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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5196_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
177
BOX 10.1 Objectives of the National
Council on Radiation Protection and
Measurements
Objectives 4 to 7 are identified in the charter of the
council (Public Law 88-376) as follows:
“To:
4. Collect, analyze, develop and disseminate in the public
interest information and recommendations about (a)
protection against radiation (b) radiation measurements, quantities and units, particularly those concerned with radiation protection.
5. Provide a means by which organizations concerned
with the scientific and related aspects of radiation
protection and of radiation quantities, units, and measurements may cooperate for effective utilization of
their combined resources, and to stimulate the work
of such organizations.
6. Develop basic concepts about radiation quantities,
units, and measurements, about the application of
these concepts, and about radiation protection.
7. Cooperate with the International Commission on
Radiological Protection, the International Commission
on Radiation Units and Measurements, and other national and international organizations, government and
private, concerned with radiation quantities, units, and
measurements and with radiation protection.”
From National Council on Radiation Protection and Measurements (NCRP): Limitation of exposure to ionizing radiation,
Report No. 116, Bethesda, MD, 1993, NCRP. Reprinted with
permission from the National Council on Radiation Protection
and Measurements, http://NCRPonline.org.
Ionizing Radiation. BEIR V supersedes four earlier BEIR
reports that listed studies of biologic effects and the
associated risk of groups of people who were either
routinely or accidentally exposed to ionizing radiation.
Such groups include:
• Early radiation workers
• Atomic bomb victims of Hiroshima and Nagasaki
• Evacuees from the Chernobyl nuclear power station
disaster
As previously noted, recommendations for EfD
limits and EqD limits are made by the ICRP, NCRP,
UNSCEAR, and NAS/NRC-BEIR. Based on these recommendations, limits on radiation exposure are established by congressional act or state mandates. National
and state agencies are charged with the responsibility of
enforcing standards after they have been established.
US REGULATORY AGENCIES
After radiation protection standards have been determined, responsible agencies must enforce them for the
protection of the general public, patients, and occupationally exposed personnel.
Regulatory agencies include the following:
1. Nuclear Regulatory Commission (NRC)
2. Agreement states
3. Environmental Protection Agency (EPA)
4. US Food and Drug Administration (FDA)
5. Occupational Safety and Health Administration (OSHA)
A summary of the US regulatory agencies is pre-
sented in Table 10.2.
survivors), data acquired from the Radiation Effects
Research Foundation (a group run by the government
of Japan primarily to study the survivors), and research
conclusions to derive radiation risk assessments for radiation-induced cancer and genetic (hereditary) effects.
National Academy of Sciences/National Research Council Committee on the Biological Effects of Ionizing Radiation (NAS/NRC-BEIR)
NAS/NRC-BEIR is another advisory group that reviews
studies of biologic effects of ionizing radiation and risk
assessment. This group formulated the 1990 BEIR V
Report, Health Effects of Exposure to Low Levels of
Nuclear Regulatory Commission
The Nuclear Regulatory Commission (NRC), formerly
known as the Atomic Energy Commission, is a federal
agency that has the authority to control the possession,
use, and production of atomic energy in the interest of
national security. This agency also has the power to enforce radiation protection standards. However, the NRC
does not regulate or inspect x-ray imaging facilities. The
primary function of the NRC is to oversee the nuclear
energy industry. This agency supervises the:
• Design and working mechanics of nuclear power
stations
• Production of nuclear fuel
• Handling of expended fuel
• Supervision of hazardous radioactive waste material
178
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
TABLE 10.2 Summary of US Regulatory
Agencies
Agency Function
Nuclear Regulatory
Commission
(NRC)
Agreement states Enforce radiation protection
Environmental
Protection
Agency (EPA)
US Food and Drug
Administration
(FDA)
Occupational
Safety and Health
Administration
(OSHA)
Oversees the nuclear energy
industry, enforces radiation
protection standards, publishes
its rules and regulations in Title
10 of the US Code of Federal
Regulations, and enters into
written agreements with state
governments that permit the
state to license and regulate
the use of radioisotopes and
certain other material within
that state
regulations through their
respective health departments
Facilitates the development and
enforcement of regulations
pertaining to the control of
radiation in the environment
Conducts an ongoing product
radiation control program, regulating the design and manufacture of electronic products,
including x-ray equipment
Functions as a monitoring agency
in places of employment,
predominantly in industry
Additionally, the NRC controls the manufacture and
use of radioactive isotopes formed in nuclear reactors
(also known as by-product materials) and used in:
• Research
• Industry
• Nuclear medicine imaging procedures
• Therapeutic treatments
Users of such radioactive materials must be formally
licensed by the NRC and will receive periodically unannounced inspections by NRC staff to determine whether
these users comply with the provisions of their licenses.
