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Biological Effects of Radiation

Learning Objectives

• Interaction of radiation in biological systems

• Cellular, DNA, and Chromosome damage by radiation

• Effects on Biomolecules

• Methods for determining DNA and cell damage

• Cellular responses to radiation

• DNA repair mechanisms

Interaction of Radiation with Biological Systems

• Measures both direct and indirect actions of radiation in biological systems.

Direct Effects of Radiation

• Involves direct interaction with critical biomolecules:

  • DNA: Structural damage includes single-strand or double-strand breaks and mutations.

  • Proteins: Denaturation can cause loss of function.

  • Lipids: Membrane disruption leads to altered cell signaling or cell death.

• Charged ionizing radiation examples: alpha particles, beta particles, positrons, protons, and heavy ions.

• Occurs when radiation directly ionizes atoms by ejecting electrons.

Indirect Effects of Radiation

• Occurs when radiation interacts with water molecules, producing reactive oxygen species (ROS) that indirectly damage biomolecules.

• ROS include hydroxyl radicals (·OH) and superoxide (O₂⁻) that attack DNA, proteins, and lipids, causing oxidative stress.

High-Energy Photons and Their Effects

• Charged particles typically cause direct effects while uncharged particles lead to indirect effects.

• High-energy photons (X-rays, gamma rays) interact in three ways:

  • Photoelectric effect: Photon ejects an electron causing secondary ionization.

  • Compton scattering: Photon transfers energy to an electron, knocking it out.

  • Pair production (high energies): A photon transforms into an electron-positron pair.

• Most biological damage from photons is indirect as they ionize water, forming ROS.

Linear Energy Transfer (LET)

• LET: The amount of energy radiation particle deposits per unit length in a material.

  • High LET: High energy deposited over short distances (alpha particles, heavy ions).

  • Low LET: Lower energy spread over longer distances (X-rays, gamma rays).

• High LET radiation creates clustered damage, making it more severe and difficult for cells to repair.

• Conversely, low LET causes widespread but sparse damage, often more manageable.

Bragg Peak

• Observed in charged particles like protons; LET and energy deposition increase dramatically just before the particle stops.

• Critical for radiation therapy as it allows maximum energy deposition within tumors while sparing healthy tissues.

• In conventional X-ray therapy, energy is distributed across tissues, affecting both tumor and healthy cells.

Radiation and DNA Repair

• Following radiation-induced damage, DNA repair mechanisms activate to restore genetic integrity.

• Key repair processes include:

  • Base excision repair

  • Nucleotide excision repair

  • Homologous recombination

• Effective repair is crucial to prevent mutations and maintain cellular function.

Conclusion

• Understanding radiation's biological effects and the mechanisms of DNA damage and repair are essential for developing effective therapeutic strategies in radiation therapy.