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FNA Quiz 30 A 20 year-old woman with a childhood history of cranial irradiation for a posterior fossa tumor presented with a 5.0 cm soft right-sided thyroid swelling and fine needle aspiration biopsy was performed and illustrated below. Based on the microscopic findings, the BEST cytologic diagnosis is:

A.  Benign thyroid nodule.

B.  Papillary thyroid carcinoma.

C.  Hyalinizing trabecular adenoma.

D.  Poorly differentiated (insular) thyroid carcinoma.

E.  Follicular lesion, favor follicular neoplasm.

Discussion: The Diff-Quik- (Panels A, B) and Pap- (Panels C, D) stained smears are hypercellular and show both flat and microacinar groups of follicular cells with moderate amounts of cytoplasm and central round nuclei with fine chromatin and small nucleoli. No colloid is visible in the background, and cytomorphologic features of papillary thyroid carcinoma (psammoma bodies, intranuclear inclusions, nuclear grooves, etc) are not present. The cell block section (Panel E) shows a lesion composed of tightly packed microfollicles and the overall features most favor a follicular thyroid neoplasm. The cells of hyalinizing trabecular adenomas may resemble those of papillary or medullary thyroid carcinoma but the smears characteristically contain metachromatic stromal material in the background. While aspirates of poorly differentiated (insular) thyroid carcinomas are also hypercellular, they usually contain numerous single cells and show greater nuclear atypia and/or evidence of necrosis. Surgical resection in this case revealed a well-differentiated follicular carcinoma.