The clinical presentation of an adolescent with knee pain and the classic radiological findings of a 'sunburst' periosteal reaction and Codman's triangle in the metaphysis of a long bone (distal femur) are highly characteristic of osteosarcoma. Osteosarcoma is the most common primary malignant bone tumor, typically affecting adolescents and young adults, often around the knee joint. Ewing sarcoma usually presents in younger children, often in the diaphysis, with an 'onion-skin' periosteal reaction. Chondrosarcoma is a malignant cartilage tumor more common in older adults and flat bones. Osteochondroma is a benign bone tumor with a cartilage cap, typically presenting as a painless mass.
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