THE ASSOCIATION OF WELL-DIFFERENTIATED THYROID-CARCINOMA WITH INSULAR OR ANAPLASTIC THYROID-CARCINOMA - EVIDENCE FOR DEDIFFERENTIATION IN TUMOR PROGRESSION

The sequence of tumorigenesis in the thyroid is unclear. It has been proposed that anaplastic carcinomas of the thyroid develop by dedifferentiation in pre-existing differentiated carcinomas. We reviewed all anaplastic and insular (poorly differentiated) thyroid carcinomas in a consultation practice of thyroid pathology that included more than 400 thyroid cancers. Sixteen tumors (4%) were classified as anaplastic or insular (poorly differentiated) thyroid carcinomas. We examined these cases to determine whether these carcinomas were associated with well-differentiated neoplasms of follicular cell derivation. Ten patients were women and 6 were men, and ages ranged from 29 to 85 years; 10 patients with anaplastic carcinomas and 2 with insular carcinomas were 56 years or older, whereas 3 of the 6 patients with insular carcinomas were 31 years or younger. Four tumors were composed exclusively of anaplastic carcinoma; all were represented only by incisional biopsies. One insular carcinoma infiltrated and destroyed all underlying thyroid tissue. In the remaining total, subtotal, or hemithyroidectomy specimens, areas of well-differentiated papillary or follicular carcinoma were found. Some differentiated papillary lesions had a wide spectrum of morphologies. including Hurthle cell, tall cell, and columnar cell features. In the literature, simultaneous or previous occurrence of well-differentiated thyroid carcinomas with anaplastic carcinomas is extremely variable, ranging from 7-89% of cases. In experimental animals, serial transplantation of differentiated thyroid tumors has been shown to lead to anaplastic transformation. Our findings suggest that the majority of anaplastic thyroid carcinomas in humans arise from well-differentiated tumors. However, only a very small number of differentiated carcinomas progress to anaplastic lesions, the factors underlying this phenomenon remain to be identified.