Lesson Video: Nuclear Radiation in Medicine Physics

Video Transcript

In this video, we will be discussing how nuclear radiation can be used in medicine. And when we say medicine, we don’t just mean the tablets that people ingest. What we’re talking about is medicine, the subject. The study of how to treat people and their illnesses. Now, nuclear radiation can have positive uses, both in diagnosing certain illnesses and also in treating them. Here, we will look at three uses for nuclear radiation in medicine: radioactive tracers, gamma-ray beam therapy, and radioactive implants. So let’s start by looking at radioactive tracers.

A tracer is a radioactive substance that’s either injected into the bloodstream of a patient or ingested by the patient in some form of food or drink. That then ends up tracing the flow of either blood or some other substance, such as water, throughout the body. Because, remember, the tracer is radioactive. And this means that as the tracer moves throughout the body, it emits radiation, which can be detected outside the body. And this is done by detectors placed outside the body. In other words, as the tracer moves around in the body, the radiation detectors on the outside can detect where the tracer actually is.

And because the tracer is flowed with either blood or water or some other substance in the body, the reading on the detectors will help us to find out exactly where in the body the blood or the water or whatever substance we’re following is flowing correctly and where exactly there’s something going wrong. For example, if there’s a blockage of some sort in the bloodstream and a lot of blood is collecting in this region of our patient, then a large amount of the radioactive tracer will collect there, therefore emitting lots of radiation from that point. And the detectors will be able to pick this up. So we can figure out exactly where there’s a blockage in the body or where the substance that’s flowing is not flowing properly.

Now, an important point to be made here is that the radiation emitted by the radioactive tracer must be able to be detected outside the body. And because the tracer itself is inside the body, the type of radiation that the tracer must emit should have a high penetrating power, which basically is the ability of the radiation to penetrate through a material. And in this case, the material that the radiation needs to penetrate through is the human body.

Now, let’s recall that out of the three most commonly discussed types of ionizing radiation — so that’s alpha radiation, beta radiation, and gamma radiation — the one which has the highest penetrating power is gamma radiation. It’s the one that can travel the furthest through materials, which means that is the most likely type of radiation that will be able to get out of the body when emitted by the radioactive tracer. And an added benefit of using gamma radiation for radioactive tracers is that out of these three types of ionizing radiation, gamma is the most weakly ionizing, which means that it’s the least likely of the three types of radiation to interact with cells in the body and end up damaging them. Because if we used alpha or beta radiation, then they would cause a lot of damage to the cells inside the body and often would not even escape the body. So detecting alpha and beta outside the body would be difficult. And on top of that, they would cause a lot of damage to cells in the body. So we’re best-off using gamma radiation.

Now, another point to be made is that a radioactive substance will have some half-life, where half-life is defined as the amount of time taken for half of the radioactive substance to decay. Now, the half-life of the radioactive substance that we use as a tracer needs to be appropriate. So what do we mean by this? Well, firstly, the half-life of the substance that we use as a radioactive tracer should not be too long. Because when we inject a radioactive tracer into the human body or ingest it for that matter, we want to be able to detect the radiation coming from it for a period of maybe a few minutes or at most a few hours, because that’s how long it’ll take the doctors to set up the detectors and scan the patient and also allow enough time for the tracer to flow through the body normally. But if the half-life is too long, then the tracer which will actually remain inside the body after the doctors have finished all their scans will still continue to emit radiation. And so, our patient will remain radioactive for a very long period of time. And that could be days or months or even years, depending on the half-life of the substance that we used as a radioactive tracer.

In other words, our radioactive tracer substance must have a half-life of, at most, a few hours. But then, the half-life can’t be too short, either. If the half-life is on the order of a few seconds, then all of the radioactive substance which is being injected into the body will decay away very quickly, sometimes even quicker than the time taken for the substance to flow around the body. And so from this, we can gather that the half-life of the substance that we use as a radioactive tracer must be at least a few minutes. But ideally, it’ll be a few hours and realistically should not be more than a few hours. And so, that’s where we can deduce about radioactive tracers.

Now, notice that radioactive tracers are used for diagnostic purposes because we send a radioactive substance around the body in order to diagnose any problems that a patient might have, for example, a blockage. The tracer itself is not used to treat any problems that the patient might have. So let’s take a look at another use of nuclear radiation in medicine that is used for treatment as a post-diagnosis.

This second use of nuclear radiation and medicine is known as gamma-ray beam therapy. Now, to understand this use of radiation, let’s first imagine that we’ve got a patient here that has a cancerous tumor somewhere in their body. Let’s say they’ve got a cancerous tumor here, somewhere in their stomach. Well, gamma-ray beam therapy is used to treat a tumor like this. And of course, as the name suggests, we use a radioactive isotope that emits gamma rays. Specifically, the isotope is placed inside a machine, such that when the radioactive isotope emits gamma rays, the machine only allows a narrow beam of gamma rays to pass out of the machine. And the beam itself can be very accurately directed towards the tumor.

