Tumors of thyroid follicular epithelium: where have we been and where are we going?.

In a time that was much simpler but not so long ago, we believed there were two main types of differentiated thyroid cancer, papillary and follicular, and diagnostic criteria were relatively straightforward. Papillary carcinomas were characterized by papillary architecture, local invasiveness, and a relatively indolent course, while follicular carcinomas had a follicular architecture, were most often encapsulated, and were more prone to distant metastasis.