Monarch Disease Ontology id MONDO_0006468 (thyroid gland undifferentiated (anaplastic) carcinoma) is 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 usually aggressive. Also known as: Dedifferentiated thyroid gland carcinoma, anaplastic carcinoma of the thyroid, anaplastic carcinoma of the thyroid gland, anaplastic carcinoma of thyroid, anaplastic carcinoma of thyroid gland, anaplastic thyroid cancer, anaplastic thyroid carcinoma, anaplastic thyroid gland carcinoma.