Anaplastic thyroid cancer (ATC), also known as anaplastic thyroid carcinoma, is an aggressive form of thyroid cancer characterized by uncontrolled growth of cells in the thyroid gland. This form of cancer generally carries a very poor prognosis due to its aggressive behavior and resistance to cancer treatments.[1] The cells of anaplastic thyroid cancer are highly abnormal and usually no longer resemble the original thyroid cells and have poor differentiation.

Reference
Wiki: Anaplastic thyroid cancer