A 44-year-old man comes to the office because he has had pain and swelling in his right upper arm for the past month. There is no history of trauma to the arm. On physical examination, there is tenderness to palpation of the arm and a mass is noted. X-ray studies show a fracture of the humerus, mottled appearance of bone, and a large, isolated soft-tissue mass in the upper arm. Based on these findings, which of the following is the most likely suspected diagnosis?