Also known as undifferentiated thyroid cancer, the disease involves the rapid division of cancer cells that don't or barely resemble or act like normal thyroid cells. Anaplastic cancers can grow as quickly as several weeks and can cause severe compression of the structures in the neck, interfering with breathing, swallowing, and speaking.

Anaplastic thyroid cancer makes up only 1%-2% of all types of thyroid cancer. But it accounts for as much as half of all thyroid cancer deaths in the country. The word anaplastic describes cancer cells that multiply quickly and that are very different from normal cells. Anaplastic thyroid cancer is also called undifferentiated thyroid cancer.

A small subset of thyroid cancer, known as anaplastic thyroid cancer (ATC), is undifferentiated, and nearly incurable with a median survival of only six months. Because of its dismal prognosis, it is responsible for 40–50% of total thyroid cancer-related deaths in the United States.