Poorly differentiated oncocytic (hürthle cell) follicular carcinoma: an institutional experience

The proposed diagnostic criteria for poorly differentiated thyroid carcinoma (known as the Turin classification) were defined based on growth pattern (solid, trabecular, or insular) and high-grade morphologic features (nuclear pleomorphism, mitoses including abnormal forms, and coagulative tumor necrosis). The development of this classification specifically did not include tumors with oncocytic or Hürthle cell cytology, and only sparse literature describing poorly differentiated oncocytic carcinomas is available. In this study, we examined a cohort of 284 cases of oncocytic follicular carcinoma/Hürthle cell carcinoma (OFC/HCC) and identified 17 cases of oncocytic variant of poorly differentiated carcinoma (OV-PDTC) based on Turin criteria. Compared to minimally invasive and angioinvasive OFC/HCC, these tumors arose in older patients (range 44-88 years; average 71 years), were larger (average size 4.5 cm), and all had extensive vascular invasion (5-15 foci), and coagulative tumor necrosis and the tumor cells were arranged in a trabecular or solid growth pattern. All showed an admixture of oncocytic follicular/Hürthle cells arranged in solid and trabecular growth pattern. Aggregates of small sized cells with minimal eosinophilic cytoplasm, comprising 10-20% of the entire tumor mass were also seen in 16/17 cases. Clinical follow-up was available in 12 cases and ranged from 6 to 120 months (average 41 months). Distant metastases were seen in 10/12 (83%) patients; two had lung and one had bone metastases at the time of thyroid surgery, and four subsequently developed cervical lymph node metastases. Two patients died of disease, and ten are alive either with or free of tumor. The OV-PDTC is a distinct entity which can be identified based on Turin criteria and the presence of a distinct "small cell" component. It is frequently associated with regional recurrence and distant metastases and can lead to tumor-related demise.