The reasoning
- Direct action: radiation ionises DNA itself (dominant for high‑LET radiation such as alpha particles and neutrons).
- Indirect action: radiation ionises water → hydroxyl free radicals → DNA damage. About two‑thirds of X‑ray damage is indirect, and it is oxygen‑dependent (oxygen "fixes" the damage). This is why hypoxic tumours resist radiotherapy (Concept 23).
- The lethal lesion is the unrepaired or misrepaired double‑strand break.