 Randomized, placebo-controlled, multicentric, phase II study -This is a double-blind placebo-controlled trial, with 4 cohort: cohort A: Osteosarcoma cohort B: Ewing sarcoma cohort C: Chondrosarcoma cohort D : chondroma. Cohort A, B and C will involve a total of 36 patients 24 Regorafenib 12 placebo. And cohort D a total of 24 evaluable patients 16 Regorafenib 8 placebo 132 patients who meet the eligibility criteria will be randomly assigned in a 2:1 ratio to the following treatment groups : 