A 30-year-old female presents with pain and swelling around her knee. X-rays reveal an eccentric, purely lytic lesion in the epiphysis of the distal femur, extending to the subarticular bone, without significant periosteal reaction. Histopathology shows multinucleated giant cells. What is the most probable diagnosis?

Explanation

Giant Cell Tumor (GCT) is a benign but locally aggressive bone tumor typically found in the epiphysis of long bones (most commonly around the knee, i.e., distal femur or proximal tibia) in young adults (20-40 years old). It presents as an eccentric, purely lytic lesion that often extends to the subarticular bone. The presence of multinucleated giant cells on histopathology is a hallmark. Chondroblastoma is also epiphyseal but typically occurs in younger individuals (adolescents) and has a chondroid matrix. Enchondroma is a benign cartilaginous tumor, usually diaphyseal/metaphyseal. Aneurysmal bone cyst can be lytic but is often expansile and multiloculated, and less specific to the epiphysis in this age group.