Intraperitoneal Small Round Cell Tumor Suggestive of Extraskeletal Ewing Sarcoma in an Eight-Year-Old Girl: Conversion to Resection and Adjuvant Volumetric Modulated Arc Therapy

Intraperitoneal extraskeletal Ewing sarcoma (EES) is rare in young children and raises specific diagnostic and therapeutic challenges.An eight-year-old girl presented with fatigue, changes in bowel habits, and weight loss.Contrast-enhanced computed tomography (CT) revealed a large peritoneal and retroperitoneal mass (12.3 × 11 cm) encasing the major abdominal vessels, abutting the liver, with a separate small hepatic lesion.Core biopsy showed a small round cell tumor with diffuse CD99 positivity and negative staining for CD45, myogenin, desmin, chromogranin A, and synaptophysin, suggestive of EES.Testing for t(11;22) and EWSR1 rearrangement was unavailable and was not performed; the diagnosis was therefore considered probable EES pending molecular confirmation, and desmoplastic small round cell tumor (DSRCT) could not be excluded.After six cycles of VIDE (vincristine, ifosfamide, doxorubicin, and etoposide), CT showed a partial response, and the hepatic lesion had regressed.Because the mass remained unresectable, four cycles of VAI (vincristine, actinomycin D, and ifosfamide) were given before surgery.On preoperative CT, the primary mass measured 4.7 × 4.0 cm in the axial plane, a 62% decrease in its longest axial diameter.The resected specimen measured 6 × 4 × 3.5 cm and showed more than 90% treatment-related necrosis.The closest margin was less than 1 mm, and the resection was classified as microscopically incomplete (R1).Adjuvant volumetric modulated arc therapy was delivered; the planned boost was omitted, and the dose was limited to 45 Gy in 25 fractions to respect the dose constraints of adjacent organs.Treatment was completed with grade 1 acute gastrointestinal toxicity (nausea, diarrhea, and epigastric pain), and the patient had no evidence of recurrence at her last visit after a short follow-up.This case illustrates the diagnostic uncertainty when molecular testing is unavailable and the balance between local control and the risks of abdominal irradiation in a child; longer follow-up is needed to assess treatment efficacy.