Answer: Papillary carcinoma and adjacent hyalinizing trabecular adenoma

Histology: Two adjacent tumors are apparent in this low power field. One exhibits complex papillary architecture and the papillary fronds are lined by cells with classic nuclear atypia of papillary carcinoma. The other tumor exhibits trabecular architecture with prominent intra- and extra-cellular hyaline deposition. The cells forming the nests also exhibit some atypical features including nuclear elongation, intranuclear grooves, nuclear pseudo-inclusions and tumoral calcification. This tumor nodule is well demarcated and lacks an infiltrative component.

Discussion: Hyalinizing trabecular adenoma (HTA) is a proliferation of follicular epithelial cells that is set apart from other benign and malignant thyroid tumors by it prominent trabecular growth pattern and its intra- and extra-cellular hyaline deposition. It is sometimes included in the differential diagnosis of other neoplasms that take on a nested/trabecular pattern of growth including medullary carcinoma, paraganglioma, trabecular follicular adenoma, and trabecular variant of papillary carcinoma. Distinction from trabecular variant of papillary carcinoma is particularly treacherous as they share similar nuclear features including elongated nuclei, intranuclear grooves, nuclear pseudo-inclusions and tumoral calcification (stromal calcifications rather than true psammomatous calcification present within the tips of papillae). Unlike true papillary carcinomas, HTAs are circumscribed lesions that do not exhibit invasive growth.

Since its original description, HTA of the thyroid has been a controversial entity. Some have considered it a unique entity, some have considered it a nonspecific pattern that may be seen with a variety of thyroid lesions including follicular adenomas, and still others have regarded it as a variant of papillary carcinoma (PC).

This thyroid pathologist is not yet ready to accept the latter view that HTA represent a low grade papillary carcinoma. The supportive evidence seems flawed. First, those studies that have allegedly demonstrated metastases have shown images that do not fit the description of HTA as originally portrayed. I am unaware of any documented case of a metastasis showing classic features of HTA. Second, those molecular studies showing changes of PC have used methods that are not specific. Third, the argument that HTAs often occur in the setting of multifocal papillary carcinoma does not take into account the fact that well differentiated carcinomas are very common and are often incidentally noted in thyroids removed for benign nodules. In the absence of infiltrative growth, the pathologist who encounters a HTA exhibiting classic morphologic features should be very comfortable in diagnosing these tumors as benign.