Topic 6 – Radioactivity — Edexcel GCSE Physics
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Topic 6 – Radioactivity explained
Both routes aim to deliver a high dose to tumour cells while sparing healthy tissue, because ionising radiation damages DNA and dividing cells die.
Read the full explanation
External treatment uses a machine outside the body directing a beam at the tumour; the beam passes through skin and healthy tissue, so several angled beams can converge on the tumour to concentrate dose there, and the patient is not radioactive afterwards. Internal treatment places a sealed source or unsealed radiopharmaceutical close to or inside the tumour, so the dose falls off with distance and nearby healthy tissue receives less, but the patient may emit radiation for a time and needs radiation-protection precautions. Compare penetration, targeting precision, dose to healthy tissue, repeatability, patient isolation and which tumours suit each route.
Your focus
- Compare how external and internal radiotherapy deliver dose to a tumour
- Contrast the dose received by healthy tissue in each method
- Justify a choice of treatment route using physical principles
Topic 6 – Radioactivity exam tips
Marking Points
- Compares external and internal treatment by stating that external beams pass through healthy tissue before reaching the tumour whereas an internal source sits close to or within it
- Contrasts the dose distribution by explaining that an internal source gives a steep fall-off with distance so surrounding healthy tissue receives a lower dose
- States that external radiotherapy uses a machine outside the body and that the patient is not left radioactive after treatment
- Explains that internal treatment may leave the patient emitting radiation for a period, requiring radiation-protection precautions such as limited contact time
- Compares targeting by noting that converging external beams concentrate dose at the tumour while internal placement relies on proximity
- Links both methods to the same underlying mechanism of ionising radiation damaging tumour cell DNA so that dividing cells are killed
Examiner Tips
- 💡Organise the answer as a series of paired comparisons rather than two separate descriptions
- 💡Name the physical quantity being compared, such as dose to healthy tissue or penetration depth, in each comparison
- 💡Finish with a judgement about which route suits which tumour situation and justify it from the physics
Common Mistakes
- States that external radiotherapy makes the patient radioactive; correct this by explaining that the beam source stays in the machine and the patient does not become a source
- Claims internal treatment gives a uniform whole-body dose; correct this by explaining that dose falls steeply with distance from the implanted source
- Treats the two methods as unrelated rather than comparing them; correct this by organising the answer around shared aim and differing dose distribution