Fine‐needle aspiration cytology of the solid variant of papillary thyroid carcinoma: A study of 13 cases with clinical, histologic, and ultrasound correlations

BACKGROUND The solid variant of papillary thyroid carcinoma (SVPTC) comprises approximately 3% of thyroid cancers, and there are conflicting reports about its behavior in the literature. The cytology of SVPTC is limited to 3 single case reports, a review article, and a monograph. We present the first cytologic study of SVPTC. METHODS Fine‐needle aspiration smears obtained with ultrasound guidance from 13 patients with histologically pure SVPTC were reviewed, and the cytologic features recorded. Ultrasound images were retrieved from radiology and were correlated with low‐power histology images. Intratumor vascularity on Doppler imaging was correlated with cellularity in cytology samples. RESULTS Three cytomorphologic patterns of SVPTC were identified: cohesive, syncytial‐type tissue fragments; microfollicles/trabeculae; and dyshesive single cells. All 3 SVPTCs in the first group were encapsulated without invasion. Two of 6 SVPTCs in the second group had a single lymph node metastasis; 4 were encapsulated, and 2 had pushing borders. Ultrasound images in the first and second SVPTC groups were similar, with the majority revealing a well defined, solid nodule with minimal intranodular vascularity. All 4 SVPTCs in the third group had infiltrative borders; and, with the exception of one 0.8‐cm tumor, all had multiple lymph node metastases. Ultrasound in the third group revealed irregular borders. RET/PTC1 and RET/PTC3 mutations were found in 2 cases of the third group. CONCLUSIONS SVPTCs are heterogeneous tumors. The cohesive, syncytial tissue‐fragment pattern can be recognized as SVPTC in smears and is associated with encapsulation and indolent behavior. The microfollicular/trabecular pattern is indistinguishable from that of the follicular variant of papillary thyroid carcinoma and has intermediate behavior. The dyshesive single‐cell pattern correlates with infiltrative tumor growth and may not be unique to SVPTC. Cancer (Cancer Cytopathol) 2015;123:71–81. © 2015 American Cancer Society. The solid variant of papillary thyroid carcinoma comprises a heterogeneous group of tumors. The tumors that have cytomorphologic patterns of cohesive, syncytial‐type tissue fragments or microfollicules/trabeculae are circumscribed and indolent, whereas tumors that have dyshesive single cells are infiltrative and potentially aggressive.