A 16-year-old boy has noted pain in his left knee after each hockey practice

session for the past month. On examination, there is tenderness to palpation of

his left knee, with a reduced range of motion. X-Ray of the left leg reveals a

mass of the proximal tibial metaphysis that erodes bone cortex, where reactive

new bone is apparent. Microscopic examination shows atypical, malignant cells

with hyperchromatic nuclei in an osteoid stroma. Which of the following

neoplasms is he most likely to have?