Failure to pass these inspections can result in significant
fines and even license suspension. Until 2008 the NRC
did not regulate the use of radioactive substances
that either are naturally occurring, like radium, or are
produced outside of a reactor by high-energy particle
accelerators, such as cyclotrons. These materials are
designated as NARM (“naturally occurring and/or ac-
celerator produced materials”). Two common examples
of cyclotron-produced radioisotopes are:
• Thallium-201 (
201
Tl) used in nuclear medicine for
heart stress tests
• Palladium-103 (
103
Pd) used for therapeutic prostate
seed implants
NARM materials were formerly solely regulated by
state bureaus of radiation protection. In 2008 the NRC
expanded its definition of by-product substances to include NARM materials. This meant that all facilities in
nonagreement states (i.e., those states that have decided
to maintain their own self-designed independent radiation protection program for radioactive materials),
must be in full compliance with NRC regulations and
additionally would have to amend their NRC radioactive materials license to include all NARM materials that
they are currently using.
The NRC writes rules and regulations. The US Office
of the Federal Register prepares and distributes these
rules in Title 10 of the US Code of Federal Regulations.
Radiation protection standards governing occupational
radiation exposure may be found in Part 20 of Title 10,
abbreviated 10 CFR 20.
Agreement States
The NRC has the authority to enter into written contracts with state governments. These agreements permit
the contracting state to undertake the responsibility of
licensing and regulating the use of radioisotopes and
certain other radioactive materials within that state.
Most states in the United States have entered into
such “agreements” with the NRC, thereby also assuming
responsibility for enforcing radiation protection regulations through their respective health departments.
These states are known as agreement states. In non-
agreement states, both the state and the NRC jointly inspect and enforce radiation protection regulations by
sending agents at different times to health care facilities.
Hospitals are evaluated to determine whether they comply with existing radiation safety regulations. Individual
states may also legislate regulations regarding radiation
safety to be above and beyond those mandated by the
NRC. Inspection of nuclear reactors and assurance of
adherence to federal radiation safety regulations in the
agreement or nonagreement states fall solely under the
jurisdiction of the NRC.
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
179
Environmental Protection Agency (EPA)
The EPA, established in 1970 through the reorganization
plan of US President Richard M. Nixon, was created to
bring several departments under one organization that
would be responsible for protecting the health of humans and for safeguarding the natural environment
from industrial practices and harmful waste disposal.
The EPA, as part of its general overseer responsibilities, also facilitates the development and enforcement of
regulations pertaining to the control of radiation in the
environment. Specifically, it:
• Directs relevant federal agencies
• Oversees the general area of environmental monitoring
• Has oversight authority for specific areas such as
determining the action level for radon
US Food and Drug Administration (FDA)
Under Public Law 90-602, the Radiation Control for
Health and Safety Act of 1968, the FDA conducts an
ongoing product radiation control program, regulating
the design and manufacturing of electronic products,
including diagnostic x-ray equipment.
A more detailed explanation of the Radiation Control for Health and Safety Act of 1968 is discussed later
in this chapter.
To determine the level of compliance with standards
in a given radiology facility, the FDA conducts on-site
inspections of x-ray equipment, particularly mammography units. Compliance with FDA standards ensures
the protection of occupationally and nonoccupationally
exposed persons from faulty manufacturing.
Occupational Safety and Health Administration (OSHA)
OSHA functions as a monitoring agency in places of employment, predominantly in industry. OSHA regulates occupational exposure to radiation through Part 1910 of Title
29 of the US Code of Federal Regulations (29 CFR 1910).
The agency is responsible for regulations concerning an
employee’s “right to know” about hazards that may be present in the workplace. A series of statutes passed by the individual states requires that employees be made aware of
these potential risks in the workplace. The act covers:
• Hazardous substances
• Infectious agents
• Ionizing radiation
• Nonionizing radiation (e.g., ultraviolet, microwaves,
etc.)
