The WHO Classification of Soft Tissue and Bone Tumours currently recognizes four categories of undifferentiated small round cell sarcoma: Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions including NFATc2 and PATZ1, CIC-rearranged sarcoma, and sarcoma with BCOR genetic alterations. These neoplasms frequently pose significant diagnostic challenges due to rarity and overlapping morphologic and immunohistochemical findings. Further, molecular testing, with accompanying pitfalls, may be needed to establish a definitive diagnosis. This review summarizes the clinical, histologic, immunohistochemical, and molecular features of these neoplasms. In addition, differential diagnosis and areas of uncertainty and ongoing investigation are discussed.

BACKGROUND: Ewing sarcoma (EWS) is a highly malignant tumor of bone and soft tissue that occasionally arises from viscera. Visceral EWS (V-EWS) is challenging to manage given its varied organ distribution and often late-stage presentation. We aimed to characterize our institutional experience with V-EWS, focusing on its surgical management, and to compare V-EWS outcomes against those with osseous (O-EWS) and soft tissue EWS (ST-EWS).
METHODS: Retrospective review of all EWS patients ≤21 years presenting to a single institution between 2000 and 2022. Patient- and disease-specific characteristics were compared. Overall and relapse-free survival were estimated using Kaplan Meier methods and log-rank test.
RESULTS: 156 EWS patients were identified: 117 O-EWS, 20 ST-EWS, and 19 V-EWS. V-EWS arose in the kidney (n = 5), lung (n = 5), intestine (n = 2), esophagus (n = 1), liver (n = 1), pancreas (n = 1), adrenal gland (n = 1), vagina (n = 1), brain (n = 1), and spinal cord (n = 1). No significant demographic differences were detected between EWS groups. V-EWS was more frequently metastatic at presentation (63.2%; p = 0.005), yet no significant overall or relapse-free survival differences emerged between EWS groups, with similar follow-up intervals. While V-EWS required multiple unique operative strategies to gain primary control, no significant difference in treatment strategies appeared between groups. Surgery-only primary control was associated with improved overall and relapse-free survival in all groups.
CONCLUSIONS: V-EWS presents unique management challenges in children and adolescents given its variable sites of origin. This large cohort is the first to describe the surgical management and outcomes of V-EWS, demonstrating more frequent metastatic presentation, while achieving similar survival across groups.
METHODS: Level 2 - Cohort Study.

BCOR-rearranged sarcomas are rare and belong to the Ewing-like sarcomas (ELS). Their morphology and histopathological features make the diagnosis challenging. We present a case, initially diagnosed as an unusual extraskeletal myxoid chondrosarcoma (EMC). A 54-year-old male patient developed an asymptomatic swelling of the lower leg. Imaging showed a 9.5-cm large intramuscular soft tissue mass. Due to its morphological and immunohistochemical profile on biopsy, it was initially diagnosed as an EMC. The patient was treated by complete resection and adjuvant radiotherapy and remained free of tumor at 7 years follow-up. Using next-generation sequencing (NGS), we retrospectively identified RGAG1-BCOR gene fusion (confirmed by RT-PCR), which has not been described in somatic soft tissue tumors so far. This finding broadens the spectrum of partner genes in the BCOR-rearranged sarcomas in a tumor with a well-documented, long clinical follow-up.

Sarcoma with EWSR1-PATZ1 gene fusion is an exceedingly rare and newly described Ewing-like sarcoma harboring EWSR1 rearrangements involving fusion partners other than ETS family genes. The clinical, histopathologic and immunophenotypic features of cases reported in literature are fairly diverse and not specific. We report a new case report posing real challenges for histological and molecular diagnosis.

Ewing sarcoma family of tumors are mainly aggressive sarcomas of bone and also arising in soft tissues, which share common features: morphological features of basophilic round cell tumors, immunohistochemical features by expression of membrane CD99 protein, and genetic features with a translocation involving EWS and FLI1 in approximately 90% of cases. The discovery of this translocation has made it possible to unify in a single entity several lesions such as PNET, neuropitheliomas, Askin tumors, Ewing sarcomas… Since then, the extensive use of molecular/genetic methods has helped to identify an increasing number of molecular anomalies in unclassified round cell sarcomas, these sarcomas often harboring an atypical morphology and a less frequent CD99 positivity. Besides the rearrangements between the FET family of genes (EWS or FUS) and the wide ETS family of genes (FLI1, ERG, FEV, ETV…), new partner genes are gradually identified: cases with EWS-non ETS partners are extremely rare, but there are more important groups which are CIC-DUX4 and BCOR-CCNB3 translocation-positive sarcomas. These findings raise the problem of the nosological borders of the Ewing/PNET entity and its links with new \"Ewing-like\" groups of tumors, and raise the therapeutic problems. The forward-looking identification of new round cell sarcomas should enable studies of wider series to try to answer these questions.