Undifferentiated carcinoma of thyroid gland

**Definition:** A primary carcinoma of the thyroid gland composed of undifferentiated cells. The malignant cells demonstrate evidence of epithelial differentiation, either by immunohistochemistry or electron microscopic studies. Microscopically, in the majority of cases there is a mixture of spindle, epithelioid, and giant cells. The vast majority of the patients present with a rapidly enlarging neck mass. The clinical course is invariably aggressive.

**Inclusions:** - anaplastic carcinoma of thyroid gland