External treatment using high-energy X-rays. Methods used to limit…

The idea

External radiotherapy directs high-energy X-rays, typically in the megavolt range, at a tumour from outside the body. These photons are ionising and damage DNA, so rapidly dividing cancer cells are killed preferentially, but healthy cells in the beam are also affected. To limit exposure to healthy tissue, the beam is shaped and aimed so that it converges on the tumour while sparing surrounding structures. A multi-leaf collimator shapes the field, and the patient is immobilised in a reproducible position. Several beams from different directions, or a rotating beam, each deliver a smaller dose so the tumour receives a high total dose while healthy tissue receives less. Treatment planning uses CT or MRI images and dose calculations, and fractionation spreads the total dose over many sessions so healthy cells can repair between exposures.

What earns the marks

• High-energy X-rays are ionising and can damage DNA; because cancer cells divide rapidly they are less able to repair this damage than most healthy cells, so they are preferentially killed.
• A multi-leaf collimator shapes the X-ray beam to match the tumour outline, reducing the volume of healthy tissue irradiated.
• Using several beams from different directions, or a rotating beam, concentrates dose at the tumour where the beams overlap while spreading the entrance and exit dose over more healthy tissue.
• Fractionation divides the total dose into many smaller treatments, allowing healthy cells time to repair sub-lethal damage between fractions while tumour cells accumulate damage.
• Accurate patient positioning, immobilisation and imaging-based treatment planning ensure the beam targets the tumour and avoids critical organs.
• The beam energy is chosen so that the maximum dose is deposited at the tumour depth rather than at the skin surface, reducing skin and superficial tissue damage.

Common mistakes and how to put them right

• Thinking that healthy cells are completely unaffected.

  Put it right: They do receive dose, and the aim is to limit it while accepting some exposure.

• Confusing external beam radiotherapy with brachytherapy or with diagnostic X-ray imaging.

  Put it right: External treatment uses a beam from outside the body at therapeutic energies.

• Assuming that a single high dose is always best.

  Put it right: Fractionation is used precisely to allow healthy tissue repair and improve the therapeutic ratio.

• Believing that the collimator focuses X-rays.

  Put it right: It absorbs parts of the beam to shape the field, it does not refract or focus the radiation.

Exam tips

• Link each exposure-limiting method to its mechanism: collimation reduces irradiated volume, multiple beams spread dose, fractionation allows repair.
• Use the term 'therapeutic ratio' when discussing the balance between tumour control and healthy tissue damage.
• When describing beam direction, explain that the tumour lies at the intersection of beams so it receives the summed dose while each healthy region receives only one beam's dose.
• Refer to treatment planning with imaging and dose calculation rather than implying that the beam is aimed by eye.