Answer: Hyalinizing trabecular adenoma

Histology: The nodule is not encapsulated but it is sharply circumscribed. It demonstrates a highly nested/trabecular pattern of growth. There is prominent intracellular and extracellular deposition of collagen imparting a pink glassy appearance to the trabeculae. Within some of the nests, the stroma has undergone degenerative changes including calicification, and cystic change with pseudopapillary formations. The cells themselves are notable for enlarged and elongated nuclei with longitudinal grooves and prominent inclusions.

Discussion: Hylanizing trabecular adenoma (HTA) is a low grade neoplasm of the thyroid that is celebrated for the way it can closely mimic papillary thyroid cancer (PTC) in several respects. At the morphologic level, the lesion can show psammoma-like calcifications and pseudopapillary formations as is illustrated in the case shown. Most troubling of all is the way the nuclear alterations (e.g. elongation, chromatin clearing, intranuclear inclusions, etc…) mirror those of true PTC. HTA usually is encountered in glands that also harbor multifocal PTC. HTA and PTC also overlap at the genetic level. Several studies have shown that some HTAs share the same genetic alterations that were thought to be specific for PTC. Because HTA and PTC share these common characteristics, there is some uncertainty regarding the true nature of HTA. One view is that HTA represents a particularly low grade form of PTC. To reflect this uncertainty, some prefer the designation hyalinizing trabecular “neoplasm” over hyalinizing trabecular “adenoma”. If there is any confusion with medullary carcinoma based on the nested pattern of growth, immunohistochemistry will resolve the dilemma. HTAs are consistently thyroglobulin positive and calcitonin negative.