PET-CT results showed a metabolically active right femoral mass, which was considered to be possibly due to osteosarcoma or eosinophilic granuloma. Osteosarcoma is a common malignant bone tumor in children. Due to the high proliferation and metabolism of malignant tissues, osteosarcoma often appears as a metabolically active mass on PET-CT. Eosinophilic granuloma also belongs to a type of malignant bone tumor, which behaves similarly to osteosarcoma on PET-CT, and therefore the two diseases cannot be definitively differentiated on the basis of PET-CT findings alone. These patients often require further diagnostic biopsy to obtain pathologic tissue for a more accurate determination of tumor type. For patients with metabolically active right femoral mass on PET-CT, who are considered to be possibly due to osteosarcoma or eosinophilic granuloma, it is recommended to seek prompt medical attention for further detailed examination and early treatment.