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24.4: Applications of Radioactivity

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Lesson Objectives

  • Describe sources of radiation in our environment.
  • Describe the effects of nuclear radiation on living systems.
  • Describe the use of radioisotopes in the diagnosis and treatment of various diseases.

Lesson Vocabulary

  • cancer cell: A cell in which the control processes that regulate cell growth and division are not working properly.
  • PET scan: A medical technique known as positron emission tomography which is used to study processes in the brain.

Check Your Understanding

  • What is the relative penetrating power for each type of radioactive decay?
  • What happens to a positron when it collides with an electron?

Introduction

Previously, you learned about certain devices such as a Geiger counter which are used to measure exposure to radiation. In this lesson you will learn about the negative effects of different radiation sources in our lives, as well as some positive ways in which radiation can be used in the medical field.

Environmental Sources of Radiation

We are all exposed to a small amount of radiation in our daily lives. Much of this exposure is due to naturally occurring radioactive substances and cosmic radiation (literally high energy particles flying in from space). For example, radon is a colorless, odorless gas formed from the decay of various uranium and thorium isotopes, which are found in the soil throughout much of the U.S. As a noble gas, radon is chemically inert, but it is also radioactive and can easily be inhaled into the lungs. Radon exposure is highest in homes that lack good air circulation, which would allow the gas to be cycled out of the residence. Fortunately, there are a number of inexpensive approaches to decreasing your exposure to radon.

Effects of Radiation

Radiation can seriously harm living organisms, including humans. In order to better understand how nuclear radiation causes damage on the cellular level, we should first understand the basics of how the cell works. DNA in the nucleus is responsible for protein synthesis and for the regulation of many cellular functions. In the process of protein synthesis, DNA partially unfolds to produce messenger RNA (mRNA). The mRNA leaves the nucleus and interacts with ribosomes, transfer RNA, amino acids, and other cellular constituents in the cytoplasm. Through a complex series of reactions, proteins are produced to carry out a number of specialized processes within the organism. Anything that disturbs this flow of reactions can damage to the cell.

The most harmful situation is when nuclear radiation does something to alter the structure of the DNA. If this prevents the production of a crucial protein, the cell will malfunction or die. In an even worse scenario, some changes to DNA will cause the cell to become cancerous. In a cancer cell, the control processes that regulate cell growth and division are not working properly. As a result, they grow and divide rapidly, often interfering with the functioning of nearby healthy cells. For example, over time internal radon exposure can lead to the development of lung cancer. This is especially problematic for smokers, who already have exposed their lungs to significant amounts of carcinogens. Tissue damage is also common in people with severe exposure to radiation.

Radioisotopes in Medical Diagnosis and Treatment

Radioisotopes are widely used to diagnose, and sometimes treat, various diseases. For diagnosis, the isotope is administered to the patient and then located in the body using a scanner of some sort. The source of the decay product (often gamma emission) can be located by the scanner, and a map of where the isotope was transported in the body can be generated. This information is often very valuable for diagnosing certain medical problems.

For example, a radioactive isotope of iodine (I-131) is used in both the diagnosis and treatment of thyroid cancer. The thyroid will normally absorb some iodine to produce iodine-containing thyroid hormones. An overactive thyroid gland will absorb a larger amount of the radioactive material. If this is the case, more and more radioactive iodine can be administered, where it will cluster in the diseased portion of the thyroid tissue and kill some of the nearby cells. Cancer treatments often cause patients to feel very sick, because while the radiation treatment kills the unwanted cancer cells, it causes damage to some healthy cells in the process.

Technetium-99m is perhaps the most widely used radioisotope in diagnosis and treatment (the “m” stands for metastable indicating a very short half-life). This isotope decays to Tc-99 by gamma emission. If a very low dose of the isotope is administered, the radiation will be of a very low intensity, so cellular damage will be minimal. Additionally, gamma radiation has a very high penetrating power, so most of it will reach the detector in the scanner. The half-life of Tc-99m is about six hours, so it will remain in the body for some time.

Tc-99m is often used to look at cardiac damage. If there is less blood flow in the heart, there will be less of the isotope concentrated in the heart muscle. Similar information can be obtained for blood flow in the brain.

There are presently over 25 different isotopes in use for diagnosis and medical treatment. A very partial list can be seen in Table below:

Isotope Half-Life Application
Cr-51 28 days labeling red blood cells
Fe-59 446 days study iron metabolism in spleen
Xe-133 5 days study lung function
Ho-166 26 hours cancer treatment

PET Scans

One of the more interesting and useful medical applications of radioisotopes is positron emission tomography (PET), often referred to as a PET scan. This technique is especially useful for studying processes in the brain. Many compounds do not enter the brain because of the blood-brain barrier, which is a particularly selective filter that prevents many substances in the blood from being transported into brain tissue.

In order to get a good picture of what is happening in the brain, radiolabels (radioactive trackers) are attached to different compounds that are known to enter the brain. Since the brain accounts for about 25% of the body's glucose consumption, this molecule is often labeled with a positron emitter, such as F-18 (half-life of 109.8 minutes), to study brain function in general. Other radiolabels are attached to specific compounds that will localize in certain areas of the brain to look at specific structures.

The PET scanner detects gamma emissions from the collision of a positron with an electron. A positron has the same mass but opposite charge of an electron. As the positron is released from the nucleus of the atom, it will collide with an electron. This meeting of matter (electron) with antimatter (positron) results in annihilation of both particles and the release of two gamma photons that travel in exactly opposite directions. The scanning apparatus detects these gamma rays and stores the data in a computer. From this information, a detailed picture of the brain can be developed.

One useful application of PET scanning is in the diagnosis of Alzheimer’s disease. This debilitating condition associated with memory loss primarily occurs in elderly individuals. A protein known as beta-amyloid gradually forms deposits, or plaques, in the brain. Severe memory loss and impaired movement appear to be direct results of the plaque growth.

The compound known as "Pittsburgh compound B" is often used to identify areas of plaque in the brain. The C-11 atom bound to the nitrogen has a half-life of just 20.38 minutes, so administration and detection must be accomplished very quickly.

The label attaches to plaques in the brain and can be observed using a PET scanner.

The computer translates the intensity of the decay from the radioactive isotope into a color scale, with red indicating a high level of radioactivity and yellow indicating somewhat less activity. We can see from the scans that the cognitively healthy individual shows the presence of very little plaque in the brain, whereas the individual with Alzheimer’s has high concentrations of beta-amyloid in numerous areas of the brain.

Other studies have used PET scans to look at certain regions of the brain in drug addicts. One of the theories about drug addiction involves activity related to the molecule dopamine, a chemical that helps carry certain nerve impulses from one brain cell to the next. Studies on dopamine activity in the brain have been helpful in understanding the biochemical processes behind addiction.

Figure above shows the accumulation of radioactive compounds that bind to dopamine receptors. The non-addicted individuals have large numbers of receptors for dopamine. The addicted persons show less binding to these receptors, indicating that fewer receptors are present.

Lesson Summary

  • Everyone is exposed to low amounts of background radiation from medical procedures and naturally occurring radioactive substances.
  • Radioactive emissions can alter the structure of DNA, which can sometimes lead to cell death or even cancer.
  • Radioisotopes are widely used in medical diagnosis. These isotopes need to have short half-lives so that they can be administered in very low doses, thus minimizing damage to cells.
  • Radioactive isotopes are also used in the treatment of certain diseases, such as cancer. These isotopes are administered to only the diseased tissue, with the goal of destroying the unhealthy cells.
  • PET scans are very useful in looking at brain structure and chemistry.

Lesson Review Questions

  1. How are radioactive materials harmful to cells?
  2. Why are cancer cells so detrimental in the body?
  3. Why should radioisotopes given to patients have short half-lives?
  4. Explain how the radioactive isotope of iodine is used to diagnose and treat thyroid cancer.
  5. How is Tc-99m used to diagnose bone cancer?
  6. Why is PET scanning so useful in studying the brain?

Further Reading/Supplementary Links

Points to Consider

  • Can you think of any historical events in which people may have been exposed to dangerous amounts of radioactivity?

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Date Created:

Sep 09, 2013

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Sep 05, 2014
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