Radiotherapy Combined with Immune Checkpoint Inhibitor on Murine Fibrosarcoma and a Narrative Review of Clinical Studies

<b>Purpose:</b> The objective of this study was to evaluate the synergistic effect of combining an immune checkpoint inhibitor (ICI) with radiotherapy (RT) on murine fibrosarcoma and to conduct a narrative review of clinical studies on soft tissue sarcoma (STS). <b>Materials and Methods:</b> Forty male C3H mice (aged 5 weeks) were injected intramuscularly with 2 × 10⁵ FSaII cells into the right thigh and randomly assigned to four groups: (1) control; (2) RT; (3) InVivoMab™ (CD279) (mouse anti-PD-1 antibody) (ICI group); and (4) combined treatment with InVivoMab™ (CD279) and RT (combination group). On day -1, ICI was administered intraperitoneally. On day 0, RT (10 Gy, single fraction) was delivered locally to the tumors in the right hind limb. Subsequently, ICI was injected twice weekly (a total of 8 times). On day 26, all mice were euthanized, and the results were analyzed. In addition, a narrative review was conducted to identify clinical evidence. <b>Results:</b> On day 26, mean gross tumor volumes were 3578.13 ± 407.32 mm³ in the control group, 1995.72 ± 970.46 mm³ in the RT group, 2729.96 ± 286.47 mm³ in the ICI group, and 1007.92 ± 197.36 mm³ in the combination group. Gross tumor growth delay was most pronounced in the combination group. Moreover, the TUNEL assay demonstrated a significant increase in apoptosis in the combination group. Analysis of the underlying immune system revealed significantly higher expression of CD4+, CD8+, and IFN-γ in the combination group. The literature search identified only 12 case reports and 3 prospective studies involving patients with STS treated with the combined treatment of ICI and RT, suggesting potential synergism with acceptable toxicity. <b>Conclusions:</b> The current study demonstrated a synergistic effect of combining an ICI with RT in murine fibrosarcoma. There was limited data in the clinical setting. Further investigations are warranted to determine the optimal combination strategy of ICI and RT for STS.