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?