Answer: Poorly differentiated carcinoma

Histology: On this representative slide of the thyroid mass, one can appreciate that the mass is solid with two histologic patterns. Part of the lesion is a follicular-pattern lesion comprised of small or medium sized follicles containing bright pink (bubblegum-like) colloid material. The follicles are lined by cells with cytologic atypia including nuclear clearing, irregular nuclear contours and nuclear grooves which is better appreciated on other slides of the tumor; overall this component meets criteria for follicular variant of papillary thyroid carcinoma. The majority of the lesion on this slide consists of a solid pattern lesion with less cytologic atypia than in the papillary thyroid component; the nuclei are round, hypochromatic, and overlapping. The mitotic rate is increased at up to 5 mitoses per 10 high power fields. There is no necrosis. There is extensive vascular invasion throughout the lesion.

Discussion: This lesion represents a poorly differentiated thyroid carcinoma arising in a follicular variant of papillary thyroid carcinoma (PTC). Poorly differentiated carcinomas of the thyroid ironically have less cytologic atypia than seen in the well differentiated papillary thyroid carcinomas. The Turin criteria to diagnose a component of poorly differentiated carcinoma are:
(1) presence of a solid/trabecular/insular pattern of growth,
(2) absence of the conventional nuclear features of papillary carcinoma, and (3) presence of at least one of the following features: convoluted nuclei, mitotic activity >or=3 x 10 HPF, and tumor necrosis.

By immunohistochemistry, poorly differentiated thyroid carcinomas retain positivity for TTF1 and thyroglobulin, whereas in contrast, the anaplastic carcinomas lose TTF1 and thyroglobulin labeling (but retain Pax8 and are p53 positive). Patients with poorly differentiated carcinoma have a worse prognosis than those with well differentiated carcinomas, but the prognosis is better than for those with anaplastic carcinoma.

Reference(s):
– Volante M1, Collini P, Nikiforov YE, Sakamoto A, Kakudo K, Katoh R, Lloyd RV, LiVolsi VA, Papotti M, Sobrinho-Simoes M, Bussolati G, Rosai J. Poorly differentiated thyroid carcinoma: the Turin proposal for the use of uniform diagnostic criteria and an algorithmic diagnostic approach. Am J Surg Pathol. 2007 Aug;31(8):1256-64.