3. Radiation physics

From greek.doctor
  • 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