Abstract
Infiltration of the capsule, vascular invasion, and/or neoplastic extension into the adjacent parenchyma are regarded as prerequisites for the diagnosis of follicular carcinoma. In modern practice, most of these tumors fall into the category of follicular carcinoma, minimally invasive (FCMI) characterized by evidence of limited capsular or vascular invasion with an excellent long-term prognosis and a good patient outcome. Notwithstanding the wide acceptance of the diagnostic criteria established by the World Health Organization for the classification of follicular carcinomas in particular, they have been difficult to apply and have led to a great deal of confusion. This confusion is compounded when applied to "low-grade" or "minimally invasive" follicular carcinoma because of the poor reproducibility of the classification and the variable results reported in the literature. Our surgical colleagues face a similar lack of a standardized treatment for low-grade follicular carcinomas, which leads to unnecessary surgical treatment. Standardization of histologic criteria is necessary to promote confidence and uniformity in the therapeutic approach of these tumors. We believe that a FCMI is defined as an encapsulated follicular tumor (not papillary), with only small to medium vessel invasion within or immediately adjacent to the tumor capsule and/or up to full-thickness capsular transgression without accompanying extension into the thyroid parenchyma with intervening fibrosis. By using these criteria, patients can be managed with conservative surgical excision to yield an excellent long-term patient outcome.