High-Energy X-Ray Therapy Revision Notes | A Level AQA Physics

1. Why High-Energy X-Rays Can Destroy Tumours

Cancer cells divide uncontrollably. The goal of radiotherapy is to deliver enough energy to those cells to permanently damage their DNA — stopping division in its tracks — while keeping the surrounding healthy tissue as intact as possible. That tension between killing the tumour and sparing the patient is the central challenge this lesson unpacks.

X-rays used in radiotherapy are far more energetic than the X-rays used in diagnostic imaging. Diagnostic X-rays typically operate in the range of 20–150 kV; therapeutic X-rays (produced by linear accelerators, or linacs) operate at 4–25 MV — where MV refers to the peak accelerating voltage of the linac, not the photon energy directly. At these energies, each photon carries enough energy to ionise atoms throughout the cell; it is damage to DNA in the cell nucleus that is the critical effect leading to cell death.

When an X-ray photon is absorbed in tissue, it ejects a high-energy electron. That electron then ionises further atoms along its path, producing reactive free radicals — particularly hydroxyl radicals from water molecules in the cell. These free radicals attack the DNA double helix, causing strand breaks. If both strands break at the same location (a double-strand break), the cell cannot repair itself and undergoes programmed cell death (apoptosis) or fails to divide successfully.

Tumour cells are often more sensitive to radiation than healthy cells because they divide more rapidly and have less efficient DNA repair mechanisms. However, healthy cells in the beam path are still damaged — which is why the methods used to limit that exposure are just as important as the beam itself.

2. The Core Problem: Healthy Tissue in the Beam Path

Here is the fundamental difficulty. To reach a deep-seated tumour — say, one in the lung or prostate — the X-ray beam must pass through layers of healthy skin, muscle, and organ tissue on the way in. If a single beam is used at full dose, the healthy tissue along that entry path receives the same dose as the tumour. That causes radiation burns, organ damage, and serious side effects.

The solution is not to reduce the beam energy — that would make it less effective at the tumour. Instead, clinicians use a combination of geometric strategies and physical shielding to ensure the tumour receives a high cumulative dose while each individual path through healthy tissue receives only a fraction of that dose.

3. Method 1 — Multiple Beams from Different Angles

The most important technique for protecting healthy tissue is the multiple-beam approach. Instead of one beam, the linac rotates around the patient (or is repositioned) so that several beams, each at a different angle, all converge on the tumour.

Picture it this way: if you use three beams, each beam delivers one-third of the total dose as it passes through healthy tissue on its entry path. But at the point where all three beams overlap — the tumour — the doses add together to give the full therapeutic dose. The tumour is irradiated at high dose; the healthy tissue on each entry path receives only a fraction.

In modern radiotherapy, the linac rotates continuously around the patient in a technique called volumetric modulated arc therapy (VMAT). The beam intensity and shape are adjusted dynamically as the machine rotates, spreading the entry-path dose across a very large volume of healthy tissue — so no single region receives a harmful dose — while the tumour remains at the intersection of all beam paths throughout the rotation.

The chart above illustrates the principle. With a single beam, healthy tissue on the entry path receives 100% of the dose — the same as the tumour. With three beams, each entry path receives roughly one-third. With arc therapy, the entry-path dose per unit area of healthy tissue drops further still, because the dose is spread over the full arc of rotation.

4. Method 2 — Collimation and Beam Shaping

Even with multiple beams, each individual beam must be shaped to match the cross-sectional profile of the tumour. If the beam is wider than the tumour, healthy tissue at the edges of the beam receives unnecessary radiation. Collimation solves this.

A collimator is a set of dense, radiation-absorbing leaves — typically made from tungsten — arranged inside the linac head. By adjusting the position of these leaves, the beam aperture can be shaped to match the irregular outline of the tumour as seen from each beam angle. This is called a multi-leaf collimator (MLC).

Because tumours are three-dimensional and rarely spherical, the shape of the beam aperture changes as the linac rotates to a new angle. The MLC leaves reposition automatically between (or during) beam delivery, ensuring that from every angle the beam footprint matches the tumour's projected outline. Healthy tissue just outside the tumour boundary is shielded by the tungsten leaves and receives a greatly reduced dose.

5. Method 3 — Patient Positioning, Immobilisation, and Treatment Planning

All the geometric precision of multiple beams and collimation is worthless if the patient moves between or during treatment fractions. A tumour that shifts even a few millimetres out of the planned position could mean the high-dose region misses the tumour and irradiates healthy tissue instead.

Patients are immobilised using custom-made shells or moulds — for example, a rigid plastic mask for head and neck treatments — that lock the patient's position reproducibly for every session. Tattoo marks or surface markers are placed on the skin to align the patient with laser positioning guides in the treatment room, ensuring the same geometry is reproduced each day.

