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Appendix F
Ionizing Radiation Exposure to the
U.S. Population, with a Focus on
Radiation from Medical Imaging
1
Rebecca Smith-Bindman, M.D.
University of California, San Francisco School of Medicine
DEFINITION OF IONIZING RADIATION AND UNITS
Radiation is energy in the form of high-speed particles and electromag-
netic waves. Radiation from electromagnetic waves is characterized by the
wavelength and the amount of energy they transfer. In general, the shorter
the wavelength, the greater the energy of the radiation, and the greater the
potential for biological damage. The types of electromagnetic radiation and
examples of sources of this radiation are shown in Figure F-1.
Ionizing Radiation
Radiation with enough energy to remove tightly bound electrons from
their orbits (and enough energy to break chemical bonds) is called ionizing
radiation (reflected by wave lengths to the right in Figure F-1, shown for
electromagnetic radiation, including ultraviolet waves, X-rays and gamma
rays). Wavelengths to the left of Figure F-1 (i.e., microwaves, radio waves,
and low-frequency cell phone waves, which are extremely low frequency
and energy) do not have this amount of energy and cannot break chemical
bonds and are non-ionizing radiation. This does not confirm that they are
safe, just that they do not cause biological damage through the mecha-
nism of breaking chemical bonds. There are three main kinds of ionizing
1 The responsibility for the content of this article rests with the author and does not neces-
sarily represent the views of the Institute of Medicine or its committees and convening bodies.
409
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410 BREAST CANCER AND THE ENVIRONMENT
FIGURE F-1 Energy spectrum of radiation.
SOURCE: NASA (http://mynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg).
Figure F-1 800 px-EM_Spectrum3-new.eps
bitmap, landscape
radiation: alpha and beta particles (not electromagnetic), gamma rays, and
X-rays.
There are many different naturally occurring sources of radiation, and
radiation is used in many areas of industry and medicine. A few typical
doses (exposure levels) and impacts of radiation are shown in Figure F-2.
Ionizing Radiation Used in Medical Imaging
Ionizing radiation used in medical imaging includes gamma rays and
X-rays. Gamma rays are delivered through nuclear medicine examinations,
when a small amount of radioactive material is inhaled, injected, or swal-
lowed by a patient, and the resulting gamma waves that are emitted by the
radio-pharmaceutical from within the patient are detected. A variety of
imaging tests use X-ray technology, including radiographs (also known as
conventional X-rays, plain films, and sometimes just X-rays for short even
though each of these sources use X-rays), fluoroscopy, angiography, and
computed tomography (CT). Ultrasound and magnetic resonance imaging
utilize ultrasound waves and magnetic waves respectively, and neither deliv-
ers ionizing radiation.
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FIGURE F-2 Sample doses of and exposures to ionizing radiation.
411
Figure F-2.eps
bitmap, landscape
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412 BREAST CANCER AND THE ENVIRONMENT
Biological Effect of Ionizing Radiation
Ionizing radiation passes through air and will deposit energy into the
tissue that absorbs it. Ionizing radiation deposits a relatively large amount
of energy into a small area, and damage caused by the radiation varies
with the dose. At low doses, cells repair the damage with no lasting effects.
At moderate doses, cells can be changed permanently, leading to cancer
or other abnormalities such as birth defects. At high doses, such as those
delivered through radiation treatment for cancer or following the immedi-
ate effect of the atomic bombs dropped on Japan, tissues fail to function,
severe burns result, and people who are exposed at these levels may die.
Predictability of Response to Ionizing Radiation
The severity of responses to high-dose radiation (generally considered a
dose above 1 sievert, Sv) increases directly in proportion to the dose. There
is generally a threshold for damage from the radiation: below a certain level
the damage will not occur, and above the threshold, the higher the dose, the
more extensive and severe the injury. These responses tend to be predict-
able, and are termed deterministic. Many of the earlier radiology pioneers
developed erythema, burns, and radiation sickness from their exposures,
as did individuals who lived near the atomic bomb explosions, and these
are deterministic effects. At lower doses of radiation, generally below 1 Sv,
the effect of the radiation is much less predictable, and no clear threshold
exists. There is a chance that a person will experience an adverse outcome
if exposed to the radiation, but it is not certain, and while the probability
increases with increasing exposure, the severity may not. These effects tend
to be all-or-none effects (either a cancer occurs or it does not). These effects
are called stochastic effects. The types of effects included in this group are
carcinogenesis and birth defects.
