Answer: Follicular variant of papillary carcinoma, well-differentiated

Histology: The tumor is thinly encapsulated throughout. It exhibits a predominant follicular pattern of growth. The follicles contain bright pink colloid and are lined by atypical cells with enlarged, optically clear and overlapping nuclei. This follicular-patterned component transitions to a nested or “insular” growth component. These nests are surrounded by thin delicate vascular channels. The tumor cells within these nests have more uniform cellular features and exhibit an elevated mitotic rate. Zones of cellular necrosis are present with this insular component.

Discussion: Insular carcinoma is probably not a distinct thyroid cancer, but rather represents a poorly differentiated component within a well differentiated carcinoma. In this case, the well differentiated thyroid carcinoma is a follicular variant of papillary carcinoma. Paradoxically, the cells that form the tumor nests often take on a more uniform appearance and lose the hallmark features of papillary carcinoma.

An insular growth pattern by itself does not warrant the diagnosis of “insular” or “poorly differentiated”. The diagnosis also requires the presence of an elevated mitotic rate and tumor necrosis. In the absence of increased mitoses and tumor necrosis, the presence of nested growth is of no prognostic relevance. The presence of a true insular component should be noted in the surgical pathology report as this component is associated with a more aggressive clinical behavior.