Answer: Papillary oncocytic adenoma

Histology: The right lobe of the thyroid harbors a solitary and discrete 1 cm encapsulated nodule. The encapsulated tumor demonstrates a prominent papillary growth pattern. The cells lining the complex papillary structures have abundant pink granular cytoplasm and enlarged nuclei with prominent nucleoli. The surrounding capsule is intact. There is no evidence of transcapsular invasion or vascular invasion.

Discussion: Rare oncocytic tumors of the thyroid exhibit true papillary configurations. These papillary configurations are no different from those encountered in conventional papillary carcinoma. Conversely, there is a well-recognized variant of papillary carcinoma that exhibits prominent oxyphilic cytoplasm. Most now agree that the distinction between a papillary oxyphilic neoplasm and oxyphilic papillary carcinoma is made at the nuclear level. In the absence of well developed nuclear features of papillary carcinoma (e.g. elongation, optic clearing, contour irregularities, etc…), such lesions are considered as oxyphilic neoplasms (i.e. Hurthle cell adenoma or Hurthle cell carcinoma) and not papillary carcinomas. In other words, nuclear features outweigh architectural patterns in the diagnosis of neoplasms exhibiting oxyphilic cytoplasm. In the present case, the diagnosis of a papillary oxyphylic adenoma was established on the basis of Hurthle cell nuclei and the absence of tumor invasion.