Dosimetry

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In measuring radiation, it is necessary to discriminate among emission, exposure, and dose. Emission is the rate at which particles are produced or energy is released. One disintegration per second is termed a becquerel (Bq). The curie (Ci) is defined as 1Ci = 3.70 x 1010 Bq. Note that this is approximately the number of becquerels emitted by 1 g of radium. Historically, the Curie was defined in terms of radium decay, although now it is given the fixed value. Also, note that 1 curie emitted by different radionuclides can have different amounts of energy. Thus, the number of curies by itself does not indicate the amount of potential harm that might be caused by a radionuclide. Environmental concentrations of radionuclides are often expressed in picocuries: 1 pCi = 10~12Ci = 0:037 Bq. For example, radon in drinking water is regulated in terms of pCi/L. The U.S. EPA has proposed a maximum contaminant level for radon in drinking water of 300 pCi/L and has an action level in ambient air of 4 pCi/L.

Exposure is defined only for electromagnetic radiation such as g-rays. The SI unit is based on the number of charges of one sign produced by complete absorption in air. The unit is coulombs per kilogram of air. The older unit of exposure is the roentgen (R):

Exposure can occur due to external sources, such as g-, p-, or x-ray emissions from outside the body. Alternatively, they may be from internal sources, such as any radionu-clide that is ingested or absorbed and decays within the body.

The dose refers to the amount of energy actually absorbed. The SI unit is the gray (Gy), which is 1 J/kg. The older unit is the rad. The conversion is 100 rad = 1 Gy. Because of the previously mentioned average ionization energy, exposure can be converted into dosage in either system of units assuming 100% absorption of the energy:

For tissues this conversion is approximate and may vary with type of tissue and g energy. Exposure and dose have frequently been confused with each other, partly because in the units of roentgens and rads they are numerically similar. Exposure refers only to ionization that would occur in air due to electromagnetic radiation such as g- or x-rays. It is the radiation that would be measured by an external dosimeter, such as a film badge clipped to the shirt of a worker. However, it only indirectly indicates how much radiation has been absorbed by the worker. Dose applies to energy absorbed by any material and due to any type of radiation.

The differing ability of various emissions to damage living tissue is accounted for by what is called the relative biological effectiveness (RBE) in biology. It is defined as the ratio of the toxicological effect of the radiation in question to the effect produced by g radiation of the same dose. The value depends not only on the radiation but on the type of endpoint (e.g., cancer, cell necrosis). Because the RBE is difficult to determine for human health effects, the relative rate at which energy is deposited in water by the emission, compared to g radiation, is used in place of RBE. This ratio is called the radiation weighting factor (Wr; formerly the quality factor). Thus; g radiation has a Wr value of 1. The Wr of a radiation is about 20. The Wr for p is also about 1. The product of dose and Wr gives the dose equivalent (H), which is measured in sieverts (Sv); Gy x Wr = Sv. (In the older measurement units, rad x Wr =rem. Then 100 rem = 1 Sv.) An interpretation of dose equivalent is that it is the dose of g radiation that would produce the same amount of tissue damage as the radiation under consideration.

Dose equivalent, however, still does not provide enough information to estimate the risk of cancer. The reason is that all parts of the body rarely receive the same exposure, and in the case of ingested radionuclides, they often concentrate in various organs or tissues with varying sensitivities. For example, dental x-rays are concentrated on the head and neck area, and ingested radioactive iodine tends to concentrate in the thyroid gland. To account for factors such as these, dose equivalent is multiplied by a tissue weighting factor, wt, which is the ratio of the cancer risk to the organ per sievert divided by the cancer risk per sievert to the whole body. The product of dose equivalent and wt is called the effective dose equivalent (HE). The wt for the gonads is 0.20; for bone marrow, colon, lung, and stomach it is 0.12; for the bladder, breast, liver, esophagus, and thyroid it is 0.05; for the skin and bone surface it is 0.01; for the rest of the body it is 0.05. The effective dose for parts of the body are computed separately because the dose received by the different parts are rarely the same. The sum of the effective dose equivalents for all the parts of the body is used to compute overall effective dose equivalent. This value is then used to compute a person's risk of cancer. Table 21.14 summarizes the units used in radiation dosimetry.

The dosage values described so far are in units of energy deposited per unit mass of tissue. However, the dosage may decrease with time because of the elimination of the radionuclide by radioactive decay, by biological elimination, or by both. The decrease

TABLE 21.14 Units and Conversion Factors Used in Radiation Health Measurements'1

Fundamental Units Traditional Units SI Units

Disintegration Rate

1 disintegration/s 1 becquerel (Bq) 27.0 picocuries (pCi)

Exposure (g- or X-Rays): Number of Charges of One Sign Produced by Complete Absorption in Air 2.58 x 10-4 C/kg 1 roentgen (R) 2.58 x 10~4 C/kg

Dose: Amount of Energy Actually Absorbed 1 J/kg 100 rad 1 gray (Gy)

Dose Equivalent: Dose x RBE or Dose x Wr: Capability to Damage Living Tissue

100 rem 1 sievert (Sv)

Effective Dose Equivalent: Dose Equivalent x Tissue Weighting Factor

100 rem 1 sievert (Sv)

"Units on the same row are equal to each other.

is a first-order process, with an effective rate constant, kE, equal to the sum of the rate constants for radioactive decay, kR, and for biological elimination, kB. The initial rate at which energy is being deposited in a tissue is D0. Then the amount that will be deposited in the first t time units will be

The dose that is deposited over a lifetime (assumed to be 50 years for adults and 70 years for exposure to children) is called the committed effective dose (CED). For an infinite time period, or for practical purposes when the lifetime is four or more times the effective half-life (1/kE), the committed effective dose is equal to the total dose:

Internal nuclides with long effective half-lives do not change appreciably in concentration in the body over a person's lifetime. In such cases it is appropriate to expressed the dosage as a rate of energy delivered per mass of tissue per unit time, D0.

Example 21.2 131I is a p emitter with a half-life of 8 days and a 180-day biological halflife in the thyroid. Thus, kR = 0.693/8.07days = 0.0859day-1 and kB = 0.693/ 180 days = 0.00385 day-1. Then kE = 0.0897 and the effective half-life is 7.72 days. About 60% of the iodine in the body concentrates in the thyroid gland. If a person receives 3 mCi of 131I, what will be the effective dose rate to the thyroid? What will be the effective dose delivered over the first 15 days, and what will be the total dose?

Answer From Table 21.13, 131I produces 0.582 MeV per disintegration; 60% of the total dose results in (0.6)(3) mCi = 1.8 mCi dose to the thyroid. Thus, the dose rate will be o „.n / 3.7 x104Bq\ /0.582MeV\/1.6 x 10-13J\ /86,400 s\ ^ ,„4l/1

This is also the dose equivalent, H, since the radiation weighting factor can be taken as 1.0 for p emissions. Also, since we are dealing with a known dose to an organ rather than a whole-body dose, a tissue weighting factor is not needed and the dosage we are computing is the effective dose equivalent, HE. Assume that the mass of the thyroid is 25 g. Since the penetration depth of p emissions is much smaller than the thyroid, one can assume that all the energy is deposited in that organ. Thus, the initial dose rate, D0, will be

D0 = 536 x 10 4j/day = 0.0214 Gy/day = 0.0214 Sv/day 0 0.025kg 111 11

The dose delivered over the first 15 days will be

0.0214 Sv/day r 1w

D =-'—f-[1 - exp(-0.0897 day-1)(15 day)] = 0.177 Sv

0.0897 day 1

Over the long term, the total dose will be

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