The physics of proton therapy

A brief explanation of the physics behind the treatment that Ashya King’s parents wanted for their son’s cancer

Energy loss of radiation in tissue
Schematic representation of the energy deposited by various forms of radiation as they pass through tissue Photograph: Johannes Gutleber/CERN

The story of Ashya King and his parents is desperately sad. One feature is that his family want him to receive proton therapy. This is not a “magic bullet” treatment and without knowing his medical details it is impossible to say what the risks and benefits would be for Ashya. However, proton therapy is in use around the world, and indeed on the NHS, and does in some cases have some huge advantages over other treatments. This article makes no comment on the specific case, it is simply an explanation of the physics behind the therapy.

Cancer treatment with radiation, or particle, beams relies on the fact that the beams scatter off the atoms and molecules in the tissue of the tumour, and this damages, and hopefully kills, the tumour.

The problem is that the beams are dumb. They don’t know or care whether a given molecule is part of a tumour, or part of some healthy tissue, perhaps in a vital organ. As they pass through the body of the patient, they will scatter off molecules, leaving their energy behind, and damaging everything as they go. The trick is to make the beams lose most of their energy in the tumour, and as little as possible in the healthy tissue they have to pass through on the way in and out of the patient.

This can be done to a large extent by careful targeting of the beam, triangulating on the tumour. But, in the plot above, you can see the advantage that protons, or indeed carbon ions, have over X-rays. X-rays are energetic photons (quanta of light). They have no mass, and as they pass through the tissue they lose energy more-or-less continuously (see the blue line in the plot above). They also scatter around, and the beam spreads out.

Because protons, carbon ions and other hadrons are heavier, they behave differently. They interact mostly with the outer electrons of an atom or molecule, and because they are hundreds or thousand of times heavier than an electron, they hardly change direction when they hit them, and they lose very little energy. Imagine a snooker ball travelling through a cluster of table-tennis balls, perhaps.

In fact the energy lost by the proton (and thus deposited in the tissue) increases as the speed of the proton drops, roughly like one divided by the square of the speed. At some point, when the speed gets very low, the energy loss increases catastrophically, and most of the energy is lost in a small volume of tissue. This rapid loss of energy is called the Bragg peak (see the green and red lines in the plot above), and if you can make it overlap with the tumour, this is perfect. The tumour dies, and minimal damage is done to surrounding healthy tissue.

This is particularly important for cancers near the brain, and for cancers in chidren, where cell-division is still happening a lot and is very sensitive to radiation damage.

A few percent of the protons will also interact with the nuclei, sometimes smashing them up and leading to secondary radiation.

Major challenges include delivering a beam with the precise energy and angle required, measuring the position of the tumour, the beam and the Bragg peak inside the patient, monitoring secondary doses, and more.

The NHS in the UK does have a treatment centre for some cancers, and will cover treatment abroad for some others. Major new centres are planned in London and Manchester.

Rob Appleby, a particle physicist who researches in this area, wrote this about the topic for Life and Physics back in 2011.

Jon Butterworth’s book, Smashing Physics, is out now. A bunch of interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.