Radiation Safety and Health Physics

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Dose-Modifying Factors

For health physics purposes, specification of the radiation absorbed dose in grays (see Chapter 22 , Section A) is inadequate for a complete and accurate assessment of potential radiation hazards. Although the relative risk of potential injury increases with increasing absorbed dose values, several other dose-modifying factors also must be taken into account.

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  The part of the body exposed. Total-body exposure carries a greater risk than partial-body exposure. Exposure of major organs in the trunk of the body is more serious than exposure to the extremities. The active blood-forming organs, the gonads, and the lens of the eye are especially sensitive to radiation damage. A superficial dose to the skin (e.g., from an external source of β particles) is less hazardous than the same dose delivered to greater depths (e.g., from an external source of γ rays or from internally deposited radioactivity).

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  The time span over which the radiation dose is delivered. A given number of grays delivered over a short period (e.g., minutes or hours) has a greater potential for damage than the same dose delivered over a long period (e.g., months or years).

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  The age of the exposed individual. Children are more susceptible to injurious radiation effects than are adults. The developing embryo and fetus are especially sensitive.

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  The type of radiation involved. In general, densely ionizing radiation [i.e., high-linear energy transfer radiation (see Chapter 6 , Section A.4)] such as α particles, fission fragments, and other nuclear particles, are more damaging per gray of absorbed dose than is less densely ionizing radiation, such as β particles and γ rays.

The dose-modifying factors in this list are taken into account in preparing regulations and making recommendations for handling of radioactive materials. For example, regulations specify different dose limits for different parts of the body, for different time periods, and for different age groups. To account for the differing hazards of different types of radiation, the equivalent dose , defined previously (see Chapter 22 , Section A), is used. For most of the radiation encountered in nuclear medicine, the equivalent dose in sieverts (or rems) is numerically equal to the absorbed dose in grays (or rads), although it must be emphasized that equivalent dose and absorbed dose are not the same quantity and have different units. To account for differing hazards for different organs and tissue types, the equivalent dose is modified by organ-specific weighting factors to compute the effective dose (see Chapter 22 , Section B.7) to an individual.

In some older texts, and in current United States federal regulations (see Section B ), the related quantities dose equivalent (in place of equivalent dose) and effective dose equivalent (in place of effective dose) may be encountered. The conceptual difference is that equivalent dose is based on the average absorbed dose in a specific tissue or an organ, whereas dose equivalent is based on the absorbed dose at a point in tissue. There also are differences in the scaling factors used to convert the absorbed dose into these quantities. These quantities are summarized in Table 23-1 . Broadly speaking, for nuclear medicine applications, equivalent dose and dose equivalent, as well as effective dose and effective dose equivalent, have similar numerical values.

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Exposure and Air Kerma

For the purpose of describing radiation levels in a radiation environment, an additional quantity— exposure —has traditionally been used. Exposure refers to the amount of ionization of air caused by a γ-ray or x-ray source. The traditional unit of exposure is the roentgen (R), with subunits of milliroentgens (1 mR = 10 ⁻³ R), microroentgens (1 µR = 10 ⁻⁶ R), and so on. An exposure of 1 R implies ionization liberating an amount of charge equal to 2.58 × 10 ⁻⁴ coulombs/kg of air, or approximately 2 × 10 ⁹ ionizations per cc of dry air at standard temperature and pressure. An exposure rate of 1 R /min implies that this amount of ionization is produced during 1 minute. The SI unit for exposure is the coulomb/kg, with no special name. Thus 1 coulomb/kg ≈ 3876 R and 1 R = 2.58 × 10 ⁻⁴ coulombs/kg.

The use of the SI units for exposure is cumbersome, and therefore in the transition to SI units, exposure is being replaced by a related quantity known as air kerma. Kerma stands for k inetic e nergy r eleased in m edi a . Exposure refers to the ionization charge produced in air, whereas air kerma refers to the amount of kinetic energy released in air ( Table 23-1 ). More precisely, the air kerma is the sum of the kinetic energy of all charged particles produced by interactions from a source of x rays or γ rays (through Compton scatter, photoelectric absorption, or pair production) per kg of air. The units of air kerma are grays (J/kg), the same as for absorbed dose. If all of the photon energy transferred to charged particles is deposited locally (in air, bremsstrahlung production is negligible, so this is a reasonable assumption), then the absorbed dose in air has the same value as the air kerma. Using the fact that 33.7 eV of energy is required to produce an ion pair in air (see Table 7-1 ), and assuming bremsstrahlung losses can be ignored, the relationship between exposure, X, and air kerma, K, can be calculated as:

The conversion between traditional units of exposure and air kerma is given by:

Exposure and air kerma are useful quantities because they can be measured using ionization chambers, which are basically ionization-measurement devices ( Chapter 7 , Section A.2). Specific instruments used for health physics measurements are described in Section E .

If the air kerma in Gy is known at a certain location, the absorbed dose in Gy that would be delivered to a person at that location can be estimated by means of a scaling factor, f. This factor is defined as the ratio of the absorbed dose in the medium of interest, D _med , to the absorbed dose in air, D _air :

The factor f depends on the mass attenuation coefficients ( Chapter 6 , Section D.1) of the medium of interest and of air and is energy dependent. Figure 23-1 shows the value of f as a function of energy for bone and for soft tissues. For soft tissues, f ≈ 1.1. The value is close to unity because the mass attenuation properties of soft tissues and air are similar. For low-energy photons ( E ≲ 100 keV), the value of f for bone is greater than unity. Because of photoelectric absorption by the heavier elements in bone (Ca and P), energy absorption in bone is greater than energy absorption by air at these energies; however, for most of the γ-ray energies commonly employed in nuclear medicine, the value of f for bone also is close to 1.

