Direct Action

  • Radiation deposits energy directly in DNA or other critical targets.

  • Produces ionizations and excited states within the DNA helix.

  • Responsible for ~30–40% of DNA damage with low-LET radiation (X-rays, γ-rays).

Indirect Action

  • Radiation interacts with water molecules (70% of cell volume).

  • Generates free radicals such as hydroxyl radicals (·OH), which diffuse and damage DNA.

  • Responsible for ~60–70% of DNA damage with low-LET radiation.

LET (Linear Energy Transfer)

  • The amount of energy deposited per unit length of track.

  • Low-LET (X-rays, γ-rays) → mostly indirect effects.

  • High-LET (α-particles, carbon ions) → mostly direct effects, clustered DNA damage.

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