In radiotherapy, the mechanism of normal tissues injury is very complex and based on dose, manifestation time, volumeof irradiated tissue and radiosensitivity can be categorized as acute and late injuries. Speaking generally, interaction of low LET ionizing radiation with normal tissues results in formation of free radicals such as reactive oxygen/nitrogen species
(ROS/RNS) that cause oxidative stress and activation of some transcription factors, pro-inflammatory molecules and cytotoxicity by inducing DNA damage, alteration of cell function/
phenotype, resulting in chronic inflammation, organ dysfunction, and ultimate fibrosis and/or necrosis.