Summary of the pathways and mechanisms involved in cell and tissue response to radiotherapy. The interaction of ionizing radiation with tissues leads to multiple types of DNA damage (for example, base damage, single-strand breaks, double-strand breaks). Double-strand breaks are harder to repair and are the most important DNA lesion induced by radiation. Radiation also produces reaction oxygen (ROS) and nitrogen (RNOS) species that stimulate cytokine, growth factor and chemokine responses. There are multiple interconnected signaling networks that respond to radiation damage that can lead to cell death, cell senescence, genomic instability, mutations and inflammatory response. Some of the key genes involved in the processes are shown. The information taken from Bentzen (2006) , Jeggo and Lavin (2009)  and Bhatti et al. . HR, homologous recombination; NHEJ, non-homologous end joining; NOS, nitric oxide synthase; SOD, superoxide dismutase.