[Anaplastic transformation of well differentiated carcinoma of the thyroid]

Four cases of thyroid cancer with anaplastic transformation are reported. The anaplastic transformed cancers were found as a metastatic lesion 3-37 years after the resection of well-differentiated thyroid cancers. In one case, anaplastic cancer was mixed with well-differentiated cancer in an old metastatic focus but not present in the partially removed hyalinized primary lesion. The role of radiation therapy in anaplastic transformation was discussed.