An algorithmic approach to overcome diagnostic challenges in FNAC in large-cell poorly differentiated thyroid carcinomas

Abstract

Poorly differentiated thyroid carcinoma (PDTC) is a distinct entity and a rare carcinoma of thyroid follicular origin with an intermediate prognosis. It is defined by the Turin criteria set in 2007. Although this entity is well known, with widely available literature on histological features, specific studies describing the cytological features of PDTC (especially the large cell type) are lacking. In this study, wedescribe the cytological and clinical features of PDTC showing large cells (PDTC-LC) with abundant cytoplasm. Twelve cases of PDTC showing abundant cytoplasm between 2007 and 2016 were retrieved from the departmental archives and studied. The cases occurred predominantly in women with a mean age of 54.3 years. The mean tumor size was 4.3 cm. Fine-needle aspiration cytology (FNAC) smears showed singly scattered large cells with abundance of cytoplasm admixed with microfollicular and insular pattern. Lymph node metastasis was noted in 7 cases and distant metastasis to bone and visceral organs were also seen in 7 cases. Microfollicular pattern may lead to these cases being misinterpreted as a differentiated follicular neoplasm on FNAC, and the dissociated large cells may mimic Hürthle cell neoplasm. Immunocytochemistry is not helpful in this scenario, although it does resolve the diagnostic dilemma when the differential diagnoses include medullary thyroid carcinoma and metastatic tumors. It is important to identify these tumors on FNAC as this facilitates proper management.