Fig. 1

The choice of therapeutic agent in combination with radiotherapy may depend on the TME. Radiotherapy can drive the expansion and activation of T-cells in tumours with a T-cell rich TME. This leads to upregulation of PD-L1 and these tumours respond to ICIs. Conversely, tumours with a myeloid rich TME do not respond to ICI, RT drives further expansion and suppressive myeloid cells and may need to be combined with IO agents which re-programme myeloid cells and/or enhance T-cell priming and infiltration. ICI immune checkpoint inhibitor, IO immuno-oncology, M2 M2-polarised macrophage, mAb monoclonal antibody, MDSC myeloid-derived suppressor cell, RT radiotherapy, TME tumour microenvironment