Answer: Hurthle cell carcinoma

Histology: The tumor is multinodular, and shows extensive vascular invasion at the tumor’s edge. The tumor is composed of cells with finely granular, dense eosinophilic cytoplasm, and the nuclei are vesicular with prominent nucleoli. The tumor has a somewhat papillary architecture, but importantly, the nuclei are at the apical surface of the cells, so these are pseudopapillae. Compression of these papillae leads to long linear arrays of slit-like spaces within long angulated glands. This architecture creates a mimic of Papillary Carcinoma of the thyroid, but the cytology is distinctive.

Discussion: Renal Cell Carcinoma and Medullary Carcinoma of the thyroid can be excluded by immunohistochemistry; namely, Medullary Carcinoma would label for calcitonin, whereas Renal Cell Carcinoma would label with the RCC marker. Papillary Carcinoma is separated from Hurthle Cell Carcinoma mainly on cytologic grounds. Papillary Carcinoma would have ovoid nuclei which overlap, show clearing, nuclear pseudoinclusions, and nuclear grooves.