Abstract
Undifferentiated pleomorphic bone sarcoma (UPS-B) is a rare high-grade sarcoma of bone that is usually seen in advanced age. There is no specific line or pattern of differentiation, and the diagnosis is often made by exclusion. It is especially important to differentiate it from osteosarcoma and dedifferentiated chondrosarcoma. It tends to be located most frequently in the lower extremity, particularly the femur. It is treated with the osteosarcoma protocol. In this study, a case of undifferentiated high-grade pleomorphic bone sarcoma located in the distal femur was reported. It aimed to discuss the patient's clinical presentation, diagnostic approach, treatment, and follow-up process in light of the literature. In this case, the tumor was resected with wide margins after neoadjuvant chemotherapy. He died of diffuse lung metastases during the adjuvant chemotherapy process. Unlike the literature, the patient was only 29 years old. The patient died 8 months after the first diagnosis.