The act requires employers to evaluate their workplaces for harmful agents and to provide training and
written information to their employees. OSHA also
regulates training programs in the workplace.
RADIATION SAFETY PROGRAM
Requirement
Facilities providing imaging services shall have an active and detailed radiation safety program to ensure
adequate safety of patients and radiation workers. The
implementation of an effective program begins with
the administrative personnel of the facility. Individuals
in executive positions must provide the resources
necessary for creating and maintaining this program.
They can:
• Delegate operational funds in the budget
• Oversee the development of policies and procedures
• Provide the equipment needed for starting and for
continuing the program
Radiation Safety Committee and Radiation
Safety Officer
The NRC mandates that a radiation safety committee
(RSC) be established for the facility. This committee
imparts guidance for the program and facilitates its
ongoing operation. A radiation safety officer (RSO)
should also be selected to:
• Oversee the program’s daily operation
• Provide for formal review of the program each year
An RSO is usually a medical physicist, health physicist, radiologist, or other individual qualified through
adequate training and experience. This person is designated by a health care facility and approved by the NRC
and the state.
Responsibilities of the Radiation Safety Officer. The
RSO is responsible for developing an appropriate radiation safety program for the facility that follows internationally accepted guidelines for radiation protection.
This individual is also charged with ensuring that the
facility’s operational radiation practices are such that all
persons, especially those who are or could be pregnant,
are adequately protected from unnecessary exposure. To
fulfill this responsibility, the management of the facility
must grant the RSO the authority necessary to implement and enforce the policies of the radiation safety
program.
180
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
The RSO must also review and maintain radiationmonitoring records for all personnel and be available
to provide counseling for individuals (e.g., those who
receive monitor readings above allowable limits).
Required Training and Experience for a Radiation
Safety Officer.
The necessary training and experience for
an RSO is described in Part 35.50 and Part 35.900 of Title
10 of the Code of Federal Regulations. The NRC publishes regulatory guides to accompany its rules. Although
legally health care facilities do not need to comply with
the guide, they frequently choose to do so to facilitate
the chances of a successful outcome of an NRC inspection or approval of license changes. The guide is the
NRC’s interpretation of how to implement its own rules.
Three pathways exist for obtaining the training and
experience required for the RSO position (identified in
Box 10.2).
Authority of the Radiation Safety Officer. 10 CFR
35.24 requires that the licensee provide the RSO:
• Sufficient authority
• Organizational freedom
• Management prerogative to perform specific duties
These tasks are identified in Box 10.3. The licensee
must establish, in writing, the authority, functions, and
responsibilities of the RSO. Because the RSO is responsible for the day-to-day supervision of the facility’s radiation safety program, this individual must have independent authority to stop operations that are considered
unsafe. Also, the RSO must be given adequate time and
resources and have a sufficient commitment from management to ensure that radioactive materials are used in
BOX 10.2 Allowable Pathways for a
Nominated Radiation Safety Officer to Meet
Training and Experience Requirements as
Described in 10 CFR 35.50 and 10 CFR 35.900
1. Certification by one of the professional boards ap-
proved by the Nuclear Regulatory Commission (NRC)
2. Didactic and work experience as described in detail
in the regulations
3. Identification as an authorized user, authorized med-
ical physicist, or authorized health physicist on the
license, with experience in the types of uses for
which the individual has radiation safety officer
(RSO) responsibilities
Box 10.3 Duties That 10 CFR 35.24
Requires the Licensee to Freely Provide
the Radiation Safety Officer to Perform
1. Identify radiation safety problems.
2. Initiate, recommend, or provide corrective action.
3. Stop unsafe operations involving by-product material.
4. Verify implementation of corrective actions.
a safe manner. The NRC requires the name of the RSO
on the facility’s radioactive materials license to ensure
that licensee management has identified a responsible,
qualified person who can directly interact with the NRC
during inspections and also concerning any inquiries
regarding the facility’s safety program. Usually, the RSO
is a full-time employee of the licensed facility; however,
the NRC has authorized individuals who are not employed by the licensee (e.g., a consultant) to fill the role
of an RSO or to provide support to the facility’s RSO.
Training for this role is included in 10 CFR 35. A list of
these requirements can be found in Appendix H.
