Neoadjuvant radiation therapy and surgery improves metastasis-free survival over surgery alone in a primary mouse model of soft-tissue sarcoma.

This study aims to investigate whether adding neoadjuvant radiotherapy (RT), anti-programmed cell death protein-1 (PD-1) antibody (anti-PD-1), or RT + anti-PD-1 to surgical resection improves disease-free survival for mice with soft tissue sarcomas (STS). We generated a high mutational load primary mouse model of STS by intramuscular injection of adenovirus expressing Cas9 and guide RNA targeting Trp53 and intramuscular injection of 3-methylcholanthrene (MCA) into the gastrocnemius muscle of wild-type mice (p53/MCA model). We randomized tumor-bearing mice to receive isotype control or anti-PD-1 antibody with or without RT (20 Gy), followed by hind limb amputation. We used micro-CT to detect lung metastases with high spatial resolution, which was confirmed by histology. We investigated if sarcoma metastasis was regulated by immunosurveillance by lymphocytes or tumor cell-intrinsic mechanisms. Compared to surgery with isotype control antibody, the combination of anti-PD-1, RT, and surgery improved local recurrence-free survival (p=0.035) and disease-free survival (p=0.005), but not metastasis-free survival. Mice treated with RT, but not anti-PD-1, showed significantly improved local recurrence-free survival and metastasis-free survival over surgery alone (p=0.043 and p=0.007, respectively). The overall metastasis rate was low (~12%) in the p53/MCA sarcoma model, which limited the power to detect further improvement in metastasis-free survival with addition of anti-PD-1 therapy. Tail vein injections of sarcoma cells into immunocompetent mice suggested that impaired metastasis was due to inability of sarcoma cells to grow in the lungs rather than a consequence of immunosurveillance. In conclusion, neoadjuvant RT improves metastasis-free survival after surgery in a primary model of STS.