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Phd student Matthew Nicol describes radiotherapy using X-rays and proton beams.
Radiotherapy is a medical application in nuclear physics, where we irradiate diseased tissue cells with beams of particles. There are several different types of radiotherapy, but today I’m going to talk to you about proton therapy. That’s where we use a beam of protons to irradiate diseased tissue – for example, a cancerous tumor. Tissue cells in our bodies are made up of molecules. If we zoom in on a molecule, we will see that it is simply when a group of atoms are bonded together. Now, if we zoom in on that atom as well, we’ll see in the very center a nucleus, and orbiting around are negatively charged electrons. Protons have a positive electric charge.
You probably already know that when we have a positively charged particle and a negatively charged particle, they are attracted to one another. So when a proton from our proton beam comes close to the atom, it can drag away some of those negatively charged electrons. And we call that process ionisation. Ionisation damages the molecules inside tissue cells. When DNA is damaged, the cell cannot work properly. More importantly, it cannot replicate and therefore can’t multiply. So one way to treat cancer is to fire a beam of protons at the cancerous tissue. The ionisation damages those cancerous cells and stops them from multiplying any further.
BUT THERE’S A PROBLEM: What is to stop the beam of protons damaging healthy tissues, as well as the diseased ones? This is where we use a property of proton radiation called penetration distance. You’ve probably heard of x-rays. They’re a type of radiation made up of photons of light. They ionise atoms and damage the tissue all the way along their path, making it difficult to avoid those healthy tissue cells. So to kill cancer cells with x-rays, we have to overlap beams from several different directions. That way the cancer cells get enough exposure to suffer damage, but the healthy cells only get a low dose. But protons do most of the damage over a small distance, and then stop.
We call this the penetration distance. This prevents them from damaging healthy tissue cells further along their path. Now here’s the really important part. This penetration distance is dependent on the energy of a proton beam. By carefully selecting this energy, we can choose how far into the body the proton does its work before stopping. We can accurately target the protons to cause maximum damage to the disease cells while causing as little damage as possible to those healthy cells. To produce a beam of energetic protons, we first need a source of protons. The simplest source is hydrogen. It’s just a proton and an electron, and we can use an electric field to strip off those electrons, leaving just a proton behind.
To get the protons to high energies we need to accelerate them. There are two main ways we do this, a cyclotron or a synchrotron. Both methods use an electric field to accelerate the protons and a magnetic field to guide them. Cyclotrons gradually accelerate protons in a spiral path, getting closer to the outside edge until they’re ready to be extracted. Cyclotrons are great at producing a large continuous current of protons, but they can only produce it of a fixed energy. Synchrotrons guide the protons around in a fixed diameter loop. Because of this, they cannot produce a continuous stream. The benefit is that we can produce protons of different energies, making them more adaptable to different patients’ needs.

As well as playing an important role in developing medical imaging technologies, particle physics is also vital in developing treatments, particularly for diseases such as cancer.

In the video above, PhD student Matthew Nicol introduces radiotherapy – a form of cancer treatment that kills cancer cells by depositing large amounts of energy into the cells, thus ionising the atoms in the cell DNA. X-ray photons or protons are used as sources of ionising radiation.

The X-rays used for radiotherapy are highly penetrating and highly ionising. They have enough energy to radicalise (tear an electron away from) oxygen and hydrogen pairs (known as hydroxyl groups) in cell DNA. This radicalisation eventually leads to the cell not being able to replicate or repair itself and the cell dies. This is desirable when the cell is a cancer cell, but one of the problems with X-ray radiotherapy is that the ionisation (or radicalisation) is not exclusive to cancer cells. The X-rays damage healthy cells as well and, as X-rays are highly penetrating, they deposit energy and ionise atoms over a large area.

Proton therapy involves firing highly energetic protons at target cancer cells. These protons directly damage the cancer cell by depositing a large amount of energy into them and, due to their relatively high mass, they scatter only a very short distance before stopping. This minimises damage to the surrounding tissue. Just like X-rays, high energy proton beams ionise atoms as they pass through the body but, unlike X-rays, the doses are relatively low until right before they stop (where they will deposit the majority of their energy). This creates the peak in the relative dose versus depth in tissue, called the Bragg Peak effect.

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Frontier Physics, Future Technologies

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