A 13-year-old boy presents with a 3-month history of progressive pain and swelling of the left femur. Imaging reveals an aggressive diaphyseal lesion with "onion-skin" periosteal reaction and a large soft tissue mass. Histopathology shows sheets of small round blue cells with scant cytoplasm and round nuclei. CD99 membrane staining is strongly positive. The molecular finding marked **D** in the diagram is identified on FISH analysis. Which of the following best describes the significance of this molecular abnormality in confirming the diagnosis?

Explanation

## Why option 1 is right The t(11;22)(q24;q12) translocation resulting in EWSR1-FLI1 fusion oncogene is the classic and pathognomonic molecular hallmark of Ewing sarcoma. This translocation is present in ~90% of Ewing sarcoma cases and is detected by FISH, RT-PCR, or karyotyping. The presence of this fusion oncogene, combined with the histologic findings of small round blue cells and CD99 positivity, definitively confirms the diagnosis of Ewing sarcoma family of tumors (ESFT), which includes peripheral PNET and Askin tumor. This molecular finding is essential for diagnosis and prognostication (Robbins 10e Ch 26; Nelson 21e). ## Why each distractor is wrong - **Option 2 (TP53 mutation)**: While TP53 mutations occur in some sarcomas and Li-Fraumeni syndrome predisposes to multiple malignancies including sarcomas, TP53 mutation is not the defining molecular abnormality of Ewing sarcoma. The EWSR1-FLI1 translocation is the pathognomonic finding. - **Option 3 (MYCN amplification)**: MYCN amplification is the hallmark of neuroblastoma, not Ewing sarcoma. Although both are small round blue cell tumors in children, neuroblastoma shows chromogranin and NSE positivity, elevated urinary VMA/HVA, and MYCN amplification—not EWSR1-FLI1 translocation. - **Option 4 (PAX3-FOXO1 fusion)**: PAX3-FOXO1 fusion is the defining translocation of alveolar rhabdomyosarcoma, a different small round blue cell tumor. Rhabdomyosarcoma shows myogenic markers (MyoD1, myogenin, desmin positivity), not the EWSR1-FLI1 translocation seen in Ewing sarcoma. **High-Yield:** EWSR1-FLI1 t(11;22) is pathognomonic for Ewing sarcoma; variant EWSR1-ERG exists but both define ESFT. Confirm by FISH/RT-PCR when small round blue cell tumor + CD99+ in diaphyseal long bone or pelvis of child/adolescent. [cite: Robbins 10e Ch 26; Nelson 21e]