[Pediatric Ewing sarcoma in the rare sites: a clinicopathological analysis of eight cases]

Objective: To investigate the clinicopathological characteristics of pediatric Ewing sarcoma in the rare sites. Methods: Seven surgical resection specimens and one consultation case diagnosed at Shanghai Children's Medical Center (4 cases), Shanghai Jiaotong University, Shanghai; Zhejiang Children's Hospital (2 cases), Hangzhou and Jiangxi Children's Hospital (2 cases), Nanchang, China from January 2019 to June 2024 were collected. The tissues were subject to histological examination and immunohistochemistry using EnVision system. The fluorescence in situ hybridizations (FISH) for EWSR1::FLI1 gene fusion and EWSR1 gene-breakapart were performed. The paraffin sections were used for next-generation sequencing (NGS). Results: There were 8 pediatric patients (4 boys and 4 girls). Their ages ranged from 7 to 14 years, with a median age of 12.5 (10.0, 13.5) years. The tumors were located in the right submandibular gland (1 case), tail of the pancreas (1 case), prostate (1 case), small intestine (1 case), nasal vestibule (1 case), adrenal gland (1 case), and right kidney (2 cases). Histologically, the tumors showed relatively uniform small round cells with an invasive growth pattern. The adamantinoma-like Ewing sarcoma occurring in the right submandibular gland had obvious characteristics of epithelial differentiation. Immunohistochemistry showed diffuse positivity for CD99, NKX2.2 in most tumors, partial or diffuse positive for epithelial and neuroendocrine markers in some cases, as well as complete negativity for desmin in all tumors. Molecular genetic study showed EWSR1 gene translocation and FUS::FLI1 gene fusion using FISH and NGS. All cases underwent chemotherapy or adjuvant radiotherapy. The follow-up for 10 to 44 months found that two patients were dead, one had recurrence and the others were free of disease. Conclusions: Extraskeletal Ewing sarcoma is rare. Careful histologic evaluation supplemented by immunophenotyping and molecular studies facilitates its diagnosis and differential diagnosis.