Radiotherapy

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.