When radiation enters the body, it will interact with the atoms and molecules of the cells (mainly made of water) to produce free radicals and molecules that are able to diffuse far enough to reach the critical target in the cell, the DNA, and damage it indirectly through some chemical reaction. This is the main damage mechanism for photons as they are used for example in external beam radiation therapy.
Typically, the radiolytic events that lead to the damage of the (tumor)-cell DNA are subdivided into different stages that take place on different time scales: 
* The physical stage (), consists in the energy deposition by the ionizing particle and the consequent ionization of water.
* During the physico-chemical stage () numerous processes occur, e.g. the ionized water molecules may split into a hydroxyl radical and a hydrogen molecule or free electrons may undergo solvation.
* During the chemical stage (), the first products of radiolysis react  with each other and with their surrounding, thus producing several reactive oxygen species which are able to diffuse.
* During the bio-chemical stage ( to days) these reactive oxygen species might break the chemical bonds of the DNA, thus triggering the response of enzymes, the immune-system, etc.
* Finally, during  the biological stage (days up to years) the chemical damage may translate into biological cell death or oncogenesis when the damaged cells attempt to divide.