Fig. 1.

Adamantinoma-like Ewing Sarcoma arising in the thyroid of a 20 year-old female. Sheets, islands and trabeculae of tumor cells (a, b, c, d, 100×, 400×, 200×, 400×, respectively, hematoxylin and eosin) showing colonization of thyroid follicles (b) and necrosis (d). Tumor cells were set in a fibrous/hyalinized and myxoid stroma (c, d). Cells were uniform, small to medium in size, with round nuclei, fine chromatin and variably prominent nucleoli (b). Immunohistochemical stains revealed strong, membranous expression of CD99 (e, 400×) and diffuse nuclear expression of p40 (f, 200×) and FLI-1 (g, 200×). RT-PCR revealed a EWSR1-FLI-1 fusion. Immunostains for thyroglobulin, TTF1, PAX8, Calcitonin, CK20, SOX10, ERG, SMA, and NUT were all negative (not shown)