A 47-year-old man has had dull, constant pain in the midsection of the right thigh for the past 4 months. On physical examination, there is pain on palpation of the anterior right thigh, which worsens with movement. The right thigh appears to have a larger circumference than the left thigh. A radiograph of the right upper leg and pelvis shows no fracture, but there is an ill-defined soft-tissue mass anterior to the femur. MRI shows a 10x8x7 cm solid mass deep to the quadriceps, but it does not involve the femur. Karyotypic analysis of tumor cells reveals t (12;16) (q13; p11) with amplification of the MDM2 gene. What is the most likely diagnosis?

A large, deep soft-tissue mass suggests cancer, most likely a sarcoma. Liposarcomas are located in deep soft tissues, can be indolent, and can reach a large size. They are the most common sarcomas of adulthood. Chondrosarcomas can be seen over a wide age range, but they arise within the bone. Carcinomas are far more common in adults than sarcomas and can metastasize, but such a large lesion in soft tissue is unlikely to be a metastasis. Nodular fasciitis is a reactive fibroblastic lesion of young adults, usually on the upper extremities and trunk, and can develop several weeks after local trauma. Osteosarcomas generally occur in individuals younger than 20 years and typically arise in the metaphyseal region of long bones. Rhabdomyosarcoma occurs in children and is most often a tumor of the head and neck, genitourinary tract, or retroperitoneum.