Before treatment begins, a CT scan of the tumour and surrounding anatomy is used to create a three-dimensional dose plan. Physicists and oncologists use treatment planning software to calculate the dose distribution across the patient's body for a given beam arrangement. The plan is optimised so the tumour receives the prescribed dose and critical structures — such as the spinal cord, optic nerves, or kidneys — receive doses below their known tolerance thresholds.

6. Method 4 — Fractionation

Rather than delivering the entire therapeutic dose in one session, radiotherapy is almost always delivered in fractions — typically one session per day, five days per week, over several weeks. This strategy is called fractionation, and it exploits a biological difference between tumour cells and healthy cells.

Between fractions, healthy cells repair sub-lethal radiation damage more efficiently than tumour cells. By spacing out the dose, healthy tissue has time to recover between sessions, while the cumulative damage to the tumour builds up. The total dose delivered over the course of treatment can therefore be much higher than would be safe in a single session.

A typical radical (curative) radiotherapy course might deliver 60–66 Gy (gray — the SI unit of absorbed dose) in 30–33 fractions. Each fraction delivers around 2 Gy. The tumour accumulates lethal damage across all fractions; healthy tissue repairs between them.

The methods above do not operate in isolation — they are combined in every modern radiotherapy treatment. The following walkthrough shows how they work together for a patient with a lung tumour.

7. Putting It Together — A Worked Clinical Scenario

Worked example: Radiotherapy Planning for a Lung Tumour

A patient has a 3 cm diameter tumour in the right lung. Describe the sequence of steps taken to deliver a high dose to the tumour while limiting exposure to the surrounding lung, heart, and spinal cord.

1. A CT scan is taken with the patient in the treatment position. The tumour volume and all critical structures (heart, spinal cord, oesophagus) are outlined on the scan by the oncologist. — The CT provides three-dimensional anatomical data. Without this, the dose distribution cannot be calculated accurately. The outlined tumour volume becomes the target; the outlined critical structures become the dose constraints.
2. Treatment planning software arranges, for example, five beams at different angles (e.g. 0°, 72°, 144°, 216°, 288°) all converging on the tumour. Each beam delivers one-fifth of the total dose through its entry path. — At the tumour, all five beams overlap and their doses sum to the full prescribed dose. Along each entry path, healthy tissue receives only one-fifth of that dose. The spinal cord is positioned so no beam passes directly through it.
3. For each of the five beam angles, the multi-leaf collimator leaves are positioned to match the projected outline of the tumour as seen from that angle. Healthy lung tissue at the beam edges is shielded by the tungsten leaves. — Because the tumour is not spherical, its projected shape changes with each beam angle. The MLC adjusts for each angle, ensuring the beam footprint matches the tumour and does not irradiate healthy tissue unnecessarily at the margins.
4. The planning software calculates the dose received by the spinal cord, heart, and healthy lung. If any structure exceeds its tolerance dose (e.g. spinal cord maximum ~45 Gy), the beam angles or weights are adjusted until all constraints are satisfied. — This optimisation step is critical. It may require accepting a slightly lower tumour dose if a critical structure cannot otherwise be protected. The plan is only approved when both the tumour dose and the healthy-tissue constraints are acceptable.
5. The total prescribed dose (e.g. 60 Gy) is delivered in 30 daily fractions of 2 Gy each, Monday to Friday over six weeks. The patient is immobilised in the same position for each fraction using a body mould and laser alignment. — Fractionation allows healthy lung tissue to repair sub-lethal damage between sessions. The tumour accumulates lethal damage across all 30 fractions. Immobilisation ensures the geometry is reproduced accurately each day so the dose is always delivered to the correct volume.

Answer: The combined use of multiple beams (dose spreading), MLC collimation (beam shaping), CT-based dose planning (optimisation), and fractionation (healthy-tissue repair) allows a high cumulative dose to be delivered to the tumour while keeping doses to the spinal cord, heart, and healthy lung below their tolerance thresholds.

8. Exam Technique and Common Mistakes

Questions on this topic appear most often as 'explain' or 'evaluate' questions worth 3–6 marks. The examiners are looking for precise physical reasoning, not vague statements. Here is how to structure your answers and avoid the traps that cost marks.

Every method covered in this lesson — multiple beams, collimation, fractionation, and immobilisation — addresses the same fundamental specification requirement: external treatment using high-energy X-rays, and the methods used to limit exposure to healthy cells. In an exam, demonstrating that you understand why each method works (the physical or biological mechanism) will always score more marks than naming the method.