DNA is currently thought to be the target of ionising radiation - that is, the reason cells die after irradiation.
There are two ways ionising radiation can cause DNA damage.
- The direct action of radiation involves ionisation of the DNA molecule itself
- The indirect action of radiation involves ionisation of water molecules, leading to the formation of hydroxyl radicals. These hydroxyl radicals interact with and damage DNA.
The type of action that occurs depends on the incident particle.
Heavier ionising particles such as protons and alpha particles have a large direct action component. By directly damaging the DNA molecule, they do not require the presence of oxygen to 'fix' damage and therefore have similar efficacy in hypoxic and oxic cells.
Smaller particles and photons are thought to cause DNA damage indirectly through formation of hydroxyl radicals. The damage to the DNA molecule caused by interactions of hydroxyl require fixation by oxygen. Hypoxic cells are therefore significantly more resistant to these types of radiation.