The differential diagnosis for an intra-articular lesion in the knee of a pediatric patient is broad. Diagnostic considerations include pigmented villonodular synovitis (PVNS)-the most common intra-articular tumor-and a variety of both benign and malignant tumors, including lipomas, hemangiopericytomas, nodular fasciitis, parosteal osteosarcomas, and fibromyxoid sarcomas. If there is concern ov...