Radiotherapy (AQA A Level Physics)

Use of High Energy X-rays

• 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

Use of Radioactive Implants

• 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