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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, ensure you provide clear, step-by-step reasoning. For comparison and evaluation questions, clearly identify and explain both similarities and differences, as well as pros and cons.
What the command words demand
- Explain
- Provide a detailed account or interpretation of how something works or why it happens.
- Describe
- Give a detailed account of characteristics, features, and processes without necessarily providing an explanation.
- Compare
- Identify similarities and differences between two or more items or concepts.
- Contrast
- Highlight the differences between two or more items or concepts.
- Evaluate
- Assess the advantages and disadvantages, benefits and limitations, of a particular method or technique.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for each question to allow time for careful reading, planning, and writing a detailed response.
- The tumour is placed at the centre of rotation of the X-ray source.1 mark
- As the source rotates, the beam always passes through the tumour, so the tumour's dose builds up continuously.1 mark
- Each region of surrounding healthy tissue is crossed by the beam only briefly during the rotation.1 mark
- Therefore the tumour receives a high total dose while the healthy tissue receives a much lower dose.1 mark
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.
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain how using several X-ray beams from different directions reduces the dose to healthy tissue compared with a single beam.
- A single beam strong enough to treat the tumour would deposit the same high dose in the healthy tissue in front of and behind it.1 mark
- Instead several weaker beams are aimed from different directions, all converging on the tumour.1 mark
- At the tumour the beam doses add up to the required high dose.1 mark
- Each beam passes through different healthy tissue, so any one region receives only a small fraction of the total dose.2 marks
Final answer: Several beams are directed from different angles so that they all converge on the tumour. Their doses add together at the tumour to give the required high dose, but each beam passes through different healthy tissue, so no single region of healthy tissue receives more than a fraction of the dose.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Saying the tumour is at the centre of rotation but not explaining why healthy tissue is spared
Why it happens: Students learn that the source rotates about the tumour but stop there, without making the dose argument that earns the marks.
Fix: 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
Why it happens: Students reach for the radiobiology (rapid division, poor repair) instead of the physics method AQA expects for external X-ray treatment.
Fix: 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
Why it happens: Students focus on rotation alone and overlook that the beam must also be confined to the tumour region.
Fix: 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
Why it happens: Students state that X-rays are used but do not link the high energy to penetration of deep tumours.
Fix: 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
Why it happens: Students mention beams from several directions but miss that the doses must add up at the tumour while each healthy-tissue path gets only a fraction.
Fix: 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
Why it happens: Students recall IMRT, proton therapy, SRS/SBRT or MLCs from wider reading and present them as the AQA answer.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Explain Radiotherapy Dose | Explain how rotating the X-ray source concentrates dose on the tumour while sparing healthy tissue. | 4 |
| Explain X-ray Therapy | Explain how converging multiple beams delivers a high tumour dose while sparing healthy tissue. | 5 |
| Explain Radiotherapy Choice | Explain why high-energy X-rays are suited to treating deep-seated tumours. | 4 |
| Explain Radiotherapy Principles | Explain why high-energy X-rays suit deep tumours and how source rotation spares healthy tissue. | 4 |
| Explain Beam Collimation | Explain how shaping the X-ray beam reduces radiation dose to surrounding healthy tissue. | 4 |
| Total across these question types | 21 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.