Cytopathology of Solid Variant of Papillary Thyroid Carcinoma: Differential Diagnoses with other Thyroid Tumors

Objectives: Solid variant of papillary thyroid carcinoma (SVPTC) is rare, differing from classical PTC (cPTC) in architecture and outcome. We evaluated the cytomorphology of SVPTC cases to assess the feasibility of a preoperative diagnosis. Study Design: SVPTC cases were evaluated for architecture, nuclear features, and Bethesda category and were compared with noninvasive follicular thyroid neoplasm with papillary-like nuclear features/follicular variant of PTC (NIFTP/FVPTC), cPTC, and poorly differentiated thyroid carcinoma (PDTC). Results: Nine SVPTCs, 29 NIFTP/FVPTCs, 12 cPTCs, and 4 PDTCs were included. The predominant architecture in most SVPTCs was solid fragment, which is helpful in differentiating them from NIFTP/FVPTC (p < 0.001) and cPTC (p = 0.006) but not from PDTC. The presence of microfollicles led to misinterpretation as NIFTP/FVPTC/follicular neoplasm in 4 patients. All but 1 SVPTC showed diffuse nuclear features. Intranuclear pseudoinclusions (INIs) were seen in 67% of SVPTCs as compared to 83% of cPTCs, 14% of NIFTP/FVPTCs (p = 0.005), and none of PDTCs. SVPTC cases were commonly (78%) categorized as intermediate/suspicious. Conclusions: The presence of solid fragments and lack of true papillae are helpful in differentiating SVPTC from cPTC. Solid fragments, trabeculae, the extent of nuclear features, and INIs should be looked for in cases with prominent microfollicles for distinguishing SVPTC from NIFTP/FVPTC. None of the features were helpful in differentiating SVPTC from PDTC.