Radiation therapy induces an abscopal effect and upregulates programmed death‐ligand 1 expression in a patient with non‐small cell lung cancer

Abstract Radiation therapy (RT) activates the antigen presentation of dendritic cells and priming of cancer‐specific cytotoxic CD8 + T cells, occasionally resulting in a systemic immune response to the tumor outside of the treatment field. The phenomenon of tumor regression at the site distant from irradiated fields is known as the abscopal effect. Several case reports have indicated a potential role of RT in overcoming primary and acquired resistance against immune checkpoint inhibitors in non‐small cell lung cancer (NSCLC) and melanoma patients. We herein report an NSCLC patient who developed acquired resistance to an RT‐induced abscopal effect and subsequently experienced reactivation of the systemic antitumor immune response by pembrolizumab, an antiprogrammed death 1 antibody. In this case, RT not only induced an abscopal effect but also upregulated the programmed death‐ligand 1 expression outside of the irradiated field when the patient developed resistance to the abscopal effect. This case can facilitate our understanding of the mechanism underlying the RT‐induced systemic immune response against cancer cells and adaptive resistance mechanism of cancer cells from immune surveillance. These findings highlight the promising results of current clinical trials combining RT and immune checkpoint inhibitors. Ongoing clinical trials will further establish evidence supporting combination therapy with RT and immune checkpoint inhibitors.