Medical Uses of Radiation (Edexcel GCSE Physics)

Radiotherapy

Radiotherapy

• Radiotherapy is the name given to the treatment of cancer using radiation

• Although radiation can cause cancer, it is also highly effective at treating it

• Radiation can kill living cells

  • Some cells, such as bacteria and cancer cells, are more susceptible to radiation than others

• During external radiotherapy, beams of gamma rays are directed at the cancerous tumour

  • Surrounding healthy tissue tends to be shielded to avoid causing any damage

During radiotherapy, the beams are moved around to minimise harm to healthy tissue whilst still being aimed at the tumour

• During internal radiotherapy, small pellets of radioactive materials can also be inserted into a tumour exposing it directly to radiation

PET Scanners and Tracers

Medical Tracers

• A tracer is a radioactive isotope that can be used to track the movement of substances, like blood, around the body

• Gamma emitters are usually used for this purpose

  • Gamma rays are highly penetrating and so will be able to pass through the body and be detected outside the body

• This allows an internal image of the body to be created

Iodine-131 is an example of a radioactive tracer

• Since gamma rays are less ionising than some other forms of radiation, the harm caused to the patient is also minimised

• As well as choosing a gamma emitter:

  • The amount of isotope used is kept to a minimum to reduce people’s exposure to radiation

  • Isotopes are chosen that have short half-lives of around a few hours: Long enough to carry out the procedure, but not so long that they cause long term harm

Positron Emission Tomography (PET)

• In PET scanning positrons are emitted by the decay of the tracer

• They travel a small distance and annihilate when they interact with electrons in the tissue

• This produces a pair of gamma-rays (also called gamma (γ) photons) which can be detected outside the body

The photons (produced by the annihilation of the electron and positron) are detected outside the body. keV are a unit of energy.

Isotope Production

• When carrying out procedures involving the use of radioactive isotopes, such as the PET scan, the amount used is kept to a minimum to reduce people’s exposure to radiation

  • Isotopes are chosen that have short half-lives of around a few hours: Long enough to carry out the procedure, but not so long that they cause long term harm

• Because of the short half-life of the isotopes involved, the isotopes have to be produced nearby – otherwise they would have decayed too much by the time they reached the hospital and be unusable