Answer: Hurthle cell adenoma with papillary features

Histology: The lesion is seen grossly as a 3 cm solitary discrete rounded mass within the thyroid parenchyma. Microscopically, the tumor is thinly encapsulated throughout without evidence of invasive tumor growth. Much of the nodule exhibits a complex papillary architecture. Within these areas, fibrovascular cores support the proliferation of cells with abundant pink granular cytoplasm, centrally placed vesicular nuclei, and prominent central nucleoli. Although the nuclei are enlarged and show partial chromatin clearing, they are evenly spaced without crowding and overlapping.

Discussion: Not every lesion of the thyroid that demonstrates papillary architecture is a papillary carcinoma. Florid papillary formations may be encountered in a variety of neoplastic and non-neoplastic conditions including Grave’s disease, multinodular hyperplasia, and hurthle cell neoplasms. The diagnosis of a hurthle cell neoplasm exhibiting florid papillary architecture is complicated by the fact that some papillary carcinomas exhibit prominent oxyphilic features. In other words, the pathologist must be able to make the distinction between a hurthle cell variant of papillary carcinoma and a papillary variant of a hurthle cell neoplasm. When dealing with this differential diagnosis, the nuclear features trump architectural features. In other words, tumor classification is based on the nuclear characteristics of the tumor. In the present case, the nuclei are evenly spaced without crowding and overlapping; and they are uniformly round to oval with only partial chromatin clearing. These features are quite characteristic of hurthle cell lesions and uncharacteristic of papillary carcinoma.

As with other hurthle cell neoplasms, malignancy is based on the presence of invasive tumor growth. In the absence of invasive tumor growth through its surrounding capsule and into intracapsular blood vessels, this tumor was classified as a hurthle cell adenoma with papillary features.