X-ray photons are widely used in external beam radiotherapy to target and destroy cancerous tumours. However, as the beam passes through healthy tissue to reach the tumour, some energy is inevitably absorbed, damaging healthy cells. An X-ray machine produces a spectrum of photon energies.
Table 1 shows the linear attenuation coefficient of liver tissue for photons of energy 200 keV200\text{ keV}200 keV and 500 keV500\text{ keV}500 keV.
Table 1
| Energy / keV\text{keV}keV | Linear attenuation coefficient of liver tissue / cm−1\text{cm}^{-1}cm−1 |
|---|---|
| 200 | 0.18 |
| 500 | 0.11 |
Deduce whether photons of energy 200 keV200\text{ keV}200 keV or 500 keV500\text{ keV}500 keV are better suited for treating a deep liver tumour located at a depth of 12 cm.
Metallic filters are placed in the path of the X-ray beam to filter out lower-energy photons, reducing the radiation dose absorbed by superficial healthy tissues. Table 2 provides linear attenuation coefficient data for two possible filter metals, Copper and Tin.
Table 2
| Energy / keV\text{keV}keV | Linear attenuation coefficient of Copper / cm−1\text{cm}^{-1}cm−1 | Linear attenuation coefficient of Tin / cm−1\text{cm}^{-1}cm−1 |
|---|---|---|
| 200 | 1.85 | 4.80 |
| 500 | 0.72 | 1.25 |
Discuss whether it is more appropriate to use Copper or Tin to filter the X-ray beam in this scenario. No calculations are required.
State and explain one other method used to limit the exposure of healthy surrounding tissues during X-ray radiotherapy.
23 exam-style questions on AQA A Level Physics Radionuclide imaging and therapy (A-level only), covering Imaging techniques (A-level only), Half-life (A-level only), Gamma camera (A-level only), Use of high-energy X-rays (A-level only), Use of radioactive implants (A-level only), and Imaging comparisons (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.