A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: External Treatment Using High-Energy X-Rays and Methods to Limit Exposure to Healthy Cells — mark scheme explained
The short answer
External beam radiotherapy (EBRT) is a crucial method in medical physics used for treating cancer. This technique involves the use of high-energy X-rays to target and destroy cancerous cells while minimizing damage to surrounding healthy tissue.
The question
Explain how rotating the X-ray source about the tumour limits the dose received by healthy tissue. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
The tumour is placed at the centre of rotation of the X-ray source.
- S2
As the source rotates, the beam always passes through the tumour, so the tumour's dose builds up continuously.
- S3
Each region of surrounding healthy tissue is crossed by the beam only briefly during the rotation.
- S4
Therefore the tumour receives a high total dose while the healthy tissue receives a much lower dose.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
The tumour is placed at the centre of rotation of the X-ray source.
- S2
As the source rotates, the beam always passes through the tumour, so the tumour's dose builds up continuously.
- S3
Each region of surrounding healthy tissue is crossed by the beam only briefly during the rotation.
- S4
Therefore the tumour receives a high total dose while the healthy tissue receives a much lower dose.
Final answer: The X-ray source is rotated about the patient with the tumour at the centre of rotation. The tumour stays in the beam throughout the rotation and so receives a high cumulative dose, while any given region of healthy tissue is only in the beam for a small fraction of the rotation and receives a much lower dose.
Common mistakes
- Saying the tumour is at the centre of rotation but not explaining why healthy tissue is spared — Always state that because the tumour is at the centre of rotation it stays in the beam continuously and receives a high cumulative dose, whereas each region of healthy tissue is in the beam only briefly during the rotation and so receives a much lower dose.
- Relying only on 'cancer cells are more sensitive' as the reason healthy tissue is spared — Lead with the physical method: rotating the source (or multiple beams from different angles converging on the tumour). Differential sensitivity is only background context, not the main marking point for 3.10.6.4.
- Forgetting the role of collimation/beam shaping — Mention that the beam is collimated and may be shaped to match the tumour outline so that radiation is confined to the tumour region, further reducing dose to surrounding healthy tissue.
- Not explaining why high-energy X-rays are chosen — Explain that high-energy X-rays (4 MV to 25 MV) are highly penetrating, so they can deliver a sufficient dose to deep-seated tumours.
- Describing multiple beams without saying they converge/overlap at the tumour — State that the beams are directed from different angles so they all converge on the tumour: the doses add at the tumour while healthy tissue along any one beam receives only a fraction of the total dose.
- Bringing in off-spec clinical techniques as the required method — For 3.10.6.4 the required answer is the rotating-source/multiple-beam geometry plus collimation. Mention advanced techniques only as optional context, not as the main method.
Where the marks go
- Full worked solution (all marking points)4 marks