MRI assessment of abscopal effect enhanced by immune checkpoint blockade

Radiation therapy (RT) on primary tumor rarely induces regression of non-irradiated metastatic lesions (abscopal effect), which is reported to be enhanced by immune-checkpoint blockade. In this study, we examined the physiological changes induced by abscopal effect using MRI-based imaging modalities such as EPR oximetry, DCE MRI, and 13C DNP MRI. Hypoxic fraction < 10 mmHg (HF10), permeability, perfusion, and CD8+ T cell infiltration in metastatic tumor increased after the combination of RT and PD-1 blockade. Interestingly, higher permeability/perfusion and lower HF10 in primary tumor before the treatment were associated with slower growth of the metastatic tumor after the treatment.