Now this is important because we want to reduce the exposure of the rest of the patient’s body to any radiation because at the end of the day, radiation is going to kill living cells. And so, we want to direct it so that it maximally kills tumor cells, cancer cells, whilst minimizing the damage to the rest of the human body. In fact, if we zoom in a little bit to where the patient’s tumor is, and we imagine that the tumor is somewhere inside the patient’s body rather than on the surface on their skin, then a rather clever thing that can be done is to use multiple gamma-ray beam therapy machines to direct multiple beams of gamma rays at the tumor. And the reason that this is done is because then each beam of gamma rays can be weakened massively so that as they pass through the body, each beam of gamma rays causes minimal damage to the healthy cells of the body. But because all of the gamma ray beams are targeted at the tumor, they cause maximum damage within the tumor itself and hopefully end up shrinking and destroying the tumor, thus effectively treating the cancer in the patient.

So now that we’ve learned what gamma-ray beam therapy is, let’s once again very quickly discuss the half-life of the substance that’s used to generate these gamma rays. Let’s imagine that inside our machine, we’ve got a lump of substance that generates gamma rays. Now, this lump of substance must have an appropriate half-life because if the half-life is too short, then it takes very little time for most of the substance to decay. And if most of the substance decays very quickly, then lots of gamma rays are released in a very short period of time. This means that the machine would have to absorb most of them. And only a few would be able to be emitted as part of the gamma ray beam. And so, this is highly wasteful. Plus, the sample won’t last very long. And we’ll have to replace it very, very quickly because most of it is decayed away in a short period of time.

Conversely, the half-life of our substance cannot be very long because if it is, then only a few particles in that substance would decay per unit time. And so, only very few gamma rays will be released per unit time, which means we might not be able to get a strong enough beam to emanate from the machine. And hence, there’s a balancing act here as well. The half-life must be just right. It cannot be too long or too short. Anyway, let’s move on to looking at another use for radioactive substances in treating illness. We will be looking at radioactive implants.

Let’s imagine once again that we’ve got a patient who has a tumor somewhere in their stomach. Well, a doctor can choose to place radioactive implants very close to this tumor, sometimes even inside the tumor, where the radioactive implants are represented by these small blue dots. Because these implants, obviously made up of some radioactive substance, are usually quite small, shaped like seeds or little rods. Now, these implants are placed in the body for an extended period of time, maybe something like a few days or a few weeks even. And because they’re placed very close to a tumor or even inside the tumor, what they do is that they emit radiation over an extended period of time, a few days or a few weeks and are hence attacking the tumor over a long time. This means that the implants can release small amounts of radiation constantly for a few days or weeks. And that is how the tumor can be treated.

As opposed to gamma-ray beam therapy, where lots of radiation is sent towards the tumor in a short period of time, radioactive implants emit small amounts of radiation over an extended period of time. Now, because radioactive implants are placed very close to the tumor itself, the type of radiation they emit doesn’t need to be massively penetrating because we don’t need it to come out of the body or, for that matter, to go very far into the body, away from where the implants are. So if we think back once again to the three most commonly discussed types of ionizing radiation, alpha, beta, and gamma, we can recall that out of the three, gamma has the largest penetrating power, as we’ve seen earlier, and alpha has the lowest penetrating power. Conversely, gamma has the weakest ionizing power, and alpha has the strongest ionizing power.

Now, for this particular purpose for radioactive implants, either beta-emitters or sometimes gamma-emitters are used. But hang on! Didn’t we say that we wanted something that wasn’t strongly penetrating and ideally was highly ionizing so that it could kill the tumor quickly? And therefore, shouldn’t we be using Alpha radiation? Well, we might think that makes logical sense. But in reality, alpha radiation is far too dangerous to have inside the body. Because if we were to implant out alpha radiation in the body, then it would cause a lot, a lot of damage, not just to the tumor around it, but possibly to healthy cells as well.

And then, there’s the matter of actually implanting the implants into the body because they have to get there somehow. The doctor has to put them there. And in the process of putting them there, for example, if they were to surgically place them inside, then in that period of time the radioactive substance that was used as an implant would still be emitting radiation. And so, all of this alpha radiation would really, really damage healthy cells in the human body. And therefore, we do not place alpha-emitters in the human body. That’s why in most cases we stick with beta-emitters because beta radiation has a medium ionizing power and a medium penetrating power. And in cases where the tumor is small, we can sometimes use a gamma-emitter as well.

At which point, we can finally discuss the half-life of the substances used as radioactive implants. Now, like we’ve mentioned already, the implants need to be emitting small amounts of radiation over a long period of time, something like a few days or a few weeks. And hence, we need to use a substance that has a half-life of a few days or a few weeks, once again, not too short or not too long for the particular purpose that we need to use it for. And it’s an important point to make: regardless of whatever purpose we want to use a radioactive isotope for, we need to make sure we use the radioactive isotope that has an appropriate half-life.

And with all of that being said, we’ve now looked at three uses for nuclear radiation in medicine. So let’s now get some practice with this by looking at an example question.

What type of radiation does a radioactive isotope need to emit to be useful as a radioactive tracer? A) Gamma and beta radiation. B) Neutron radiation only. C) Beta radiation only. D) Alpha radiation only. E) Gamma radiation only.

Okay, so to answer this question, let’s first recall what we mean by a radioactive tracer. Well, a radioactive tracer is a radioactive substance that’s injected into the human body or ingested with some food or drink that then flows around the body. For example, if it’s injected into the body, then it flows with the blood. And because the substance is radioactive, it gives off radiation as it moves through the body. Because of this, we can place a detector outside the body that detects the radiation coming from inside the body. And that allows us to see that the flow of the substance that we’re testing — in this case, the blood — is normal inside the body.

If something is wrong, however, for example, if we say that there’s a blockage in the flow of the blood here in the body of the patient, then when we place a detector in that region, we will find that lots of radiation is coming from there as lots of blood piles up there and therefore lots of radioactive tracer parts up there. And in other regions of the body, for example, after the blockage, we might only get a few tracers of radiation. So that’s the purpose of a radioactive tracer.

Now, importantly, the radiation coming from the radioactive tracer needs to pass through the body so that it’s detected outside the body. In other words, the type of radiation that the tracer must emit must have a high penetrating power. It must be able to penetrate quite deep into any material because remember, the radiation itself needs to penetrate through a lot of flesh and skin in order to escape the body. Now, at this point, we can see that four different kinds of radiation have been mentioned in the options given to us in the question. We’ve seen alpha radiation, beta radiation, gamma radiation, and neutron radiation, otherwise known as free neutrons.

Well, we can recall that alpha radiation has a very low penetrating power. It cannot penetrate very deep into materials before it interacts with them and is absorbed by material. And on top of this, alpha radiation is highly ionizing. But if we were to inject a substance into the body that produces highly ionizing radiation, then the radiation would very quickly cause a lot of damage inside the body. It would ionize a lot of the cells in the body. And as well as this because alpha radiation cannot penetrate very far, most of it would not be able to escape the body. And so, our detectors would be useless at that point. Therefore, we do not want anything that emits alpha radiation.

And for a similar reason, we can eliminate beta radiation as well. Because even though beta radiation can penetrate further than alpha radiation and is not quite as strongly ionizing as alpha radiation, it’s still not ideal because quite a lot of beta radiation would be absorbed before it got out of the body. And beta radiation is still strongly ionizing enough that it would cause a fair amount of damage in the body. And so, we can eliminate anything that produces beta radiation as well.

Now, when we think about free neutrons, well different substances interact differently with a beam of free neutrons. Nevertheless, neutrons are generally known to have a high penetrating power. They can get quite far through a material. So we might think that a neutron-emitter is a useful isotope in this case. But actually, it turns out that neutrons are quite hard to detect. So even if neutrons were to make their way out of the body, detecting them would be quite difficult. And as well as this, some neutrons would be absorbed by the body, which means that these neutrons would interact with the atoms in the body.

This is problematic because sometimes when neutrons interact with some material, they cause other forms of radioactive decay. In other words, a neutron interacting with the body could result in alpha or beta or gamma radiation being emitted. And this is problematic because, as we’ve already seen, we do not want any alpha or beta radiation within the body for the purposes of simply diagnosing how well a particular substance is moving around inside the body. And for these reasons, we do not want neutron-emitters to be used as radioactive tracers.

And so, it looks like gamma radiation is the best of all worlds. It’s highly penetrating, so we’re likely to detect it outside the body, and also relatively weakly ionizing, so it won’t cause a lot of damage. And hence, our answer is that a radioactive isotope used as a radioactive tracer must emit gamma radiation only.

So now that we’ve looked at an example question, let’s quickly summarize what we’ve talked about in this lesson. We firstly learned about radioactive tracers, which are gamma-emitting isotopes injected in the body of a patient to study the flow of substances, for example, blood. And radioactive tracers are used for diagnosis, in other words, to figure out if there’s some sort of problem with the patient. Their purpose is not to treat illness. Secondly, we saw that gamma-ray beam therapy consists of beams of gamma rays directed into the body of a patient to treat a cancerous tumor. And finally, we saw that radioactive implants are beta- or gamma-emitters placed in the body near a tumor to irradiate that tumor over a few days or weeks. We also saw that in each case, it is important to choose an isotope with an appropriate half-life. And so, that has been an overview on how nuclear radiation can be used in medicine.