- 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