Poorly Differentiated ("Insular") Thyroid Carcinoma on Fine-Needle Aspiration

Poorly differentiated thyroid carcinoma (PDTC) is a distinct entity of thyroid follicular origin (without follicular or papillary differentiation), with high-grade features and an aggressive clinical behavior intermediate between that of well-differentiated and undifferentiated thyroid carcinoma. It accounts for 4% to 7% of thyroid malignancies. Poorly differentiated thyroid carcinoma often presents at an advanced stage and tends to metastasize to regional lymph nodes, lungs, and bones. A variety of histological patterns exists for PDTC and the corresponding cytomorphological features are also varied and depend on the growth pattern of the individual neoplasm. We report the cytomorphology of PDTC sampled by fine-needle aspiration and review previously reported cases in the literature. While PDTC do not possess specific cytomorphological features that would allow for a definitive diagnosis on fine-needle aspiration, the presence of certain features may suggest the possibility of this rare neoplasm. In particular, cells with overlapping bland nuclei found both in groups and individually are common features. Lesions are often cellular and lack colloid, providing the sense of a neoplasm. It may be difficult to exclude the more common possibility of a papillary thyroid carcinoma or follicular neoplasm; however, familiarity with this uncommon entity allows one to include it in the differential diagnosis.