Squamous differentiation in thyroid carcinoma. With special reference to histogenesis of squamous cell carcinoma of the thyroid.

Squamous differentiation of thyroid carcinoma was studied clinicopathologically and immunohistochemically in 29 autopsy cases. Tumor cell nests with squamous differentiation (CNSD), which histologically resembled squamous cell carcinoma, were found in 6 cases (20.7%). All of these 6 cases with CNSD had areas of undifferentiated carcinoma, representing 31.6% of 19 cases with undifferentiated carcinoma, and all but one case also showed coexisting papillary carcinoma. The CNSD were histologically associated with undifferentiated carcinoma in 5 cases, and with papillary carcinoma in one case; the CNSD were occasionally intermingled with these types of carcinoma, and there were findings suggesting a histological transition between the CNSD and undifferentiated carcinoma or papillary carcinoma. Immunohistochemistry revealed that all the CNSD were reactive with antibodies for keratin and vimentin, whereas thyroglobulin and desmin were not expressed. It was concluded that the CNSD examined here were most probably due to extensive squamous differentiation (squamous metaplasia) in undifferentiated carcinoma and papillary carcinoma. In addition, the present results may explain the fact that cases diagnosed solely as squamous cell carcinoma sometimes show a prognosis similar to that of undifferentiated carcinoma, and may well represent extensive squamous differentiation in such tumors rather than true squamous cell carcinoma of the thyroid.