Radiation therapy or radiotherapy is a treatment that uses ionising radiation to kill or control the growth of malignant cells. It is most often used to treat cancer. This blog covers the physics and biology behind how this actually works in treating cancer along with current advancements in this area.

So what is radiation? It is formally defined as the emission of energy as electromagnetic waves or as moving subatomic particles. If this is as helpful as a porcelain hammer don’t worry, take a look at figure 1. It is a completely out of proportion sketch that shows what beams of sunshine look like as they travel towards Earth. The distance between the two points A and B is called the wavelength. The sun emits radiation with many different wavelengths. The visible light (colours) that we can see is radiation with a wavelength of about 400-700 nm which is roughly 100,000 times smaller than the diameter of a strand of hair. I did say the drawing was out of proportion. Radiation can also be in the form of particles such as electrons, protons and neutrons (subatomic particles or particles that make up the atom).

Now I’m sure you’ve heard of x-rays, and I’m not talking about the cool photo you get after falling and breaking a bone whilst ‘out for a run’ at 2am. X-rays and gamma rays are actually forms of radiation with even smaller wavelengths than visible light. A smaller wavelength equals a higher energy (the beams move at the same speed, just imagine one is on its 4th cup of coffee and the other is doing a degree in physics), and these are the forms of radiation used for radiotherapy. 

In the case of internal radiation treatment a radioactive source is placed inside the body, in or close to the tumour. During external radiation treatment radiation is generated by a linear accelerator which shoots electrons at a metal target in order to release photons. You can imagine a photon as a quantised portion of a beam. But the radiation doesn’t have to be in the form of a photon, it can also be in the form of an electron or proton. This radiation is then precisely targeted towards the area of the patient’s body in need of treatment. 

Radiation can directly or indirectly damage cancer cells. Direct damage occurs when the radiation damages the cancer cell itself. Indirect damage occurs when the radiation comes into contact with nearby water molecules which in turn causes reactions that produce free radicals. Free radicals are unstable and highly reactive atoms or molecules with a positive or negative charge. These free radicals then destroy the cancer cells.

But how do they damage the cancer cells and do they also damage healthy cells? Figure 2 shows a diagram of the structure of DNA. Cancer cells are made up of DNA, in fact all cells in the human body are made of DNA. The radiation damages the DNA by breaking a base pair, making it difficult for repair to take place. And yes, radiotherapy does also kill normal cells but they have what’s called effective DNA repair mechanisms, so up to a point, they can undo the damage that radiotherapy causes. However, cancer cells have defective DNA repair mechanisms which means they will not recover from impairment. Breaking up a patient’s treatment into different sessions (fractionation) gives the normal cells time to repair. Figure 3 displays the quantity of cancer cells vs normal cells after multiple treatments. 

And finally; what are the newest developments in this area of research? Well, in the past, most treatment has been done using photons. Recently however, advancements have been made in particle therapy (using protons or electrons). Proton arc radiotherapy employs rotating sub-beams to deliver a prescribed dose in a continuous arc. This form of therapy remains in experimental stages although its feasibility has been demonstrated. More promising is proton FLASH radiotherapy (PFT) which delivers a very large dose of radiation to the desired area in a very small amount of time (40 Gray/s rather than the current 10 Gray/min). PFT improves on current methods in a myriad of ways. It is more effective in sparing normal tissue while being comparatively effective in killing cancer cells. It is superior at preserving the functionality of organs in the body while also maintaining a shorter treatment time. The side effects felt by a patient who has undergone PFT are usually less severe than those felt by a patient who has undergone another form of radiotherapy. 

And that concludes this blog! I hope you enjoyed this read and learnt something new. Radiotherapy is a salient area of scientific research that has shown much promise in recent years and will hopefully continue to improve the lives of many for years to come.


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