Hyalinizing trabecular adenoma of the thyroid. A report of two cases with cytologic, histologic and immunohistochemical findings.

BACKGROUND: Hyalinizing trabecular adenoma (HTA) is a rare benign thyroid tumor that is easily confused with medullary carcinoma on surgical specimens and with papillary carcinoma on cytologic specimens. CASES: The fine needle aspiration biopsies (FNABs) and surgically resected specimens from two patients with HTA were studied. Nuclear grooves and nuclear pseudoinclusions were identified in both FNABs. Gross examination of the surgical specimens revealed two nodules (0.8 and 0.5 cm in diameter) in one case and a 6-cm nodule in the other. A thin, fibrous capsule surrounded each nodule. A predominant trabecular pattern was observed in the three lesions. Antibodies to thyroglobulin, carcinoembryonic antigen (CEA), epithelial membrane antigen (EMA), vimentin, chromogranin, synaptophysin, neuron-specific enolase, proliferating cell nuclear antigen (PCNA), Ki-67 and p53 were used to stain 10% buffered, formalin-fixed, paraffin-embedded sections of the surgical specimens. No immunostaining was observed with antibodies against calcitonin, synaptophysin, chromogranin, EMA, vimentin or p53. Less than 5% of cells were CEA positive in one case. The Ki-67 index was low and PCNA expression high.
CONCLUSION: HTAs occur as solitary or multiple nodules. FNABs of HTAs contain cells with nuclear grooves and nuclear pseudoinclusions but lack psammoma bodies, high cellularity and papillary structures. Immunohistochemistry using anticalcitonin and antithyroglobulin antibodies is helpful in distinguishing these tumors from medullary carcinoma. The low Ki-67 index and absence of p53 immunostaining are consistent with the benign behavior of this tumor. The significance of high PCNA immunostaining is uncertain.