Rationale and evidence to combine radiation therapy and immunotherapy for cancer treatment

Cancer immunotherapy exploits the immune system's ability to differentiate between tumor target cells and host cells. Except for limited success against a few tumor types, most immunotherapies have not achieved the desired clinical efficacy until recently. The field of cancer immunotherapy has flourished with a variety of new agents for clinical use, and remarkable progress has been made in the design of effective immunotherapeutic regimens. Furthermore, the therapeutic outcome of these novel agents is enhanced when combined with conventional cancer treatment modalities including radiotherapy (RT). An increasing number of studies have demonstrated the abscopal effect, an immunologic response occurring in cancer sites distant from irradiated areas. The present work reviews studies on the combination between RT and immunotherapy to induce synergistic and abscopal effects involved in cancer immunomodulation. Further insight into the complex interactions between the immune system and cancer cells in the tumor microenvironment, and their modulation by RT, may reveal the abscopal effect as a clinically relevant and reproducible event leading to improved cancer outcome.