<prompt> Rarest type of thyroid carcinoma isPapillaryFollicularMedullaryAnaplastic  <response>: Undifferentiated anaplastic carcinomaThis is one of the most aggressive malignancies in humans. Thankfully it is rare. It may develop de novo, or present as dedifferentiation of a papillary or poorly differentiated carcinoma. The disease is characterised by rapid growth, visceral invasion and distant metastases. The surgeons role in this disease is crucial. Thyroid lymphoma can be incorrectly diagnosed as anaplastic cancer and so biopsy is critical. This can be done using a core or open technique.Reference: Bailey Loves short practice of surgery , 27th ed page no 816 