What is anaplastic thyroid carcinoma?

Undifferentiated tumors of the thyroid follicular epithelium, accounting for less than 5% of thyroid tumors. They are aggressive, with a mortality rate approaching 100%. Patients with anaplastic carcinoma are older than those with other types of thyroid cancer, with a mean age of 65 years. Approximately one-fourth of patients with anaplastic thyroid carcinomas have a history of a well-differentiated thyroid carcinoma, and another one-fourth harbor a concurrent well-differentiated tumor in the resected specimen. Anaplastic carcinomas manifest as bulky masses that typically grow rapidly beyond the thyroid capsule into adjacent neck structures. On microscopic examination, these neoplasms are composed of highly anaplastic cells, which may be large and pleomorphic or spindle shaped and in some cases mixture of the two cell types. Foci of papillary or follicular differentiation may be present in some tumors, suggesting origin from a better-differentiated carcinoma.