Answer: Papillary carcinoma, oncocytic variant

Histology: The tumor consists of a proliferation of eosinophilic cells with abundant granular cytoplasm and papillary architecture. Although many of the tumor cells demonstrate coarse chromatin with central nucleoli, consistent with an oncocytic neoplasm, they also show nuclear features suggestive of papillary carcinoma, including marked membrane irregularity, prominent nuclear grooves, and patchy chromatin clearing. Taken in sum, these findings are consistent with a diagnosis of oncocytic variant of papillary carcinoma.

Discussion: Oncocytic variant of papillary thyroid carcinoma makes up less than 10% of all papillary carcinomas. They can demonstrate either papillary or follicular architecture. The papillary nuclear features can be relatively subtle in these tumors compared to other subtypes of papillary carcinoma, with only focal nuclear clearing, nuclear grooves, and nuclear pseudoinclusions. However, this atypia should be more marked than what is acceptable for true Hurthle cell neoplasms. The main differential diagnosis for oncocytic variant of papillary carcinoma is the tall-cell variant of papillary carcinoma, which should have more prominent papillary nuclear features and cells that are 3 times as tall as they are wide. It is essential to make this distinction because the oncocytic variant lacks the aggressive behavior of tall cell variant and instead has a risk profile similar to conventional papillary carcinoma.