Answer: Hyalinizing trabecular neoplasm

Histology: The lesion is non-encapsulated but sharply circumscribed. Its pattern of growth is nested and trabecular. The centers of the nests are acellular and hyalinized with occasional calcification. Many of the tumor nuclei are elongated, grooved, and contain prominent eosinophilic inclusions.

Discussion: Hyalinizing trabecular neoplasm, as the name indicates, is a thyroid neoplasm characterized by a trabecular pattern of growth and prominent extracellular and intracellular deposition of collagen. There is considerable confusion surrounding the malignant potential of this neoplasm. Once considered a peculiar variant of a follicular adenoma, some now believe that the HTN is more closely related to a low-grade papillary carcinoma. This shifting perspective is based on the following observations: 1) A number of cases have been reported of conventional papillary carcinoma arising from a HTN. 2) Papillary carcinoma and HTN share several key morphologic characteristics including the presence of psammoma bodies, and nuclei that are elongated, grooved, and optically clear with inclusions. And 3), at the genetic level papillary thyroid cancer and HTN also share a common alteration (i.e. activating RET/PTC gene rearrangements).

Because of the tremendous morphologic overlap, HTN may be confused with papillary carcinoma, particularly on fine needle aspirates. HTN lacks the papillary architecture of conventional papillary carcinoma, and its nuclei are less crowded and non-overlapping. The nested pattern of growth may cause confusion with paraganglioma and medullary carcinoma. Confusion with medullary carcinoma is further exacerbated by the amyloid-like appearance of the intercellular hyaline. Immunohistochemical studies and special stains are helpful in establishing the diagnosis. In general, the cells of HTN are thyroglobulin positive and calcitonin negative, and the hyaline material does not stain with Congo Red.