Figure 1.

Targeted and non-targeted biological effects in conventional external beam radiotherapy. Targeted effects are caused by one or more particles traversing irradiated cells and can be divided in DNA and non-DNA-centered effects. Non-targeted effects describes the effects observed in cells that have not been directly traversed by particles but that are close to irradiated cells, as well as long-distance effects. DNA, mitochondria, and the cell membrane are the main sensitive targets of radiation. Following targeted and non-targeted effects, cells can survive (lesions are effectively repaired), they can die (lesions are not repaired) or they can be transformed. The dose–effect relationship of targeted effects is commonly fitted by linear or linear-quadratic models. A saturation of the response to non-targeted effects has been described. For more details, see the main text.