Spindle cell transformation of papillary carcinoma: an aggressive entity distinct from anaplastic thyroid carcinoma.

Spindle cells are not routinely encountered in the context of thyroid pathology and are most often present in anaplastic thyroid carcinoma, medullary thyroid carcinoma, and benign conditions such as Riedel struma or de Quervain granulomatous thyroiditis. Only a few publications have reported papillary thyroid carcinoma admixed with a prominent spindle cell component. While these tumors are clearly distinct from anaplastic thyroid carcinoma, prognostication as to their oncologic potential is not yet established. We describe a unique case of spindle cell transformation of papillary thyroid carcinoma. The blandness of the spindle cells was so impressive as to dissuade us from a malignant diagnosis on preoperative biopsies. However, this patient unfortunately died shortly after transformation of this papillary thyroid carcinoma. We conclude that this peculiar and rare spindle cell transformation should be regarded as a potentially lethal variant of papillary thyroid carcinoma.