目的·分析儿童CIC重排肉瘤（CIC-rearranged sarcoma，CRS）的临床、病理及分子遗传学特点，以提高对2020年第5版WHO软组织和骨肿瘤分类新提出的小圆细胞肉瘤的认识。方法·收集上海交通大学医学院附属儿童医院2019年1月—2021年4月确诊的54例小圆细胞未分化肉瘤患者资料，筛选出有完整的临床、影像、病理及随访资料的儿童CRS 5例，分析其临床病理学特征。结果·5例CRS中，男性3例，女性2例，年龄为5个月~10岁。无症状患儿3例，首发症状为胸背部疼痛1例，喷射性呕吐1例。CT影像学：均表现有大小不等的囊性、实性或混合性不规则软组织团块影。病理学检查：肿瘤平均长径12.5 cm，切面灰白色或灰褐色，外观呈烂鱼肉样。组织病理学：主要为小圆形细胞弥漫片状分布。免疫组织化学：CD99呈弥漫膜阳性或局灶阳性，WT1呈弥漫核阳性，并不同程度地表达BCL2、CD56、FLI1、ERG等免疫标志物。5例荧光原位杂交（fluorescence in situ hybridization，FISH）检测显示EWSR1（22q12）阴性，CIC（19q13）基因重排，同时部分患儿还检测出CIC融合基因突变，包括2例CIC-DUX4融合和1例CIC-NUTM1融合。所有病例均行手术治疗，并术后辅助化疗。5例都获得随访，平均随访21.4个月，其中1例患儿死亡、2例带瘤生存、2例无肿瘤残留。结论·儿童CRS是临床上少见的恶性软组织肿瘤，多表现无症状或症状非特异，易转移且预后极差；对组织学形态表现为尤文肉瘤样改变的软组织肿瘤，结合免疫组织化学并加做EWSR1和CIC重排等相关分子检查有助于诊断，同时CIC融合基因的进一步检测对估计预后可能有提示意义。

Objective ·To analyze the clinical, pathological and molecular genetic characteristics of CIC-rearranged sarcoma (CRS) in children to improve the understanding of small round cell sarcoma newly proposed in the 5th edition of the WHO Classification of Soft Tissue and Bone Tumours. Methods ·Data of 54 patients with small round cell undifferentiated sarcoma diagnosed in Shanghai Children's Hospital, Shanghai Jiao Tong University School of Medicine from January 2019 to April 2021 were collected. Five cases of pediatric CRS with complete clinical, imaging, pathological and follow-up data were selected to analyze their clinicopathological features. Results ·There were 3 males and 2 females in the 5 CRS patients aged from 5 months to 10 years. There were 3 asymptomatic children, and chest and back pain and projectile vomiting were the first symptoms in 2 cases respectively. All CT images showed cystic, solid or mixed irregular soft tissue masses of varying sizes. Histopathological examination showed that the average maximum diameter of the tumor was 12.5 cm, and the cut surface was grayish white or grayish brown, with the appearance of rotten fish flesh. The histopathology was mainly distributed in small round diffuse sheets. Immunohistochemistry showed CD99 diffuse membrane positive or focal positive, WT1 diffuse nuclear positive, and BCL2, CD56, FLI1 and ERG expressed to varying degrees. Fluorescence in situ hybridization (FISH) test showed that EWSR1 (22q12) was negative and CIC (19q13) gene was rearranged. Meanwhile, some children also had CIC fusion gene mutations, including 2 CIC-DUX4 fusion cases and 1 CIC-NUTM1 fusion case. All patients were treated with surgery and postoperative adjuvant chemotherapy. Follow?up data were available in all the five cases (mean 21.4 months); one patient died of disease, whereas two patients were alive with unresectable recurrent tumour, and the remaining two patients were alive with no evidence of disease. Conclusion ·Pediatric CRS is a rare malignant soft tissue tumor with asymptomatic or non-specific symptoms, easy metastasis and poor prognosis. Immunohistochemistry combined with EWSR1 and CIC rearrangement-related molecular examination is helpful for the diagnosis of Ewing's sarcomatoid soft tissue tumors, and further detection of CIC fusion gene may be helpful for prognosis estimation.