Radiotherapy (AQA A Level Physics)

• Cancerous cells divide more frequently than healthy cells
  • As a result of this, X-rays destroy cancer cells at a greater rate than they destroy healthy cells

• Different energies of X-rays are used for treating cancer in different areas of the body
  • Lower-energy X-rays are used for treating skin cancer
  • Higher energy X-rays are used for targeting tumours deeper in the body

External treatment using low-energy X-rays

• Low-energy X-rays can be used to treat skin cancer
• These X-rays can be directed at surface tumours, but they do not penetrate deep into the body
  • Therefore, the risk of damage to deeper tissues is reduced

External treatment using high-energy X-rays

• To treat a tumour deep inside the body, high-energy X-rays can be directed at the tumour from an external source
  • This is known as external beam radiotherapy (EBRT)

• When cells divide, they are sensitive to X-ray radiation
  • In EBRT, X-rays from an external source destroy cancer cells during division

Limiting Exposure to Healthy Cells

• During high-energy X-ray radiotherapy, the risk to healthy tissue is reduced by:
  
  • Using metal filters to remove low-energy X-rays
  • Directing the X-rays from different directions

Using metal filters limits exposure because...

• During the EBRT process, which lasts a few minutes, the patient is held in a made-to-fit mould made from aluminium to ensure they do not move
  • This ensures that only the target tissue receives the radiation dose

• When X-rays are produced, they are emitted with a wide range of energies
• Filtering the beam through an aluminium sheet ensures that the low-energy X-rays are removed
  • The less energetic X-rays are likely to damage tissues close to the surface
  • This means that only the more energetic X-rays reach the tumour deep inside the body

Using X-rays from different directions limits exposure because...

• The beam of X-rays can be precisely controlled to minimise damage to healthy tissue
  • The beam of X-rays is shaped to match the exact dimensions of the tumour (called conformal radiotherapy)

• X-rays are produced in a linear accelerator (LINAC) by accelerating electrons towards a heavy metal target
  • This LINAC source can be rotated around the patient
  • The target tissue is placed at the centre of rotation and therefore receives a much greater dose of radiation than the surrounding healthy tissue

Rotating X-Ray Source

The source of high energy X-rays rotates around the target, ensuring the targeted area receives a much larger dose of radiation than the surrounding healthy tissues

• Internal radiotherapy is when a radioactive implant is placed next to, or into the tumour itself
  • This implant consists of metal 'seeds' containing radioisotopes which irradiate the tumour and a small radius of tissue around it

• The main advantage of internal radiotherapy is that the source of radiation can be placed as close to the cancer as possible
• However, the main disadvantage is that a small amount of healthy tissue is likely to be exposed to ionising radiation

What type of radioisotope should be used in an implant?

• The radioisotope used in an implant should:
  • Have a short half-life (as long as the activity is also high)
  • Have a short range (as long as it can pass through the implant casing)
  • Be highly ionising

• The radiation must not penetrate far from the implant site to avoid irradiating healthy tissue further from the tumour
  • This is why beta radiation is the most common choice, as it can penetrate the seed's metal casing, but does not penetrate further than a few mm beyond the implant site
  • Alternatively, low-energy gamma rays can be used, as they are less penetrating than high-energy gamma

• Alpha radiation is not appropriate for this form of radiotherapy
  • The alpha particles would be unable to penetrate the metal casing of the seeds
  • Even if they were able to, they would only heavily ionise the tissue near the seed
  • As a result, the seeds would be harming healthy tissue rather than irradiating cancerous tissue
  • Additionally, the alpha particles could not penetrate the whole way through the tumour