Pure squamous cell carcinoma and mixed adenosquamous cell carcinoma of the thyroid gland.

Four cases of pure squamous cell carcinoma and two cases of mixed squamous cell and glandular carcinoma are reported. Obviously, there is a possibility that squamous cell carcinoma of the thyroid gland develops from the existing squamous cells or as a result of metaplasia of follicular cells. Metastases and direct extension of squamous cell carcinoma in the thyroid gland are much more frequent and must be excluded before the diagnosis can be established. Although the gross and microscopic pathology of these lesions presents no unusual features, follicles lined by metaplastic squamous cells offer strong presumptive evidence of a primary thyroid neoplasm. These tumors are comparatively radioresistant and biologically highly malignant. The treatment of choice is radical surgical resection, and these neoplasms must be treated more aggressively in the initial stage.