Zachary S. Morris, MD, PhD
One of the hallmarks of tumorigenesis is the ability of cancer cells to suppress, or circumvent, the host immune system.1
Advances in strategies to overcome tumor cell evasion of immune detection have led to a rapid pace of preclinical and clinical development in the field of cancer immunotherapy. Most notably, in recent years T-cell checkpoint inhibitors have demonstrated therapeutic efficacy in multiple cancer types. This class of immunotherapy modulates tumor tolerance among T cells by antagonizing specific inhibitory receptor–ligand interactions, thereby enhancing T-cell activation.
A frequent observation from clinical studies of T-cell checkpoint inhibitors is that even in the context of metastatic disease, a small subgroup of patients who respond to treatment have complete and durable regression of disease. This raises the possibility that augmenting response rates to checkpoint blockade or other such immunotherapies may result in a dramatic impact on survival of patients with advanced-stage malignancy. Nevertheless, durable tumor response remains elusive for the overwhelming majority of patients with metastatic cancers, and immunotherapies are not typically effective in patients with immunologically “cold” tumors, characterized by low levels of T-cell infiltrate and few mutation-created neoantigens.
One potential approach to improving the response to cancer immunotherapies is to combine such treatments with radiotherapy (RT). Radiation may interact with the tumor immune microenvironment at a targeted site through various mechanisms, including: (1) temporary local eradication of radiation-sensitive immune lineages including suppressor and effector lymphocytes; (2) local release of inflammatory cytokines and damage-associated molecular patterns, resulting in local effects on endothelial cell expression of adhesion receptors, immune cell trafficking, and immune cell activation; (3) immunogenic tumor cell death and release of tumor-specific antigens; and (4) induction of phenotypic changes in the expression of immune susceptibility markers on tumor cells surviving radiation.2
Because of these effects, radiation may enhance antigen cross-presentation and diversification of antitumor T-cell responses.3,4
By modulating tumor immune tolerance and functional immunogenicity at a targeted site, external beam radiation therapy may serve as a method of in situ tumor vaccination, a therapeutic strategy that is intended to convert a patient’s own tumor into a nidus for presentation of tumor-specific antigens that will stimulate and diversify an antitumor T-cell response.5,6
The capacity of external beam radiation alone to elicit a systemic antitumor immune response is well documented (eg, abscopal effect) but quite rare.7
In the context of targeted immunotherapies such as checkpoint inhibitors, however, multiple preclinical studies have demonstrated that localized radiation therapy can consistently enhance a systemic antitumor immune response.4,8-10
Results from retrospective clinical studies indicate the safety of such combinations.11-14
This highlights decades of improvements in the physical targeting of radiation therapy and the molecular targeting of immunotherapies, which have rendered these treatments increasingly amenable to combined modality approaches (Figure
). However, prospective clinical studies reported to date have not demonstrated response rates greater than anticipated with checkpoint blockade alone.4,15-18
More than 100 ongoing clinical studies are now investigating combinations of radiation therapy with immune checkpoint inhibition in various clinical contexts. This unprecedented extent of clinical investigation reflects both the promise and enthusiasm for such approaches, as well as a degree of uncertainty about the circumstances, disease sites, radiation dose and fractionation, timing and sequencing, and clinical settings that may portend greatest benefit for immuno-RT combinations.
Figure. Historical Convergence of Radiotherapy and Immunotherapy Advancements