How Do We Measure the Biological Effects of Internal Emitters?
How Do We Measure the Biological Effects of Internal Emitters?The general principles just described require further refinement when applied to doses from internal emitters.
What information is needed to calculate absorbed dose of a radionuclide inside the body?Calculating the absorbed dose from a radionuclide inside the body is complex since it involves both the physics of radioactive decay and the biology of the body's metabolism. Six important factors that must be considered are these:
How varied are the types of radiation that different radionuclides emit?Radionuclides can emit several types of radiation (e.g., gamma rays, beta or alpha particles). Each radionuclide emits its own unique mixture of radiations; indeed, scientists identify radioactive materials by using these unique mixtures as if they were fingerprints. The mix of radiations for a specific radionuclide is always the same, regardless of whether the radionuclide is located on a bench in a physicist's laboratory or inside the human body. This means that the type of radiation of each radionuclide can be measured outside the body with great precision by laboratory instruments. A quality factor, discussed earlier, is used to adjust for the difference in the biological effects of different types of radiation.
What determines how long a radionuclide will irradiate the body?The combination of the physical and biological half-life (the effective half-life) determines how long a radionuclide will continue to pump out energy into surrounding tissue. If the physical and biological half-lives of a particular chemical form of a radionuclide are very long, the radionuclide will continue to expose an individual to radiation over his or her lifetime. The total lifetime radiation exposure, expressed in rem, is called the committed dose equivalent.
The physical half-life is the length of time it will take for half of the atoms in a sample to decay to a more stable form. The physical half-life of each radionuclide can be measured precisely in the laboratory. A shorter half-life means that the miniature power source will "run down" sooner. Sometimes, however, a radionuclide will not decay immediately to a stable form, but to a second, still unstable, form. A full calculation, therefore, must also include the types of radiation and physical half-lives of any decay products.
The biological half-life does not depend on the radionuclide but rather on the chemical form of the radionuclide. One chemical form of the radionuclide might be rapidly eliminated from the body whereas other chemical forms may be slowly eliminated.
To measure the biological half-life of a particular chemical form of a radionuclide, that chemical form needs to be studied in animals. Since the biological processes of different animals vary considerably, an accurate determination of the biological half-life requires that each chemical form of the radionuclide be studied in each animal of interest. Prior to studying a chemical form of a radionuclide in a human being, animal studies are performed to get some idea of what to expect.
Once the results of animal studies are available, scientists are able to predict what amount of that chemical form of the radionuclide can be safely injected into humans. An accurate determination of what fraction of each chemical form of the radionuclide accumulates in each organ and how long it stays in each organ in humans can only be determined by studying humans. These type of studies are called biodistribution studies.
What is the tissue weighting factor?Some chemical forms of radionuclides are highly concentrated in one small organ (e.g., iodine in the thyroid gland). When this happens, that organ will absorb most of the radiated energy, and little energy will be deposited in the remainder of the body. Thus, for each chemical form of a radionuclide, there is an organ that will receive the highest dose from that radionuclide. Since organs also vary greatly in their sensitivity to radiation, the biological consequences of the radiation dose differ depending on the organ. This difference in sensitivity to radiation is represented by what is called a tissue weighting factor.
What is the difference between committed equivalent dose and committed effective dose?An estimate of the risk posed by a radionuclide in the body depends on its chemical form, its biodistribution, its physical properties (how it decays), and the sensitivity of the organs exposed. When all these factors are considered in the calculation of risk for a single radionuclide, the total lifetime exposure is called the committed equivalent dose. If more than one radioisotope is present, the sum of all the committed equivalent doses is called the committed effective dose. Both are expressed in rem or the more modern units sieverts.108 These calculations provide a basis for comparing the risk posed by different isotopes.
How do radiation risks compare with chemical risks?It should be noted that radiation is not the only possible hazard resulting from the medical use of radionuclides. Few radioisotopes, whether intentionally or accidentally introduced into the body, enter in a chemically pure form. The radioactive atoms are usually part of a larger chemical compound. The chemical form of the radioisotope may pose its own hazards of chemical toxicity. Chemical toxicity depends upon the chemical effect of the compound on the body, quite independent of any effects of radiation. Determining chemical toxicity is an entire field of science on its own.