Radiation therapy (RT) occasionally induces regression of non-irradiated metastatic lesions, which is called abscopal effect. Immune checkpoint inhibitors enhance abscopal effects. In this study we examined the physiological changes induced by abscopal effect and identify using MRI-based imaging biomarkers which predict the successful abscopal effect. Hypoxic fraction < 10 mmHg (HF10), permeability, perfusion, and CD8+ T cell infiltration in unirradiated tumor increased after the combination of RT and PD-1 blockade therapy. Interestingly, higher permeability/perfusion and lower HF10 in irradiated tumor before treatment is associated with slower growth of the unirradiated tumor after treatment of the tumor on the contralateral side.
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