RADIATION FOR HEALTH AND SAFETY ACT OF 1968
In 1968, the US Congress passed the Radiation Control
for Health and Safety Act (Public Law 90-602) to protect
the public from the hazards of unnecessary radiation
exposure resulting from electronic products such as
microwave ovens and picture tube color televisions.
Diagnostic x-ray equipment also was included. The act
permitted the formation of the Center for Devices and
Radiological Health (CDRH). The CDRH falls under
the jurisdiction of the FDA. Essentially, it is responsible
for conducting an ongoing electronic product radiation
control program including establishing standards for
the manufacture, installation, assembly, and maintenance of machines for radiologic procedures. Further
responsibilities include:
• Assessing the biologic effects of ionizing radiation
• Evaluating radiation emissions from electronic products in general
• Conducting research to reduce radiation exposure
Code of Standards for Diagnostic X-Ray Equipment
The code of standards for diagnostic x-ray equipment
(Public Law 90-602) went into effect on August 1, 1974.
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
181
Box 10.4 Provisions Included in the
Standards for Diagnostic X-Ray Equipment
1. Automatic limitation of the radiographic beam to the
image receptor regardless of image receptor size, a
condition known as positive beam limitation.
2. Appropriate minimal permanent filtration of the x-ray
beam to ensure an acceptable level of beam quality.
Filtration provides significant reduction in the intensity of very “soft” x-rays that contribute only to the
added patient-absorbed dose.
3. Ability of x-ray units to duplicate certain radiation ex-
posures for any given combination of kilovolts at peak
value (kVp), milliamperes (mA), and time to ensure
both exposure reproducibility and linearity. Reproduc-
ibility is defined as consistency in output in radiation
intensity for identical generator settings from one
individual exposure to subsequent exposures.* A
variance of 5% or less is acceptable. Exposure linear-
ity is defined as consistency in output radiation intensity at a selected kVp setting when changing from
one milliamperage and time combination (mAs 5 mA
3 exposure time) to another. Linearity, which is de-
fined as the ratio of the difference in mR/mAs values
between two successive generator stations to the
sum of those mR/mAs values, must be less than 0.1.
4. Inclusion of beam limitation devices for spot films
taken during fluoroscopy. Such devices should be located between the x-ray source and the patient.
5. Presence of “beam on” indicators to give visible warn-
ings when x-ray exposures are in progress and both visual and audible signals when exposure has terminated.
6. Inclusion of manual backup timers for automatic
(photo-timed) exposure control to ensure the termination of the exposure if the automatic timer fails.
*Mathematically, reproducibility is specified by the coefficient
of variation C, which is by definition equal to the standard deviation (SD), of at least five successive output measurements
employing the same technique factors divided by the average,
or mean value, of those measurements. The regulation requires
that C must not exceed 0.05.
SD is an indicator of how measurements for a group, e.g., grades
in a class exam, are dispersed from the average value. A low
standard deviation means that most of the measured values are
close to the average. A high standard deviation means that the
values range much farther away from the average value.
This code applies to complete systems and major components manufactured after that date. Some relevant
provisions of the standards for diagnostic x-ray equipment are listed in Box 10.4.
Public Law 90-602 does not regulate the diagnostic x-ray
user. It is strictly an equipment performance standard.
ALARA CONCEPT
In 1954, the National Committee on Radiation Protection (later known as the National Council on Radiation
Protection and Measurements) put forth the principle
that radiation exposures should be kept “as low as
reasonably achievable” (ALARA) with consideration
for economic and societal factors. According to NCRP
Report No. 160, “The protection from radiation exposure is as low as reasonably achievable when the expenditure of further resources would be unwarranted by
the reduction in exposure that would be achieved.”
3
The ALARA concept is accepted by all regulatory
agencies. In 1987, the NCRP described it as “the continuation of good radiation protection programs and
practices which traditionally have been effective in
keeping the average and individual exposures for
monitored workers well below the limit.”4 It may also
be referred to as optimization per ICRP Publication
No. 37 and Publication No. 55. Medical imaging personnel and radiologists share the responsibility to
keep occupational and nonoccupational dose limits
ALARA.
In practice this translates into EfDs and EqDs well
below maximum allowable levels. This goal can be simply achieved through the employment of proper safety
procedures performed by qualified personnel. Such
methods should be clearly explained in a facility’s radiation safety program. To define ALARA, health care
facilities typically adopt investigation levels, defined as
level I and level II. In the United States, these levels are
traditionally one-tenth to three-tenths of the applicable
regulatory limits.
