Eli5 How does radiation therapy actually work? Like how doesnt it break down more of the body and make the cancer worse or something like that? And does it target only cancer area or everything?
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Radiographer here! I have over 5 years of experience working in radiotherapy. I will try to describe how a certain type of widely used radiotherapy works technically.
To clarify, a radiologist is, broadly speaking, a doctor who looks at medical images and diagnoses things. A radiographer is, broadly speaking, the technician operating medical imaging machines and taking the scans. I work as the latter, but no longer in radiotherapy.
I will briefly describe intensity-modulated radiotherapy, which is one type of several, but probably one of the most common these days.
For a patient to receive this form of radiotherapy, they must first have a type of CT scan taken. CT scans are useful for several things, but very simply, they give us a picture of stuff in your body, and they also tell us how dense those things are (obviously, bone is going to be more dense than liver tissue, for example). The imaging is done in “slices” so that you scan scroll through them, and look at the body from different angles.
Using this CT scan, a radiotherapist doctor will basically draw the target area for the radiotherapy, which is usually most of the tumor tissue. They will also draw a surrounding “safety” zone, in case the patient or the tumor moves a little during the time the radiation is administered. Also, they’re going to draw the critical organs that must receive as little radiation as possible. This can look like this image:
[https://bmccancer.biomedcentral.com/articles/10.1186/s12885-017-3144-5/figures/2](https://bmccancer.biomedcentral.com/articles/10.1186/s12885-017-3144-5/figures/2)
Next, a specially trained medical physicist will make a treatment plan. They do this using special software that can take the info from the CT scan (like tissue density), and calculate how best to deliver the radiation, so that most of it will be delivered to the target area, and least of it to the tissues we want to protect.
Once that’s done, the treatment can begin. The radiation can be delivered in a single occasion, a few occasions, or spread over many occasions (sometimes even 30+ days). This depends on many factors that the doctor responsible for the treatment must be familiar with. There are protocols for different types of tumors in different parts of the body.
An average linear accelerator, machines used in radiotherapy, might look like this:
[https://www.medicaldevice-network.com/wp-content/uploads/sites/23/2019/07/1l-Image-TrueBeam-Radiotherapy-System.jpg](https://www.medicaldevice-network.com/wp-content/uploads/sites/23/2019/07/1l-Image-TrueBeam-Radiotherapy-System.jpg)
The patient lays the table. Remember the CT scan that was taken before, the one necessary for the plan? The technicians will have marked the patient (using tattoos, or small metal markings, for example), creating a reference point. This point is important for many reasons; one is that it lets the radiotherapists position the patient before therapy. Then, the machine will move this table according to the treatment plan.
After this, the technicians will use the linear accelerator to take another CT scan. This one is kinda worse than expensive diagnostic CTs, but its only use is to give us a lower resolution image of the body to make sure we position the person correctly. The technican then sorta “overlaps” this new CT with the old one, checking the patient’s anatomy and matching their current position on the table to the plan position. If you do it well, the patient should be laying on the bed exactly as they were when their initial CT scan was taken. So now we know the radiation will go to the right spot. So, we take a new picture, and make sure the new picture and old picture match up.
Then, the radiation source, the big head of the machine, will start rotating. In this head is a series of individual “leaves” of highly dense material, usually tungsten, which move independently using little motors. These are so dense that they block most of the radiation if they are all closed. They can form shapes to allow radiation to pass through. The software and machine take the plan created by the physicist, and start moving these leaves in a way that, combined with the rotating head of the machine, will mean that the tissue surrounding the tumor will receive less radiation than the tumor itself. It looks like this:
[https://www.youtube.com/watch?v=msX1ypCjkK4&ab_channel=PhoenixCyberKnife](https://www.youtube.com/watch?v=msX1ypCjkK4&ab_channel=PhoenixCyberKnife)
This lasts a few minutes, and then, you’re done. For the day, at least.
Note that it’s not possible for the radiation not to go through surrounding tissue. For example, if you have a lung tumor, some of it will have to go through your skin, ribs, surrounding lung, even your heart. But because the radiation source is constantly rotating around you, and the leaves of tungsten constantly moving, you can effectively minimize how much radiation reaches critical parts of your body.
That is one type of radiotherapy, but there are many others.
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