was well to moderately differentiated, and the stroma contained an abundant inflammatory infiltrate rich in lymphocytes and eosinophils. The lack of goblet cells, extracellular mucin, and extensive stromal sclerosis excluded the diagnosis of sclerosing mucoepidermoid carcinoma with eosinophilia. Immunohistochemical staining revealed focal expression of cytokeratin 7 and diffuse labeling with cytokeratin AE1/AE3. The squamous cell carcinoma overexpressed p53 protein and showed increased proliferative activity, as evidenced by the high MIB-1 labeling index. In contrast, the tumor did not show immunoreactivity for thyroglobulin or thyroid transcription factor 1.