 1. Participants must have histological diagnosis consistent of Anaplastic Thyroid Cancer ATC. Cytologic diagnosis by fine needle aspiration alone is not sufficient. Histologic diagnosis may be made by core needle biopsy, incisional biopsy, thyroidectomy, or other surgical biopsy. Fresh tumor biopsies re-biopsy should be obtained whenever feasible. The central pathology review may take place prior to or after the participant starts treatment with lenvatinib. 