Definition of poorly differentiated carcinoma of the thyroid: the Japanese experience

The terminology and definitions pertaining to thyroid malignancies of follicular cell origin that are neither well-differentiated papillary or follicular carcinomas nor undifferentiated anaplastic carcinomas remain controversial. Against this background, we previously proposed that "poorly differentiated carcinoma" should be added to the classification of thyroid carcinoma arising from follicular epithelium. The histological criteria and biological characteristics of poorly differentiated carcinoma of the thyroid were described. In this discussion, we make a new proposal concerning the histological classification of thyroid cancers derived from follicular epithelium. According to this proposal, thyroid cancers can be divided into common types and special types. In the common types, the usual histology should be included. The common types are well-differentiated carcinoma, poorly differentiated carcinoma, and undifferentiated carcinoma. The specific types include columnar-cell carcinoma, tall cell carcinoma, cribriform carcinoma, and other rare carcinomas.