Mechanism of Carcinogenesis
The mechanism through which radiation exposure can lead to cancer
is outside the scope of this review. However, carcinogenesis is believed to
be multifactorial. Patient-related vulnerabilities are important, and a few
patient groups have been identified who are particularly vulnerable to
radiation-related carcinogenesis. Ionizing radiation has sufficient energy
to break chemical bonds, can cause DNA damage, and therefore can con-
tribute to how cancer may occur, but it is thought to act through various
mechanisms rather than a single effect. Radiation can initiate gene muta-
tions, promote the number of premalignant cells, and alter DNA repair.
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APPENDIX F
There are believed to be important differences in cancer risks by age, almost
certainly reflecting some of these differences.
Radiation Measurements
Several commonly used radiation measurements are described below,
and the measures used in subsequent discussion are summarized in Table F-1.
The roentgen (R) describes the intensity of a beam of X-rays or gamma
rays and the ionization of atoms in air as radiation passes through space.
The roentgen also is the unit used in the calibration of X-ray generating
equipment (CE Essentials, 2011).
The radiation absorbed dose (rad) describes radiation in a medium
other than air. As ionizing radiation passes through matter (e.g., human tis-
sue) it imparts energy into the medium ionizing it. One rad is equivalent to
100 ergs of energy absorbed in a gram of material (tissue). The comparable
SI unit (International System of Units, adopted by nearly all countries) is
the gray (Gy), which is expressed as joules per kilogram (1 Gy = 100 rad).
The measure of rad or Gy describes the absorption of ionizing radiation
in matter and is related to the biological damage in tissue; the greater the
amount of energy transferred to tissue by ionizing radiation, the higher the
rad or gray and the greater the biological damage (CE Essentials, 2011).
There are different types of radiation (e.g., X-rays, particle radiation),
and these are associated with different amounts of energy and potential to
cause biological damage; this variation in effect is attributed to the linear
energy transfer (LET), which has to do with the type of radiation and the
different biological effect on the tissue. Thus, while rads or grays measure
the physical aspects of the energy, they do not reference the biological
effect of various types of ionizing radiations and are not generally used as
TABLE F-1 Selected Units of Ionizing Radiation (measurements in bold
are used in this summary)
Measurement Description
RAD / Gray (=100 rad) Radiation in tissue
REM / Sievert (=100 rem) Radiation in tissue, accounting for biologic sensitivity
of tissue
Effective dose Full body equivalent dose equal to a partial anatomic
area radiation
EUS Average effective dose per individual in the U.S.
population, whether exposed to the specific exposure
or not
EExp Average effective dose to an individual in a group
exposed to that source of radiation
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414 BREAST CANCER AND THE ENVIRONMENT
measures of exposure. The sievert (Sv) is the SI unit of dose that is used
as a measure of exposure, as it attempts to reflect the biological effects of
radiation. It is also measured in joules/kilogram (as is the gray), but it incor-
porates the biological effectiveness of different types of ionizing radiation
in different types of tissues. For a given amount of radiation (measured in
grays), the biological effect (measured in sieverts) can vary considerably as
a result of the radiation-weighting factor (WR). The roentgen equivalent
man (rem) is the older unit of equivalent dose, and 1 Sv = 100 rem. Rem
remains the unit most widely used for occupational exposure.
Radiation exposures from medical imaging are rarely uniform—usually
one area of the body is exposed much more than another (compare the tis-
sue exposed to radiation from a mammogram to the tissue exposed for a
head CT). This is very different from naturally occurring radiation, which
often is more equally distributed across the body. To compare the radiation
doses associated with different types of exposures, it would be most accu-
rate to compare organ-specific exposures from different types of examina-
tions. For example, comparison of the dose to the lungs from a chest X-ray
and a chest CT would likely reflect the most accurate information regarding
the potential for these two types of tests to cause harm. However, this is
extremely impractical, as it would require creating a complex matrix of
organ-specific doses for each medical procedure. This would make it dif-
ficult to compare different exposures, as some types of exposures involve
partial-body irradiation, others full-body irradiation, and it makes it hard
to compare overall exposures in this way.
