3. Radiation physics

• Types of radiation used
  • Natural radiation (due to radioactive decay)
    • Alpha radiation (two protons and two neutrons)
    • Beta radiation (high-energy electrons)
      • Electrons (beta radiation) has low penetration power, so beta radiation is only suitable for treating tumours near the skin surface
  • Gamma radiation (high-energy photons)
    • Photon radiation (gamma and X-ray radiation) has high penetration power, so it is suitable for treating deep tumours
  • Artificial radiation
    • X-rays (gamma radiation which is artificially made)
    • Heavy ion radiation
      • Heavy ions, like carbon ions, are accelerated to high speeds
      • Proton radiation (alpha radiation) and heavy-ion radiation don’t penetrate tissues deeper than a certain point
        • This makes these types of radiation suitable for treating tumours where radiation-sensitive tissue is very close, for example inside the skull
        • They’re also suitable for treating tumours close to the skin
        • This effect is due to the so-called “Bragg peak” on the percentage depth dose distribution
• Particle therapy
  • Accelerated protons or heavy ions are sent into the tumour
  • Very expensive
• Compton-scattering is the most important radiation effect on tissues in radiotherapy
  • The photoelectric effect, pair production and coherent scattering are less important
• The inverse square law
  • The law states that the intensity of radiation decreases in an inverse square ratio with the distance from the source of the radiation
  • For example
    • If the distance from the source is doubled, the radiation intensity is reduced to one fourth
    • If the distance from the source is increased three-fold, the radiation intensity is reduced to one ninth
  • The inverse square law is important in treatment planning in brachytherapy
• Percentage depth dose (PDD)
  • The PDD is the percentage of the maximum dose which is deposited in tissue (or something, it’s confusing)
  • PDD is important in treatment planning in teletherapy
• Squamous cell cancers are generally very radiosensitive
  • Adenocarcinomas on the other hand, are not
• Radiation biology
  • Radiation induces double-stranded breaks in DNA
  • The cell is most sensitive to radiation in the M and G2 phases of the cell cycle
  • The oxygen effect
    • According to the oxygen effect, normoxic tissues and cells are more radiosenstive than hypoxic ones
    • The oxygen effect occurs because oxygen “stabilizes” or “makes permanent” the DNA damage produced by reactive oxygen species
    • In hypoxic tissues, the DNA damage is not made permanent and can therefore be repaired by the cell