Immunotherapy and stereotactic ablative radiotherapy (ISABR): a curative approach?

Abstract

Conventional radiotherapy, in addition to its well-established tumoricidal effects, can also activate the host immune system. Radiation therapy modulates tumour phenotypes, enhances antigen presentation and tumour immunogenicity, increases production of cytokines and alters the tumour microenvironment, enabling destruction of the tumour by the immune system. Investigating the combination of radiotherapy with immunotherapeutic agents, which also promote the host antitumour immune response is, therefore, a logical progression. As the spectrum of clinical use of stereotactic radiotherapy continues to broaden, the question arose as to whether the ablative radiation doses used can also stimulate immune responses and, if so, whether we can amplify these effects by combining immunotherapy and stereotactic ablative radiotherapy (SABR). In this Perspectives article, we explore the preclinical and clinical evidence supporting activation of the immune system following SABR. We then examine studies that provide data on the effectiveness of combining these two techniques - immunotherapy and SABR - in an approach that we have termed 'ISABR'. Lastly, we provide general guiding principles for the development of future clinical trials to investigate the efficacy of ISABR in the hope of generating further interest in these exciting developments.

Figure 1. Immune stimulation by SABR

Antitumour effects of stereotactic ablative radiotherapy (SABR). SABR results in immune activation by inducing tumour-cell death, modulating tumour-cell phenotype and normalizing aberrant tumour vasculature to allow for improved oxygen and drug delivery. After cell death, the release of tumour debris with associated danger signals, tumour-associated antigens (TAAs), and inflammatory cytokines are recognized by and activate dendritic cells, promoting antigen presentation to cells of the immune system. Polyclonal antigen-specific T cells are then generated, some of which can attack tumours located within the radiation field, as well as distant tumours; this response can be augmented by the addition of systemic immune-enhancement measures. GM-CSF; granulocyte macrophage colony stimulating factor; IL, interleukin; MHC, major histocompatibility complex.