Model for the ALARA Concept
The ALARA concept adopts an extremely conservative
model concerning the relationship between ionizing
radiation and potential risk. The relationship is the
linear nonthreshold model discussed in Chapter 9
(reproduced in Fig. 10.2). The central principle of
radiation protection is that in the interest of safety,
risk of injury should be overestimated rather than
underestimated.
FOOD AND DRUG ADMINISTRATION WHITE PAPER
The US FDA supports the premise that “each patient
should get the right imaging examination, at the right
182
Response to radiation
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
purpose of this federal act, under the directorship of the
Secretary of Health and Human Services, is to ensure
that regular medical and dental radiologic practices
adhere to rigorous safety provisions. Individual states
are encouraged to enact similar statutes and administer
certification and accreditation programs based on the
standards established therein. Because no legal penalty
exists for noncompliance, many states, unfortunately,
have not responded with appropriate legislation.
(expressed in biologic effects)
Radiation dose
Fig. 10.2 Dose–response curve. Hypothetical linear (straight-
line) nonthreshold curve for radiation dose–response relationship. The straight-line curve passing through the origin in this
graph indicates both that the response to radiation (in terms of
biologic effects) is directly proportional to the dose of radiation
and that no known level of radiation dose exists below which
absolutely no chance of sustaining biologic damage is evident.
time, with the right radiation dose.”5 This declaration
is clearly stated in the FDA document known as the
White Paper, published in February 2010, in which they
announced “the launch of a cooperative Initiative to
Reduce Unnecessary Radiation Exposure from Medical
Imaging.”5 Working in conjunction with its partners, the
FDA intends to take action to:
1. “Promote safe use of medical imaging devices”
2. “Support informed clinical decision”
3. “Increase patient awareness”
5
By coordinating these efforts, the FDA will be able to
“optimize patient exposure to radiation from certain
types of medical exams, and thereby reduce related risks
while maximizing the benefits of these studies.”
CONSUMER-PATIENT RADIATION HEALTH AND SAFETY ACT OF 1981
The Consumer-Patient Radiation Health and Safety Act
of 1981 (Title IX of Public Law 97-35) (see Appendix I)
provides federal legislation requiring the establishment
of minimum standards for the accreditation of educational programs for persons who perform radiologic
procedures and the certification of such persons. The
RADIATION-INDUCED RESPONSES OF CONCERN IN RADIATION PROTECTION
Categories for Radiation-Induced Responses
At present, the two main categories of radiation-induced
responses of serious concern for humans are:
1. Tissue reactions
2. Stochastic (probabilistic) effects
Changes in Terminology From the 1970s to the Present
The ICRP and the NCRP update radiation protection
terminology to recognize current scientific principles
used to describe radiation effects. This progression in
terminology is summarized in Box 10.5.
Tissue Reactions. In the preceding chapters, tissue
reactions were described as biologic somatic effects of
ionizing radiation that can be directly related to the
dose received. These reactions exhibit a threshold dose
5
5
below which the response does not typically occur and
above which the severity of the biologic damage increases as the dose increases. For example, if a specific
dose of radiation is required to cause a skin burn, a
5
BOX 10.5 Recent Changes in the
Terminology Used to Describe Radiation
Effects for the Purpose of Radiation
Protection Guidelines
Approximate Year
of Adoption Terminology
1977–1991 stochastic vs. nonstochastic
1991–2012 stochastic vs. deterministic
2012–present stochastic vs. tissue reactions
(early or late)
6–9
6
7
8,9
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
183
higher dose of radiation will cause the skin burn to be
more severe. When radiation-induced biologic damage
escalates, it does so because greater numbers of cells
interact with the increased number of x-ray photons
that are present at higher radiation exposures. In general, tissue reactions typically occur only after large
doses of radiation. However, tissue reactions could also
result from long-term individual low doses of radiation
sustained over several years. In either instance the cumulative amounts of such radiation doses are usually
much higher than those typically encountered by a
patient in diagnostic radiology.*
Early and late tissue reactions. Tissue effects may be
early, such as:
• Diffuse redness over an area of skin after irradiation
(erythema)
• A decrease in the white blood cell count (leukopenia)
• Epilation, or loss of hair
As was discussed in Chapter 8, other, and much more
severe, early consequences of radiation sickness can also
arise, such as:
• Hematopoietic syndrome
• Gastrointestinal syndrome
• Cerebrovascular syndrome
Recall that these effects usually occur within a few
hours or days after very high-level radiation exposure
to a significant portion of the body. Some late tissue
reactions due to high-level radiation exposure, though,
occur months or more afterward. They include:
• Cataract formation
• Fibrosis
• Organ atrophy
• Loss of parenchymal cells
• Reduced fertility
• Sterility caused by a decrease in reproductive cells
Early tissue reactions such as erythema and late tis-
sue reactions such as cataract formation have a high
probability of occurring when entrance radiation doses
exceed 2 Gyt.
