Case 9 -

Extraskeletal Ewing's sarcoma / Peripheral Primitive Neuroectodermal Tumor (PNET) Ricardo V. Lloyd and Arie Perry

Clinical History This 11-year old girl presented with a large pelvic mass adjacent to the urinary bladder. Multiple lung nodules were seen on imaging studies. Gross Description Two incisional soft tissue biopsy specimens, 2-2.5 cm in greatest dimension each, were obtained for histology and cytogenetics. Case 9 - Figure 1 PAS Case 9 - Figure 2 PASD Case 9 - Figure 3 CD99 Case 9 - Figure 4 FLI1-11q24 (green); EWS-22q12 (pink) Microscopic Description Sections revealed a highly cellular proliferation of small, undifferentiated cells arranged in large sheets and nests, with intervening zones of necrosis and hemorrhage. The tumor was relatively deficient in stroma and had no discernable rosettes. Tumor cells had minimal quantities of clear to amphophilic cytoplasm with round to oval nuclei containing smoky chromatin and small nucleoli. Mitotic and pyknotic figures were plentiful. There was abundant intracytoplasmic, diastase-digestible PAS-positive material, consistent with glycogen deposition. Immunostaining revealed diffuse vimentin and O13 (CD99) expression. No staining was found for cytokeratin, EMA, desmin, inhibin, or LCA. The tumor karyotype was 46,XX,t(11;22)(q24;q12) and FISH revealed fusion signals in 84% of cells using probes to EWS (22q12) and FLI1 (11q24). RT-PCR similarly found evidence for an EWS-FLI1 type 1 (EWS exon 7 to FLI1 exon 6) chimeric transcript.