In order to compare radiation doses to different body parts on an
equivalent basis, it would be helpful to have a single metric that could be
used to compare all radiation doses (even though this is imprecise as the
mammogram is truly best summarized as a breast dose and a head CT a
brain dose). The effective dose (E) is a metric for estimating a full-body
dose that would be equivalent to an individual organ dose. It is estimated
by calculating a weighted average of the doses (equivalent dose) to different
organ systems. Thus, a large dose to a single organ might be similar (with
respect to the stochastic cancer risks) to a smaller dose to the entire body.
The weighting factors used to make these calculations reflect the different
radio sensitivities of the tissues. Because of the ease of comparison, effective
dose is an extremely useful measure of radiation exposure and will be the
primary metric used in this summary.
The measurement of radiation emitted by radioisotopes used in nuclear
medicine and positron emission tomography (PET) imaging, is the curie
(Ci), which describes a quantity of radioactive material that disintegrates
per second (CE Essentials, 2011). Decay or disintegration is a process by
which a radioactive nucleus changes to another type of atomic nucleus. The
SI unit of radioactivity is the becquerel (Bq). The becquerel is the activity of
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APPENDIX F
a quantity of radioactive material in which one nucleus decays per second.
The curie is an older, non-SI unit of radioactivity equal to the activity of
1 gram of radium-226.
Measurement of Absorbed Dose, Radiation Detriment, and Impact of
Individual Factors on Radiation Detriment
The amount of radiation absorbed into the body, and the resulting
doses to different parts of the body and organs, will depend on the person’s
sex and size (including relative amounts of different tissues). Compared
with an adult, a child exposed to a particular radiation dose will absorb
more per unit of tissue, as there is less organ volume in which the radiation
can dissipate. For example, compare a teaspoon of red dye added to a cup
of water as compared with a gallon of water—the dye in the gallon of water
will become much more dilute. The radiation detriment will relate both
to the absorbed dose and to the type of tissue that has been irradiated, as
some organs are much more vulnerable to the effects of radiation than other
organs. A complete comparison of different sources of radiation exposure
would try to estimate exposures to different organ systems and how this
varies by age of exposure (as both will influence the detriment), but this
level of detail would make it nearly impossible to provide an overview. Thus
to keep the summary as simple as possible, I will use primarily the mea-
surement of effective dose to describe the U.S. population’s exposures to
radiation, variation within and across different imaging tests in the delivery
of radiation, and radiation detriment.
Metrics of Population Exposure
The most useful metric to compare the population’s radiation expo-
sure from different sources is the average exposure, measured in effective
dose, per individual in the entire population. It is calculated by dividing
the cumulative dose to the population by the number of individuals in
the population (EUS) and does not consider whether an individual person
is exposed or not. As an example, television (TV) exposes individuals to
a small amount of ionizing radiation, and the average exposure from TV
would be calculated by dividing the total radiation delivered by all hours of
TV watched in the United States by the number of individuals in the United
States. This will be an estimate (as individuals watch different amounts of
TV), but it is useful for comparing average exposure to ionizing radiation
from TV and other sources, such as the sun or radon. In contrast, certain
types of radiation are received by a much smaller group of individuals,
such as occurs with occupational exposures. For these exposures it is more
reasonable to calculate the average dose to individuals exposed to that
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416 BREAST CANCER AND THE ENVIRONMENT
source of radiation, rather than compute an overall effective dose for the
population. This measurement, effective dose to those exposed (EExp), is
calculated by dividing the cumulative dose to the individuals exposed by
the number of individuals exposed. Both metrics will be used to describe
population exposure.
WHAT IS KNOWN ABOUT CARCINOGENICITY
OF IONIZING RADIATION
Ionizing radiation is widely used in industry and medicine, occurs in
many different natural sources, and presents a health hazard. It causes
microscopic damage to living tissue, and this results in elevated risks of
cancer and birth defects at low exposures, and more substantial biological
damage, including burns, radiation sickness and death, at higher exposures.
