Papillary carcinoma of the thyroid. A discussion of its several morphologic expressions, with particular emphasis on the follicular variant.

Papillary carcinoma of the thyroid has been classically defined on the basis of the presence and even the predominance of papillary formations within the tumor. Changes in this traditional concept have evolved along two separate but related lines. The first is the realization that well-differentiated tumors having a papillary component, however minimal, will exhibit the behavior of papillary carcinoma even in the presence of extensive areas with a sclerosing, solid, or follicular pattern of growth. The second is an increased reliance on cytologic criteria (particularly the ground-glass nucleus) rather than architectural features for the identification of papillary carcinoma. Through the use of these criteria in association with various clinical features, evidence is put forward in support of the concept of a follicular variant of papillary carcinoma, i.e., a tumor type in which papillae are nil or absent but which still belongs by cell type and clinicopathologic behavior to the group of papillary carcinomas.