患儿，男，7岁，因颜面部水肿入院。超声检查提示中等量心包积液及胸腔积液，胸部电子计算机断层扫描可见纵隔内结构不清，增强扫描呈轻-中度强化，考虑纵隔占位。进一步行全身正电子发射断层显像-计算机断层扫描、纵隔病灶病理活检、胸腔积液及心包积液病理细胞学检查及免疫组化等检查，最终诊断为孤立性髓系肉瘤。病理检查是临床诊断的金标准，但在取材部位受限时，易出现漏诊、误诊，应根据病情多次多部位取材，同时结合流式细胞学及免疫组化协助诊断，避免漏诊、误诊。

The patient was a 7-year-old boy admitted with facial edema. Ultrasound indicated moderate pericardial effusion and pleural effusion. Contrast-enhanced chest computed tomography showed indistinct mediastinal structures with mild-to-moderate enhancement, suggestive of a mediastinal space-occupying lesion. Further evaluation with whole-body PET-CT, pathological biopsy of the mediastinal lesion, cytological examination of pleural and pericardial effusions, and immunohistochemistry led to the final diagnosis of isolated myeloid sarcoma. Pathological examination is the gold standard for clinical diagnosis; however, limited sampling sites can result in missed or incorrect diagnoses. Multiple and multi-site sampling should be undertaken according to the clinical context, combined with flow cytometry and immunohistochemistry to assist diagnosis and reduce missed diagnoses and misdiagnoses.