Similar to immune reactions against
antigens from bacteria or viruses, the abscopal effect requires
priming of immune cells against tumor antigens. Abscopal effects of ionizing radiation are often blocked by the immunosuppressive
microenvironment inside the irradiated tumor which prevents effective T cell priming. This explains why the effect is so rarely seen in patients receiving radiotherapy alone. In contrast, the combination of immunomodulatory drugs such as ipilimumab and pembrolizumab can partially reconstitute systemic anti-tumor immune reactions induced after local tumor radiotherapy. However, there is so far no consensus on the optimal radiation regimen needed to increase the chance of abscopal tumor regression. ==References==