The exact mechanism of cell death due to radiation
is still an area of active investigation. A large body of evidence supports double-stranded breaks of nuclear DNA as the most important cellular effect of radiation. This breakage leads to irreversible loss of the reproductive integrity of the cell and eventual cell death.
Radiation damage can be directly ionizing; however, in clinical therapy, damage is most commonly indirectly ionizing via free-radical intermediaries formed from the radiolysis of cellular water.
Radiation can also affect the processes of the cell cycle
necessary for cell growth, cell senescence, and apoptosis (programmed cell death). Many of these processes are only now beginning to be elucidated and manipulated in order to make radiation therapy more effective.