Ewing-like sarcoma/undifferentiated round cell sarcoma in an infant with APC and MSH6 variation

Rationale: Ewing-like sarcoma (ELS)/undifferentiated round cell sarcoma (URCS) is a rare type of soft tissue sarcomas (STS), especially in infants, with poor prognosis. It is a so-called "small round cell" sarcoma, and has many features of Ewing sarcoma, but lacks rearrangements in EWSR1. The diagnosis and treatment of this kind of STS remains challenging. BCOR genetic abnormalities have been found in some Ewing-like sarcomas. Patient concerns: This report presents an ELS case of a female infant, who was 2 months old when initially diagnosed, with the clinical stage of IIIA (G(2)T(2)N(0)M(0)). Histologic findings revealed an undifferentiated neoplasm composed of small round tumor cells with round, open chromatic nuclei, and scant cytoplasm in a sheet growth pattern. Fluorescence in situ hybridization (FISH) analysis showed absence of EWSR1 and ETV6 gene rearrangement. Molecular genetic testing found no established variants of clinical significance but variants of unknown significance in APC, KMT2D, and MSH6 were detected. Immunostaining revealed that the tumor cells were positive for TLE1 and BCOR, and negative for cytokeratin (AE1/AE3), Desmin, CD45, S100, CD31, HMB45, and SATB2. INI-1 was retained. Diagnosis: Ewing-like sarcoma (ELS)/undifferentiated round cell sarcoma (URCS) Interventions: After initial diagnosis, the patient received 4 cycles of combination chemotherapy for 2 months. Radical amputation of left upper extremity was performed 3 months after diagnosis. Postoperative chemotherapy was continued for 6 cycles. Outcomes: The patient died of intracranial metastasis with hemorrhage in 13 months after initial diagnosis, 5 months after the last cycle of chemotherapy. Lessons: ELS in infancy is extremely rare and has a poorer prognosis than Ewing sarcoma or infantile fibrosarcoma. APC and MSH6 variation might be related with the disease progression and predict a poorer prognosis. This rare case promotes better understanding of the disease and suggests a promising role for the combination chemotherapy regimen in treating infantile ELS. Importantly, it brings to light the possibility of intracranial metastasis, which requires proactive screening for timely detection.