Cytologic diagnostic approach to poorly differentiated thyroid carcinoma: a single-institution study

Fine-needle aspiration cytology (FNAC) is a well-established technique for the preoperative evaluation of thyroid nodules because it is minimally invasive, cost-effective, quick, efficient, and safe. Various articles have discussed differences in the cytomorphology of different types of thyroid cancer. However, review articles on the cytologic diagnosis of poorly differentiated thyroid carcinoma (PDTC) are scarce as PDTC are rare tumors. Although the histologic diagnostic criteria are well standardized, the cytologic diagnostic criteria are not yet standardized. This prompted us to study the cytomorphological features of PDTC and assess features of distinction from differentiated thyroid carcinoma (DTC) and medullary thyroid carcinoma (MTC).

This was a retrospective study of thyroid FNAC smears from 44 PDTC cases retrieved from the database of a single tertiary cancer institute (2009-2013). Papanicolaou and Giemsa smears were evaluated for 21 cytomorphologic features. Immunocytochemistry was available for 6 cases only.

The frequencies of cytomorphologic features in the 44 cases were as follows: hypercellularity, 84.1%; insular pattern, 79.5%; small cell size, 93.2%; high nuclear-cytoplasmic ratio, 93.2%; granular chromatin pattern, 95.45%; nuclear overlapping, 88.64%; mild pleomorphism, 86.36%; grooves/inclusions, 22.7%; binucleation/multinucleation, 9.1%; abrupt nucleomegaly, 34.1%; apoptosis, 45%; mitosis, 25%; necrosis, 34.1%; and colloid, 22.7%.

A high index of suspicion is necessary for an upfront diagnosis of PDTC on FNAC. Although PDTC, DTC, and MTC have overlapping features, there are distinguishing features also. The cytologic diagnostic criteria for PDTC need to be standardized by collaborative efforts among tertiary cancer centers. A prompt diagnosis is the key feature for planning multimodality treatment.