Carcinoma of the Thyroid With Ewing/PNET Family Tumor Elements

To the Editor: A case of a nonneuroendocrine small cell tumor of the thyroid displaying carcinomatous features, CD99 expression, and EWSR1-FLI1 rearrangement was reported in the International Journal of Surgical Pathology. Since the present tumor appeared almost identical to the small cell tumor of the thyroid Cruz et al reported 2 years ago under the designation of small cell (basaloid) thyroid carcinoma, we decided to search retrospectively for CD99 immunoexpression and for the EWSR1-FLI1 rearrangement. Both features were also detected in the neoplastic cells of the case reported by Cruz et al (Figure 1). We concluded one is seeing a special type of nonneuroendocrine small cell thyroid carcinoma with primary extraeskeletal Ewing family tumor (PEEFT) features and a good prognosis that we proposed to designate as carcinoma of the thyroid with Ewing family tumor elements (CEFTE). These tumors raise a number of interesting questions. First, should they be considered as an example of tumors in the carcinomatous end of the spectrum of PEEFT or as true small cell nonneuroendocrine carcinoma exhibiting the EWSR1-FLI1 rearrangement? Second, what is their most likely histogenesis? If we stick to their PEEFT nature, one may propose that they probably originate, like any other PEEFT, from mesenchymal stem cells. If one follows the alternative possibility, that is, that of a small cell nonneuroendocrine carcinoma, the situation is complex because at least 2 hypotheses can be considered: