Video: Understanding X-Ray Therapy

Do the X-rays used in X-ray therapy have more energy, less energy, or the same energy as those used in X-ray imaging?

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Video Transcript

Do the X-rays used in X-ray therapy have more energy, less energy, or the same energy as those used in X-ray imaging?

Okay, so what we’re being asked to do here is to compare the X-rays used in X-ray therapy and X-ray imaging. Now, first of all, let’s start by thinking about what X-rays actually are. X-rays are a form of electromagnetic radiation. And we can recall that on the electromagnetic spectrum, X-rays slot in very nicely between ultraviolet light and gamma rays. Where the electromagnetic spectrum that we’ve drawn here is incomplete, there’s a few other kinds of electromagnetic waves on this side. And also, we’ve arranged the electromagnetic spectrum drawn here in terms of increasing frequency as we move towards the right.

X-rays specifically are defined as electromagnetic waves, with a frequency of anywhere between three times 10 to the power of 16 hertz and three times 10 to the power of 19 hertz. At this point, we can also recall the relationship which tells us that the energy carried by one photon of electromagnetic radiation is equal to Plank’s constant, just a constant value, multiplied by the frequency.

And so because of this equation, we can see that X-rays can have a range of energies. The lowest energy that an electromagnetic wave can have and still be called an X-ray, will call this energy 𝐸 subscript low. And, it’s equal to the Plank’s constant ℎ multiplied by the lowest frequency of X-rays. Which is we’ve seen is three times 10 to the power of 16 hertz. And, the highest energy that an electromagnetic wave can have and still be called an X-ray is equal to Plank’s constant multiplied by the highest frequency of X-rays. Which is three times 10 to the power of 19 hertz. Now, the exact values of 𝐸 low and 𝐸 high don’t actually matter. But what does matter is that we see that there’s a range of possible energies that X-rays can have. And in this question, we need to try and work out whether X-rays used in X-ray therapy are higher energy than X-rays used in X-ray imaging.

To answer this, let’s look at the difference between X-ray therapy and X-ray imaging. In X ray therapy, we start with a patient who, let’s say, might have a cancerous tumor growing in their arm. In order to treat this tumor, the cancer cells that form this tumor must be destroyed. And one way to do this is to send in lots of X-rays towards that tumor. The aim is to destroy as many cancer cells as possible whilst also doing minimal damage to the surrounding tissue. So, this is how X-rays are used in X-ray therapy. They’re targeted at tumors, whether they’re on the outside of the body or the inside of the body, in order to kill the cancer cells that formed these tumors.

So, it’s important that the X-rays specifically move towards the tumor and are absorbed by the tumor. Because if they’re not absorbed by the tumor, then they’re still moving around. Because if the X-rays themselves and the energy carried by these X-rays is not absorbed by the tumor, then the X-rays could potentially move through the rest of the body and damage other parts of it. Which is not something that we want. Additionally, we want to make the energy of these X-rays as high as possible in order to maximize the chances of destroying the cancer cells that formed the tumor. So overall, we want X-rays that have fairly high energy. And also X-rays that will be easily absorbed and therefore not penetrate into the body where we don’t want it to penetrate.

X-ray imaging, however, is quite different. In X-ray imaging, the patient that is to be X-rayed, or at least the part of the body of the patient that is to be X-rayed, is placed in front of a screen. And, X-rays are passed through the patient. Now, the body of the patient will absorb different amounts of X-rays, depending on the composition of the given body part. So, for example, any bone in the patient’s body will absorb more X-rays. And so, fewer X-rays will be able to go through to the screen behind the patient. Whereas flesh will absorb few X-rays and so more is available to go through. And this is all picked up by the screen itself.

The screen reacts based on how many X-rays it’s absorbing at every single point on the screen. And, this allows us to create an image of the insides of our patient. But, the key point is this here. That X-rays should be able to travel through our patient, especially through the fleshy parts. In fact, some X-rays do even travel through the bone and still end up on the screen. It’s just that the bone still absorbs more X-rays than the fleshy parts do.

So, the point is that X-rays should be able to penetrate through our patient. And, the other point is that these X-rays are purely for imaging. We are not trying to damage any part of the body. Whether that’s healthy tissue or a tumor. Because in X-ray imaging, we’re just trying to take a picture of what’s going on inside our patient’s body. And so, these X-rays must be low enough in energy that they don’t do any serious damage to our patient.

So, when we compare X-ray therapy to X-ray imaging, we see that in X-ray therapy we want high-energy X-rays to damage the cancer cells that form a tumor. Whereas in X-ray imaging, we want low-energy X-rays that penetrate through the body and do absolutely minimal damage to the body itself. And hence, as the answer to our question, we can say that the X-rays used in X-ray therapy have more energy than those used in X-ray imaging.

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