Case Report: CBFA2T3::GLIS2-positive myeloid sarcoma with focal bone marrow involvement mimicking Ewing sarcoma in an...

CBFA2T3::GLIS2-positive myeloid neoplasms occur predominantly in infants and can closely mimic pediatric small round cell sarcomas. We describe a 14-month-old boy who presented with left leg pain, an abnormal gait, and a destructive left ischial mass. Tissue biopsy was initially interpreted as an undifferentiated Ewing-like sarcoma. Bone marrow aspiration contained approximately 2% unclassified large cells, while trephine biopsy demonstrated focal tumor infiltration. Marrow tumor cells showed strong CD56 and weak CD61 expression and were positive for WT1, CD43, ERG, CD117, and BCL2. Lysozyme, MPO, TdT, and CD71 were negative. Diagnostic bone marrow flow cytometry was not performed. Molecular studies on pelvic lesion tissue showed negative results on EWSR1 break-apart fluorescence in situ hybridization (FISH). Broad RNA fusion testing detected CBFA2T3::GLIS2, joining CBFA2T3 exon 11 to GLIS2 exon 3, without a covered Ewing or Ewing-like sarcoma-defining fusion. After one cycle of cyclophosphamide, doxorubicin, and vincristine under the provisional sarcoma diagnosis, treatment was redirected to a 10-day course of cytarabine, etoposide, and daunorubicin. Severe myelosuppression was managed with anti-infective and supportive therapy. Subsequent marrow morphology, trephine biopsy, and flow cytometry showed no evident residual tumor. Follow-up MRI demonstrated marked reduction of the pelvic mass, although residual osseous abnormalities persisted. The patient subsequently continued treatment elsewhere. Available records documented additional targeted therapy and maternal haploidentical hematopoietic stem cell transplantation (HSCT). The first reported post-transplant molecular assessment detected no CBFA2T3::GLIS2 fusion transcript. Detailed post-transfer treatment parameters and long-term outcomes were unavailable. This case highlights the value of integrated tissue and marrow assessment and broad RNA fusion testing in EWSR1-negative infantile small round cell tumors.