Abstract:
Approximately 50% of subjects with cancer have been treated with ionizing radiation (IR) either as a curative,
adjuvant, neoadjuvant or as a palliative agent, at some point during the clinical course of their disease. IR kills
cancer cells directly by injuring their DNA, and indirectly by inducing immunogenic cell killing mediated by
cytotoxic T cells; but it can also induce harmful biological responses to non-irradiated neighbouring cells (bystander
effect) and to more distant cells (abscopal effect) outside the primary tumour field of irradiation.
Although IR can upregulate anti-tumour immune reactions, it can also promote an immunosuppressive tumour
microenvironment. Consequently, radiotherapy by itself is seldom sufficient to generate an effective long lasting
immune response that is capable to control growth of metastasis, recurrence of primary tumours and development
of second primary cancers. Therefore, combining radiotherapy with the use of immunoadjuvants such as immune
checkpoint inhibitors, can potentiate IR-mediated anti-tumour immune reactions, bringing about a synergic
immunogenic cell killing effect.
The purpose of this narrative review is to discuss some aspects of IR-induced biological responses, including factors
that contributes to tumour radiosensitivity/radioresistance, immunogenic cell killing, and the abscopal effect.