For tissue reactions caused by high doses, their fre-
quency of occurrence is not linear with respect to dose
*A significant exception to this is high-dose-rate fluoroscopic
procedures. For these studies, entrance dose rates as great
as 200 mGya/min are possible. A fluoroscopic exposure of
15 minutes at this level would result in a patient entrance dose
of approximately 3 Gya.
10
but instead follows a nonlinear threshold curve that is
sigmoidal (S-shaped) with a threshold (see Fig. 9.1B).
Stochastic Effects. Since stochastic effects are non-
threshold, randomly occurring biologic somatic
changes, their chances of occurrence increase with each
radiation exposure. Examples of stochastic effects are:
• Cancer
• Genetic alterations
Stochastic responses may be demonstrated with the use
of both the linear (see Fig. 9.2) and the linear-quadratic
dose–response curves (see Fig. 9.3). Because a stochastic
event is an all-or-none, random effect, ionizing radiation
will normally induce some cancers within a large general
population, but determining beforehand which members
of that population will develop cancer is not possible. Injury may result from exposure of a single cell or from damage in a sensitive substructure, such as a gene. The assumption is that no minimal safe dose exists. The frequency of
an occurrence in a population increases in proportion to
the magnitude of the absorbed dose of ionizing radiation
delivered to the entire population. Therefore, the net effect
on the population group depends not only on the number
of individuals irradiated but also on the mean dose that
each individual receives.
A summary of both early and late tissue reactions
and stochastic (probabilistic) effects is presented in
Box 10.6.
CURRENT RADIATION PROTECTION PHILOSOPHY
Both genetic and somatic responses to ionizing radiation
were considered in developing the present EfD limiting
recommendations. The current radiation protection
philosophy is based on the assumption that a linear nonthreshold relationship exists between radiation dose and
biologic response. Thus, even the most minuscule dose
of radiation has a nonzero possibility of causing some
harm. The current philosophy also acknowledges that
ionizing radiation possesses a beneficial potential. This
philosophy proposes that, when employed, the potential
benefits of exposing the patient to ionizing radiation
must far outweigh any potential risk.
Effective Dose Limiting System
The EfD limiting system is the current method for controlling the risk of biologic damage to radiation workers
184
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
BOX 10.6 Summary of Serious
Radiation-Induced Responses of Concern
Stochastic
Tissue Reactions
Early Reactions Effects
Erythema (diffuse redness
over an area of skin after
irradiation)
Blood changes (decrease of
lymphocytes and platelets)
Epilation (loss of hair)
Acute radiation syndrome
Hematopoietic syndrome
Gastrointestinal syndrome
Cerebrovascular syndrome
Late Reactions
Cataract formation
Fibrosis
Organ atrophy
Loss of parenchymal cells
Reduced fertility
Sterility
(Probabilistic)
Cancer
Genetic (Hereditary)
Effects
Mutagenesis
(irradiation of
DNA of somatic
cells leading to
abnormalities
in new cells as
they divide in
that individual)
and the general public from radiation exposure
(Fig. 10.3). The effective dose limit (EDL) is the upperboundary dose of ionizing radiation that results in a
negligible risk of:
• Bodily injury
• Hereditary damage
EDLs may be specified for whole-body exposure,
partial-body exposure, and exposure of individual organs. Separate limits are set for occupationally exposed
individuals and the general public. The sum of both the
external and internal whole-body exposures is considered
when effective dose limits are established. Their values are
such as to minimize the risk to humans in terms of early
and late tissue reactions and stochastic effects. Natural
background and medical exposure are not included.
