Hyalinizing trabecular adenoma of the thyroid gland.

The hyalinizing trabecular adenoma, a distinctive lesion, is important because it may mimic medullary and papillary carcinoma microscopically. We describe 11 such tumors obtained from women ages 27-72 years (mean, 46 years). The initial pathologic diagnoses were carcinoma (five cases), adenoma (two cases), paraganglioma (one case), and "indeterminate" (three cases). None of the tumors recurred or metastasized (mean follow-up, 10 years). Grossly, the neoplasms measured 0.3-4 cm in diameter, and were yellow-tan and circumscribed. Microscopically, they were encapsulated or circumscribed and solid, or vaguely lobulated. The polygonal, oval, and elongated tumor cells were arranged in trabeculae, clusters, or both, and were often inserted vertically into capillaries. The sharply outlined cells had finely granular cytoplasm that was either acidophilic, amphophilic, or clear. Typical features included oval and elongated nuclei, perinucleolar vacuoles, acidophilic nuclear inclusions, fine nuclear grooving, and infrequent mitotic figures. Perivascular hyaline fibrosis and cell degeneration mimicked amyloid, but these tumors were Congo red-negative. Occasional trabeculae featured round or irregularly shaped follicles, sometimes with papillary infoldings, that were either empty or contained colloid-like material and psammoma bodies. Immunostaining of tumor cells was positive for thyroglobulin and negative for calcitonin. Among six tumors analyzed by flow cytometry, five displayed a diploid pattern and one showed an aneuploid peak.