Fig. 2

Macro- and microscopic findings of the tumor, and FISH assay of tumor cells. a The tumor cross-section is grayish-white in color with internal necrosis. b Microscopic examination with hematoxylin and eosin staining reveals uniform small round cells with round nuclei, intense chromatin staining, and a high nucleus-to-cytoplasm ratio proliferating solidly and partially showing a rosette-like structure. Immunohistochemical examination with anti-CD99 (c) and FLI-1 (d) antibodies shows strongly positive results. e A typical result of FISH assay to identify the chromosomal break point of EWSR1 in paraffin-embedded tissue reveals break-apart signals, shown as separate red and green signals (white arrowheads)