Findings and diagnosis

Key findings

• Right knee radiographs: Permeative lytic lesion at the proximal lateral tibia including the metadiaphysis to tibial plateau extending to the cortex, with probable cortical breakthrough anteriorly. Small to moderate knee joint effusion is present.

• Whole-body bone scan: Increased radiotracer uptake in the right proximal tibia with a photopenic center, representing the primary tumor.

• Right knee MRI with and without intravenous contrast: Large T1 iso-, T2 hyperintense lesion of the proximal tibia extending to the tibial eminence. This lesion is mostly non-enhancing with peripheral rim irregular enhancement. Multifocal extraosseous components including extension into the knee joint space anterior to the anterior cruciate ligament, to the anteromedial aspect of proximal tibia, along the lateral aspect of the insertion of patellar tendon. Fibular head and distal femur are grossly intact. 

Differential diagnosis

• Chondroblastic osteosarcoma
• Ewing sarcoma
• Osteomyelitis
• Chondrosarcoma
• Metastasis
• Aneurysmal bone cyst

Final diagnosis: Chondroblastic osteosarcoma

Pathology: Proximal tibial biopsy revealed high-grade cartilaginous neoplasm, most consistent with chondroblastic osteosarcoma.