Upper boundary safe radiation exposure limits for
occupationally exposed persons are associated with
risks that are similar to those encountered by employees
in other industries that are generally considered to be
reasonably safe. These industries include:
• Manufacturing
• Trade
• Civil Service
Effective Dose (EfD) Limiting System
ASSESSES
Radiation exposure
and
associated risk of biologic effects
FOR
Radiation workers
and
the general public
Fig. 10.3 Effective dose (EfD) limiting system.
Quantitative values for radiation risks are derived
from the complete injury that may be caused by radiation exposure. Because many conflicting views continue
to exist on assessing the risk of cancer from low-level
radiation exposure, the trend has been to create more
rigorous radiation protection standards.
Occupational Risk
The potential for terminal cancer, shortening of life
span because of the induction of cancer, hereditary imperfections triggered by reproductive cell mutations,
other abnormalities, and overall poorer quality of life
are collectively taken into account in formulating
occupational risk standards.
As mentioned previously, the risk to a radiographer
from radiation exposure may be equated with occupational risk in generally safe industries. That risk is estimated to be a 2.5% chance of a fatal accident over an
entire career. The lifetime fatal risk in hazardous occupations, however, is many times greater. A few examples
of such occupations include:
• Logging
• Deep-sea fishing
• Iron and steel workers
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
185
To ensure that the hazard to radiation workers is no
greater than the threat to the general public, the NCRP
proposes that radiation protection programs for radiation workers be designed to prevent individual workers
from obtaining a total external plus internal cumulative
EfD in excess of their age in years times 10 mSv.4 Consider the following situation: A worker at age 40 years
has been employed at a nuclear power plant for 10 years.
He had previously been employed as a radiation worker
in another industry, during which he received a cumulative EfD of 100 mSv (10 rem). Therefore, the radiation
protection program for his current position should
have ensured that he has not accumulated a total EfD
greater than 300 mSv (30 rem) during his 10 years of
employment.
The Vulnerability of the Embryo-Fetus to Radiation Exposure
The fact that the embryo-fetus in utero is particularly
sensitive to radiation exposure has already been established. Epidemiologic studies of atomic bomb survivors
exposed in utero provided conclusive evidence of a dosedependent increase in the incidence of severe intellectual
disability for fetal doses higher than approximately 0.4 Sv.
The most significant risk for radiation-induced intellectual disability occurred when the embryo-fetus was exposed 8 to 15 weeks after conception.
BASIS FOR THE EFFECTIVE DOSE LIMITING SYSTEM
Concept Underlying Radiation Protection
The essential concept underlying radiation protection is
that any organ in the human body is vulnerable to damage from exposure to ionizing radiation. Even though
some organs are known to be more sensitive to radiation than others, every organ is considered to be at some
risk due to the assumed random nature of somatic or
hereditary radiation-induced effects.
The EfD limiting system includes, for the determination of EqD for tissues and organs, all radiation-vulnerable human organs that can contribute to potential risk,
rather than only those human organs considered critical. In earlier recommendations such as NCRP Report
No. 39 (released in 1971), only vital organs such as the
gonads, blood-forming organs, and lung tissue were
identified.
11
Tissue Weighting Factor
Although this factor was previously discussed, a brief
description follows to reinforce greater understanding
of its importance as it relates to the EfD limiting system.
The EfD limiting system is an attempt to equate the
various risks of cancer and genetic effects on the tissues
or organs that were exposed to radiation. Because various tissues and organs do not have the same degree of
sensitivity to these effects, the system employed must
compensate for the differences in risk from one organ to
another. Therefore a tissue weighting factor (WT) is
used. This factor “indicates the ratio of the risk of
stochastic effects attributable to irradiation of a given
organ or tissue (T) to the total risk when the whole body
is uniformly irradiated.”12 Organ or Wt factors recommended by the ICRP in Report No. 60, released in 1991,
and adopted by the NCRP in Report No. 116, published
in 1993, are reproduced in Box 10.7.
BOX 10.7 Organ or Tissue Weighting
Factors for Calculating Effective Dose
0.01 0.12
Bone surface Red bone marrow
Skin Colon
Lung
0.05 Stomach
Bladder
Breast 0.20
Liver Gonads
Esophagus
Thyroid
Remainder*
*The remainder takes into account the following additional
tissues and organs: adrenals, brain, small intestine, large
intestine, kidney, muscle, pancreas, spleen, thymus, and
uterus.
