Ewing sarcoma (ES) is a round cell sarcoma that, by definition, harbors a gene fusion between a member of the FET gene family (i.e. EWSR1 or FUS) and a member of the ETS transcription factor family (FLI1, ERG, ETV1, ETV4, and FEV).

ES is the second most common malignancy of bone in children, with a peak incidence in the 2nd decade of life, and with 80% of tumors occurring in children and adolescents. ¹ Nearly 90% of tumors occur in the bone, while roughly 10% occur in somatic soft tissue and about 2% occur in visceral sites.¹ 

Patients usually present with a painful mass, sometimes with an associated pathologic fracture. ES of the bone occurs in the diaphysis or diaphyseal-metaphyseal regions of long bones, pelvic bones, and ribs, in contrast to osteosarcoma, which occurs in the epiphysis. There are characteristic imaging findings, including the so-called "onion-skin" periosteal reaction in radiographs. About 25% of patients present with metastatic disease.

ES is typically composed of sheets of monomorphic small round cells with scant cytoplasm and primitive-appearing nuclei with evenly dispersed chromatin. Some examples exhibit cytoplasmic clearing due to the presence of glycogen, and some contain pseudorosettes; tumors with the latter feature were once designated "peripheral neuroectodermal tumor", terminology that is no longer used. Rarely, ES exhibits larger cells with more copious cytoplasm.

ES shows strong diffuse membranous expression of CD99 in 95% of the cases (Fig.1). ² FLI1 immunohistochemistry is also highly sensitive, but it has limited diagnostic value due to non-specificity. More recently, NKX2.2 was identified through gene expression studies as a marker that is approximately 95% sensitive and 90% specific for Ewing sarcoma (Fig.1). ³ While patchy CD99 expression is seen in CIC rearranged sarcoma and sarcoma with BCOR alterations, these tumor types usually do not exhibit the strong diffuse expression characteristic of ES, and they are nearly always negative for NKX2.2. ⁴

Figure 1. Immunohistochemical findings in Ewing sarcoma. Ewing sarcoma characteristically demonstrates strong and diffuse membranous expression of CD99, as well as diffuse nuclear expression of NKX2.2.

ES is fatal if left untreated; however, with current regimens survival has improved substantially, with a 5-year survival of 70-80%. ^1,5 Patients who present with metastatic disease have a worse prognosis, with a 5-year survival of less than 30%. ⁶ Body site is associated with prognosis and, in particular, primary tumors of the pelvis have a poorer prognosis. ⁷

ES is usually treated with neoadjuvant chemotherapy, followed by surgery and/or radiation. The efficacy of doxorubicin-based chemotherapy regimens was established in the early 1990s;⁸ subsequent studies showed that the addition of ifosfamide and etoposide to doxorubicin-based regimens improved survival in patients with localized ES. ⁹More recently, it was shown that decreasing the interval between chemotherapy cycles improved outcomes in patients with localized disease. ¹⁰ Complete pathologic response to neoadjuvant chemotherapy is a positive prognostic factor, with an overall 5-year survival of roughly 95%. ¹¹

• The most common translocation, present in ~85% of the cases, is t(11;22)(q24;q12), which was first detected in the early 1980s.¹² In the early 1990s, this translocation was shown to be associated with EWSR1::FLI1 gene fusion. ¹³ The molecular variants are: Type I, in which EWSR1 exon 7 is fused to FLI1 exon 6 (∼55% of cases); Type II, in which EWSR1 exon 7 is fused to FLI1 exon 5 (∼25% of cases); and rare EWSR1::FLI1 molecular variants, each accounting for <3% of cases. ¹⁴ The histologic differences between tumors with different fusion variants are minimal, and the clinical differences are negligible. Therefore, information about the fusion type is not routinely used in clinical practice.^15,16

• EWSR1::ERG gene fusion is the second most frequent variant. It is not always detectable by using break-apart EWSR1 probes¹⁷, because typically it is not a balanced t(21;22)(q22;q12), but instead it is associated with an unbalanced, chromoplexy pattern, likely due to the opposite directions of EWSR1 and ERG transcription.  ES patients harboring gene fusions through chromoplexy were more likely to relapse and had a high incidence of TP53 mutations. ¹⁸ In a case of suspected ES with negative EWSR1 FISH, additional testing such as ERG FISH or RT-PCR/next generation sequencing could be considered.

• Alternative rare ETS fusion partners (<10%) i.e. ETV1, ETV4, and FEV, function as molecular equivalents of FLI1, with no apparent difference in morphologic or immunophenotypic features. EWSR1 and FUS are members of the FET family of RNA binding proteins, have similar functions, and are interchangeable in rare ES translocations, ¹⁹ However, some potential differences in clinical features and outcomes among gene fusion types in ES subsets are emerging. For example, ES with EWSR1::FEV or FUS::FEV gene fusions have a higher propensity for extra skeletal locations, older age at diagnosis, and worse clinical outcomes compared to EWSR1::FLI1 and EWSR1::ERG fusions. ²⁰ Reported tumors with EWSR1::ETV1 or EWSR1::ETV4 gene fusions have occurred in very young children (aged <2 years). ²¹

• Recurrent chromosomal aberrations include gains of chromosomes 1q, 2, 8, and 20. Chromosome 1q gain is frequently associated with chromosome 16q loss as a result of an unbalanced translocation t(1;16) and is associated with poor clinical outcomes. The most frequent deletions involve CDKN2A on chromosome 9p and are associated with poor prognosis. ¹ CDKN2A alterations are present in 10–22% of ES, and additional alterations found in a minor proportion of ES include mutations in STAG2 (15–21%) and TP53 (5–7%). ^1,22ES of the bone and extraskeletal ES show similar frequencies of EWSR1 translocations, TP53 mutations, and CDKN2A mutations or deletions. ²³