To what extent can developments in particle therapy revolutionize cancer treatment?
Teymour Taj
Cancer is a serious threat to life in every nation. In 2018, there were 17 million 1 new diagnosed cancers worldwide, placing a significant burden on the wellbeing of the patients, their friends and family, as well as on healthcare systems. The main methods of treating cancer are chemotherapy and radiotherapy, if surgery is not possible. However, in their current forms, these treatments can be highly toxic to healthy tissues and organs, giving patients unpleasant and long-term side effects which can even lead some to discontinue treatment. Fortunately, research has led to the discovery of novel cancer treatments which may be more effective at controlling the illness, as well as causing less damage to healthy cells. In this essay I will discuss the set of new radiotherapy treatments using subatomic particles, why they are needed and their viability as an effective treatment for cancer. Conventional external radiotherapy involves the use of ionizing X-ray or gamma radiation to target tumours. The radiation damages the DNA of cancer cells, preventing them from functioning or reproducing, and leading to cell death. There are two main mechanisms for this: direct damage, and indirect damage through free radicals and other reactive species. Direct damage involves gamma or X- ray photons breaking bonds in either strand of the DNA double helix. The most lethal form is a double- strand break, which results in cell death if it is not fixed. Single-strand breaks are passed on after cell division, allowing DNA damage to accumulate until the cells die. Indirect damage involves the formation of reactive species known as free radicals through the breaking of bonds in molecules such as water. The fission of covalent bonds in water can lead to the formation of hydroxyl radicals (•OH), one of the most common reactive oxygen species (ROS). This radical is the most involved in DNA damage because it targets labile hydrogen atoms, such as those in the sugar-phosphate backbone of DNA, as well as the electron-rich π -bonds present in its bases. This process is known as oxidative damage.
The main ways the radiation ionizes the molecules in the body is via the Compton and photoelectric effects. 2 These are two effects involving the interaction between photons and electrons in atoms. The photoelectric effect is when an incident photon is completely absorbed by an electron. The electron gains energy from the photon, causing it to move up energy levels in the atom and eventually freeing it completely. Any remaining energy unused in this process is stored in the free electron’s kinetic energy. The
Figure 5: A diagram of the Compton effect
Compton effect (Fig. 1) involves interactions between loosely-held outer shell electrons and incoming photons. The photon transfer some of its energy to an electron, which recoils. The photon is also scattered, and its wavelength increases due to this loss of energy. The change in wavelength depends on the original wavelength as well as the angle at which the photon is incident on the electron.
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