Fig. 1Grossly, the resected tumor, measuring 12.0×8.0×7.5 cm, is located in the mesentery of the jejunum. The cut surface of tumor demonstrates a whitish-pink solid appearance with foci of hemorrhage, extensive necrosis, and myxoid features. The border is irregular, and the tumor directly invades the wall of jejunum with focal ulceration.
Fig. 2(A) The tumor focally invades the mucosa of jejunum, resulting in mucosal erosion. (B) The tumor is composed of sheets or lobules of small round cells containing uniform vesicular, round, or oval nuclei; scanty clear or eosinophilic cytoplasms; and indistinct cytoplasmic borders. Well-formed Homer-Wright rosettes are frequently observed.
Fig. 3(A) Areas of osseous matrix with calcification and partly well-formed bony trabeculae are observed. (B) The periphery of osteoid and bone formation shows frankly malignant tumor cells, but the central portion of the bone reveals lacuna containing, benign-looking nuclei.
Fig. 4Most tumor cells are positive for CD99 (MIC2) (labeled streptavidin biotin).
Fig. 5Ewing sarcoma breakpoint region 1 (EWSR1) fluorescence in situ hybridization (FISH) on interphase cells showing split-apart signals. Interphase nuclei with fused orange and green hybridization signals are interpreted as indicative of an intact (not rearranged) copy of the EWSR1 gene. A split signal pattern (one green and one orange) seen on interphase nuclei is interpreted as indicative of a EWSR1 gene rearrangement. This case has evidence of EWSR1 rearrangement by FISH.
Fig. 6Recurrent tumor showing same morphology of tumor cells with primary tumor and more mature metaplatic bone.