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How does radiation therapy work?

By: Alex Guebert, University of Calgary

Radiation can be a scary word, conjuring images of nuclear fallout or mild-mannered physicists transforming into hulking green monsters, but radiation therapy is a valuable tool in the cancer therapy toolkit. It is estimated that one in two Canadians will be diagnosed with cancer in their lifetime, and of those, many will receive some form of radiation therapy.

Radiation is energy travelling from one place to another and it is described by how much energy the radiation has. This spectrum includes low energy radiation such as radiowaves, microwaves, infrared, and visible light, but usually when discussing radiation we are referring to higher energies, collectively referred to as ionizing radiation. Ionizing radiation includes high energy ultraviolet light (wear your sunscreen!), x-rays, and gamma rays.

These ionizing forms of radiation have enough energy to remove electrons from atoms, and these electrons become bowling balls that can smash through and break the twisted ladder structure of DNA in cells. Sometimes, cells can repair this damage, but sometimes the damage can’t be repaired or is repaired incorrectly (think grafting two mismatched ladders together). When this happens, there can be a few outcomes: 1) the cell can’t function anymore and dies, 2) the cell cannot reproduce to replace itself at the end of its life, or 3) the cell reproduces the incorrectly repaired DNA. Outcome 3 is a mutation.

Linear accelerators generate a beam of high energy x-rays (about 100x as energetic as the x-ray used to diagnose your broken arm), that is precisely shaped and calibrated to target where the cancerous cells are. The radiation waves collide with atoms in the body, releasing electrons, damaging DNA, and killing cancerous cells.

Along the way, some healthy cells are damaged as well because it is unavoidable to have the radiation pass through healthy tissue en route to the tumour, but there are techniques to keep this to a minimum. First of all, the beam of radiation is precisely shaped by sliding leaves of tungsten metal placed in the head of the linear accelerator. These leaves create a shadow, blocking the beam, and can slide to create a precisely shaped hole that the beam can pass through. Secondly, depending on where the cancer is, healthy cells are better at repairing damage than cancerous cells. To take advantage of this, we can deliver the radiation therapy over multiple days, giving healthy cells more time to repair themselves between treatments. Additionally, we can use more beams coming from different angles, so we can spread out lower radiation doses to healthy tissues while concentrating the radiation on the target. You can think of this like multiple spotlights on a stage performer: if there are multiple spotlights on the performer, the performer will be brightly lit up, where the spotlights all overlap, while the area around the performer will be dimly lit. Likewise, we can minimize the radiation dose to healthy cells by spreading out lower doses over a larger area.

Over the last 100 years, radiation therapy has come a long way, largely due to the development of computers. CT scans have provided 3D images of a patient’s anatomy that allow radiation oncologists more certainty in the location of organs and tissues inside the body. The transition from film x-rays to digital allowed for more images to be captured because film wouldn’t need to undergo time consuming developing. This allowed for x-ray imaging to be attached directly to linear accelerators, so that radiation therapists can determine if internal anatomy is aligned correctly immediately before treatment. More recently, magnetic resonance linear accelerators have appeared on the scene. These new machines are capable of taking high resolution images of the patient during treatment, and scientists are researching ways to adapt the beam during the treatment to target the therapy even better. This research uses artificial intelligence and machine learning techniques and will ultimately allow for even better and more personalized radiation therapy treatments for future patients.

Edited by B.G. Borowiec and A.E. McDonald. Header photo by Wikimedia Commons.

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