In a recent study using an osteosarcoma mouse model, the addition of checkpoint inhibitors (anti-PD-1 and anti-CTLA-4) to CIRT (10 Gy, 50 keV/μm) or X-rays (10 Gy) only slightly improved the response of primary tumors, the growth of which was efficiently suppressed by both radiation types upon monotherapy at least during the 21 days of the experiment [246]. This evidence concerns the gene CTLA4 and osteosarcoma.