Synergizing radiation therapy and immunotherapy for curing incurable cancers. Opportunities and challenges.

The combination of radiation therapy and immunotherapy holds particular promise as a strategy for cancer therapeutics. Evidence suggests that immunotherapy is most beneficial alone when employed early in the disease process or in combination with standard therapies (eg, radiation) later in the disease process. Indeed, radiation may act synergistically with immunotherapy to enhance immune responses, inhibit immunosuppression, and/or alter the phenotype of tumor cells, thus rendering them more susceptible to immune-mediated killing. As monotherapies, both immunotherapy and radiation may be insufficient to eliminate tumor masses. However, following immunization with a cancer vaccine, the destruction of even a small percentage of tumor cells by radiation could result in crosspriming and presentation of tumor antigens to the immune system, thereby potentiating antitumor responses. Learning how to exploit radiation-induced changes to tumor-cell antigens, and how to induce effective immune responses to these cumulatively immunogenic stimuli, is an exciting frontier in cancer therapy research. This review examines mechanisms by which many forms of radiation therapy can induce or augment antitumor immune responses as well as preclinical systems demonstrating that immunotherapy can be effectively combined with radiation therapy. Finally, we review current clinical trials where standard-of-care radiation therapy is being combined with immunotherapy.