Thus for practical purposes air kerma (in grays) is approximately equal numerically to the absorbed dose in grays that would be received by an individual at that location, and in turn, as described in Section A.1 , the absorbed dose in grays is numerically equal to the equivalent dose in sieverts. Because of their approximate numerical equivalence, grays and sieverts, or in traditional units, roentgens, rads and rems, are sometimes (mis)used as approximately interchangeable quantities; however, one should be aware that they represent distinctly different physical quantities.

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Nuclear Regulatory Commission Licensing and Regulations

The use and distribution of radioactive materials in the United States are under the primary control of the Nuclear Regulatory Commission (NRC). The NRC issues licenses to individuals and to institutions to possess and use radioactive materials. In addition to medical uses, industrial, research, educational, and other uses of radioactive materials also require NRC licensing. In some states, the NRC has entered into an agreement to transfer its regulatory and licensing functions to a radiation control agency within the state. Such states are called agreement states.

Medical licenses generally fall into one of two categories: specific licenses of limited scope or specific licenses of broad scope. Limited-scope licenses are for limited kinds and quantities of radionuclides, which are listed specifically in the license. They may be issued to individual physicians (e.g., in private offices) or to institutions (e.g., hospitals). Licenses issued to institutions also list the of individuals authorized to practice under the license.

Broad-scope licenses are issued to larger institutions that require greater licensing flexibility (e.g., basic research as well as medical uses in a university setting). Broad-scope licenses generally cover more radionuclides and greater quantities than do limited-scope licenses. The NRC permits the institutional radiation safety committee to authorize individuals to use radionuclides under the license rather than requiring them to be listed specifically on the license.

The NRC also issues regulations that must be observed by licensees in the use of radioactive materials. These regulations are published in Title 10 of the Code of Federal Regulations (CFR). Two of the more relevant sections of these regulations for nuclear medicine are Part 20 (10CFR20), covering radiation protection, and Part 35 (10CFR35), covering medical uses. The NRC regulations are based primarily on the recommendations of two advisory bodies, the ICRP and the National Council on Radiation Protection and Measurement (NCRP), as discussed in Section B.7 . The NRC also periodically issues regulatory guides to assist licensees in the interpretation and implementation of its regulations.

In addition to the NRC, several other government agencies are involved in the regulation of radioactive materials, such as the U.S. Department of Transportation (shipping regulations) and the U.S. Food and Drug Administration (pharmaceutical aspects).

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Restricted and Unrestricted Areas

The NRC regulations prescribe different maximum radiation limits for restricted and unrestricted areas. A restricted area is one “… access to which is controlled by the licensee for the purposes of protection of individuals from exposure to radiation and radioactive materials.” Normally, restricted areas are not accessible to the general public, and generally they are occupied only by individuals whose employment responsibilities require them to work with radioactive materials and other radiation sources. Such individuals (e.g., nuclear medicine physicians, technicians, and radiochemists) are said to be occupationally exposed. Administrative staff, janitorial personnel, and facilities maintenance personnel generally are not included in this category. Restricted areas must be clearly marked with radiation warning signs.

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Dose Limits

The dose limits specified in 10CFR20 are based on the general recommendations by the ICRP and NCRP ( Section B.7 ) that an individual's total effective dose (see Chapter 22 , Section B.7) should not exceed 50 mSv (5 rem) per year. Furthermore, 10CFR20 requires that the deep-dose equivalent (dose equivalent at a depth of 1 cm in tissue) to any individual organ or tissue (excluding the lens of the eye) should not exceed 500 mSv (50 rem) per year. The limit for shallow-dose equivalent (dose equivalent at a depth of 0.007 cm in tissue) to the skin and extremities also is 500 mSv (50 rem) per year. The most restrictive limit is to the lens of the eye, which has an annual limit of 150 mSv (15 rem). The annual occupational dose limits for minors (<18 years of age) are 10% of the annual dose limits specified for adult workers. The dose equivalent to an embryo or fetus should not exceed 5 mSv (0.5 rem).

These dose limits, which apply to occupationally exposed personnel, are called occupational dose limits. Occupational dose limits do not include radiation doses received by the occupationally exposed individual while that individual is undergoing a medical examination, nor do they include any radiation dose from natural radiation sources, such as cosmic rays and naturally occurring radioactivity in the environment.

Note that the regulations require the licensee to control radiation doses not only from licensed materials but from “other sources in the licensee's possession” as well (e.g., nonlicensed radioactive materials or an x-ray generator). Thus a licensee would be in violation of the regulations if the occupation limits were exceeded even if most of the radiation dose were caused by nonlicensed sources.

For individual members of the public, the annual effective dose equivalent limits are 1 mSv (0.1 rem). Radiation levels in unrestricted areas should deliver a radiation dose of less than 0.5 µSv/hr (0.05 mrem/hr), assuming continuous occupation of the area. Transient radiation levels of up to 20 µSv/hr (2 mrem/hr) are permitted.

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Concentrations for Airborne Radioactivity in Restricted Areas

A particular problem in nuclear medicine laboratories is the potential for leakage or escape of radioactive gases (e.g., ¹³³ Xe used in pulmonary function studies) or volatile radioactive material (e.g., concentrated ¹³¹ I solutions). The NRC regulations specify the concentrations for airborne radioactive materials that would result in the annual dose limits described in Section 3 . These calculations assume that the workers are chronically exposed to these concentrations during a 2000-hour working year and that 2 × 10 ⁴ mL of air is breathed per minute. Concentration limits are shown in Table 23-2 for radionuclides that are used in nuclear medicine.

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