Adamantinoma-like Ewing sarcoma of the thyroid.

Adamantinoma-like Ewing sarcoma (ALES) is a rare variant of Ewing sarcoma family of tumors (ESFTs). ALES is characterized by translocations of the EWSR1 (Ewing sarcoma breakpoint region 1) gene on chromosome 22, morphologic features of small round blue cell tumors with focal squamous differentiation, and a unique immunoprofile. Herein, we report a 21-year-old patient who presented with severe, acute onset back pain. Imaging revealed a large, exophytic, heterogeneously enhancing mass in the left thyroid and numerous lytic bone lesions. Fine-needle aspiration of the thyroid, revealed a cellular smear with "small round blue cell" morphology. The unique immunoprofile of positive ESFT markers (NKX2.2 and CD99), along with positive markers of squamous epithelial differentiation (AE1/AE3 and p40), led to a diagnosis of ALES. This was confirmed by fluorescence in situ hybridization, which demonstrated EWSR1 rearrangement in 74% of nuclei.