†
In extraordinary circumstances in which one of the
remainder tissues or organs receives an equivalent dose
in excess of the highest dose in any of the 12 organs
for which a weighting factor (WT) is specified, a WT of
0.025 should be applied to that tissue or organ and a
WT of 0.025 to the average dose in the other remainder
tissues or organs.
From National Council on Radiation Protection and Measurements (NCRP): Limitation of exposure to ionizing radiation,
Report No. 116, Bethesda, MD, 1993, NCRP. Reprinted with
permission from the National Council on Radiation Protection
and Measurements, http://NCRPonline.org.
†
186
CHAPTER 10 Dose Limits for Exposure to Ionizing Radiation
CURRENT NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS RECOMMENDATIONS
National Council on Radiation Protection and Measurements Reports
The NCRP periodically reiterates and updates its position on radiation protection standards and publishes
recommendations on these standards in the form of
reports. Recommendations contained in NCRP Report
No. 184 now supersede those contained in NCRP Reports No. 116, No. 91, and No. 39. A summary of some
critical issues and changes follows.
NCRP Report No. 184, Medical radiation exposure of
patients in the United States, November 2019, is an update to NCRP Report No. 160, Ionizing radiation expo-
sure of the population of the United States (2009). This
new Report revises medical radiation exposure information with data collected between 2006 and 2016. The
2009 report (No. 116) revealed a dramatic increase in
medical radiation exposure over the previous 25 years:
medical exposure comprised nearly half of all radiation
exposure to the US population, primarily due to an
increase in computed tomography (CT) scanning and
cardiac nuclear medicine procedures. A decade has
passed since the publication of NCRP Report No. 160,
and changes in technology, the emergence of campaigns
for dose reduction and optimization, indications for
specific examinations, and reimbursement appear to
have favorably affected medical radiation exposure and
dose to the US population. There has been a substantial
reduction in radiation doses to the US population since
NCRP Report No. 160 was published in 2009. The
annual nontherapeutic medical radiation dose to the
US population in 2006 was 2.92 mSv and decreased to
2.16 mSv in 2016 (Fig. 10.4).
Cumulative Effective Dose (CumEfD) Limit. A radia-
tion worker’s lifetime effective dose must be limited to
this individual’s age in years times 10 mSv. This is
known as the cumulative effective dose (CumEfD)
limit and it pertains to the whole body. Adhering to this
limit ensures that the lifetime risk for these workers
remains acceptable. CumEfD limits, however, do not
include:
• Radiation exposure from natural background
radiation
• Exposure acquired as a consequence of a worker’s
undergoing medical imaging procedures
The limits do include the possibility of both:
• Internal exposure
• External exposure
The CumEfD is therefore, the sum or total of both
the internal and external EqDs.
Medical imaging personnel seldom receive EqDs that
are a significant fraction of the annual occupational
effective dose limit. A radiation safety program that is
well structured and properly maintained will ensure
that individual occupational exposure will not remotely
approach 50 mSv (5000 mrem) in any given year.
International Commission on Radiological Protection Recommendation for Downward Revision of the Annual Effective Dose Limit.
mended that the annual EfD limit for occupationally
exposed persons be reduced from 50 mSv to 20 mSv as
a result of information obtained regarding the Japanese
atomic bomb survivors in whom the risk of radiation
from the atomic bomb detonations was estimated to be
approximately three to four times greater (more damaging) than previously estimated.13 Although the NCRP
has not changed from the 50 mSv limit, in the future it
could very well recommend a lower limit on the annual
occupational EfD because of the:
1. Revised risk estimates derived from the more recent
reevaluations of dosimetric studies on the atomic
bomb survivors of Hiroshima and Nagasaki
2. Appearance, as a result of longer follow-up time, of
increased numbers of solid tumors in the survivor
population
In the United States, lowering of the current limits
is the responsibility of the NRC, individual states, and
the FDA.
In 1991, the ICRP recom-
11
Limits for Nonoccupationally Exposed Individuals.
In addition to limits for occupationally exposed individuals, the NCRP recommends EDLs for nonoccupationally exposed individuals who are not undergoing
medical imaging procedures. The NCRP-recommended
annual EDL is 1 mSv (100 mrem) for continuous or
frequent exposures from artificial sources other than
medical irradiation and natural background and a limit
of 5 mSv annually for infrequent exposure.4 These nonoccupational values established for individual members
of the general public are designed to restrict their chances