A 16-year-old-girl came with swelling and pain over the lateral aspect of the right knee since two months. She had suffered an injury to the leg prior to the onset of the swelling.
Frontal and lateral radiographs of the right knee in an immature skeleton show a lytic lesion with a narrow zone of transition in the epiphysis of the lateral condyle of the tibia. The plain CT scan shows a well-defined lytic lesion in the lateral tibial condyle with thin sclerotic margins. The lesion demonstrates an eccentric epiphyseal location and a narrow zone of transition. MR imaging reveals a well-defined oval altered signal intensity lesion in the proximal epiphysis of the lateral condyle of the right tibia. It is hypointense on T1 and hyperintense on T2/PDFS sequences. There is mild joint effusion with mild soft tissue and muscle edema.