Which mechanism is the most common route for DNA damage from ionizing radiation?

• A. Indirect action
• B. Direct action
• C. Direct and indirect actions are equally likely
• D. Thermal effects

Answer: (A)

Most DNA damage from ionizing radiation in biological tissue comes from indirect action. The reason is that living tissue is mostly water, so the energy deposited by radiation primarily radiolyses water molecules, creating reactive free radicals such as hydroxyl radicals and hydrogen atoms. These radicals diffuse and chemically attack DNA, causing strand breaks and base alterations. Although direct action—where radiation directly ionizes the DNA itself—can occur, it’s less likely because the DNA is a relatively small target amid a sea of water molecules. Thermal effects, meaning tissue heating from the radiation, are negligible at typical diagnostic doses and do not drive DNA damage. High-LET radiation tends to cause more direct damage, but for the low-LET radiation common in medical imaging, indirect action is the dominant pathway.