Poorly Differentiated Thyroid Carcinoma

Abstract

Poorly differentiated thyroid carcinoma, accounting for less than 10 % of all thyroid cancers, occupies an intermediate position between well-differentiated thyroid carcinoma and anaplastic thyroid carcinoma in regard to morphology and biologic behavior. The typical cytologic findings consist of predominantly small follicular cells with high nuclear-to-cytoplasmic ratio, marked nuclear crowding and overlapping, frequent mitotic figures, and irregular nuclei, frequently with evidence of necrosis. However, its differentiation from other thyroid neoplasms, in particular, follicular adenoma/carcinoma, and follicular variant of papillary thyroid carcinoma, may be difficult due to overlapping of cytologic features, such as microfollicular structures, irregular nuclei, and nuclear grooves. Therefore, when the cytologic findings do not permit a definitive diagnosis of poorly differentiated thyroid carcinoma, a diagnosis of “carcinoma of follicular origin” or “carcinoma not otherwise specified” should be rendered and not as “follicular variant of papillary thyroid carcinoma” and certainly not as “follicular neoplasm.”