X-Rays and DNA Damage: Mechanism, Mutagenesis, and Cancer Risk

How Ionizing Radiation Affects DNA

X-rays are ionizing radiation—high-energy photons that eject electrons from atoms, creating reactive ions and free radicals.

Direct vs. Indirect Damage

• Direct: Photon hits DNA, breaking phosphodiester bonds.
• Indirect (predominant): Radiation ionizes water → hydroxyl radicals (·OH) → attack DNA.

Types of DNA Lesions

• Single-strand breaks (SSBs): Usually repaired accurately.
• Double-strand breaks (DSBs): Highly mutagenic; can cause chromosomal translocations, deletions.
• Base modifications: e.g., 8-oxoguanine → mispairing with adenine → G:C→T:A transversions.
• Cross-links: DNA-DNA or DNA-protein.

From Damage to Cancer

1. Unrepaired/misrepaired DSBs → genomic instability.
2. Mutations in oncogenes (e.g., RAS) or tumor suppressors (e.g., TP53).
3. Clonal expansion → malignancy (leukemia, thyroid, breast, lung cancers linked to radiation).

Risk Context

• Risk is dose-dependent (linear no-threshold model for radiation protection).
• Diagnostic X-rays: very low individual risk; justified by clinical benefit.
• ALARA principle: As Low As Reasonably Achievable.
• Children and young adults more radiosensitive.