Radiation is one of the most heavily studied carcinogens, and exten-
sive epidemiological and biological evidence supports the association of
low and moderate doses of ionizing radiation with increased cancer risk.
The epidemiological evidence of an increased risk of cancer comes from
(1) cancer development among survivors of environmental and accidental
exposures to radiation (i.e., survivors of atomic bombs, Chernobyl, and
Soviet Union and United States weapons testing) (Pierce and Preston, 1993,
2000; Land, 1995; Ron et al., 1995b; NRC, 1996, 2003, 2006; Charles,
2001; Brenner et al., 2003; Preston et al., 2003, 2007; Preston, 2008); (2)
populations medically irradiated for benign conditions such as scoliosis,
tinea capitis, and tuberculosis (Ron et al., 1995a; Little, 2001; Modan et
al., 2000; Ron, 2003); (3) patients receiving radiotherapy for malignant
disease (Little, 2001; Neglia et al., 2001, 2006; Ron, 2003; Sachs and
Benner, 2005; Sigurdson et al., 2005; Bassal et al., 2006; Ronckers et al.,
2006); and (4) those who have received occupational exposures, including
radiologists, radiological technologists, and nuclear power workers (Lewis,
1963; Matanoski et al., 1975; Muirhead et al., 1999, 2009; Berrington
et al., 2001; Cardis et al., 2007; Linet et al., 2010). All of these groups
received doses in the range of that delivered by current medical imaging
and have been shown to be at increased risk of developing cancer. Evidence
of radiation-induced cancer is strongest for leukemia, but an increased risk
of all solid cancer types (e.g., breast, lung, colon) has been associated with
exposure to doses of ionizing radiation in the same range as that delivered
by medical imaging.
A full discussion of the data on the risks of ionizing radiation is outside
the scope of the review; however, a comprehensive review of the published
literature can be found in the report of the National Research Council,
Biological Effects of Ionizing Radiation (BEIR) Subcommittee (BEIR VII;
NRC, 2006). According to the BEIR subcommittee, the most accurate
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APPENDIX F
current model of radiation carcinogenesis at low to moderate doses is the
linear and nonthreshold model, which predicts that the risk of outcomes
(primarily cancer) is directly proportional to the radiation dose received,
is additive, and does not respect a minimum-risk threshold. This model
assumes there is no safe level of radiation exposure. This theory is the basis
for radiation protection recommendations by national and international
committees tasked with ensuring radiation protection of workers and the
general population (NRC, 1996; Charles, 2001; Gonzalez et al., 2007;
NCRP, 2009). While this theory is widely endorsed within the radiation
safety and protection communities, it is challenged by some who believe
there is a threshold below which there is no cancer risk (Strzelczyk et al.,
2007). Its important to emphasize that this theory is far less relevant for
understanding the detrimental effects of radiation from many exposures,
such as CT, where directly observed evidence shows that radiation doses
delivered to patients in the range of CT are carcinogenic. The model is most
helpful to understand the carcinogenicity of very low exposures that are
below those that have been studied in epidemiological studies.
A number of different mathematical models have been developed to
estimate the cancer risks associated with exposure to ionizing radiation
across a range of doses. The dominant model that has been most widely
adopted was used by National Academy of Sciences in the BEIR VII report
(NRC, 2006). There are a large number of assumptions in this model,
among them that the risk of solid cancer is linear and follows a non-
threshold model, and that the patterns of cancer outcomes among Japanese
atomic bomb survivors are directly transferable to the U.S. population. The
impact of these assumptions is often taken into account with creation of
uncertainty limits around estimated risks.
SOURCES OF IONIZING RADIATION
EXPOSURE TO THE U.S. POPULATION
The National Council on Radiation Protection and Measurements
(NCRP, 2011) is a nongovernmental, not-for-profit corporation chartered
by Congress to collect, analyze, develop, and disseminate to the public
information and recommendations about radiation, radiation protection,
and radiation measurements. The NCRP published two reports detailing
the U.S. population’s exposure to ionizing radiation in two time periods;
Report No. 93 was published in 1987 (NCRP, 1987b), and Report No.
160 was published in 2009 as an update to this earlier report and describes
exposures through 2006 (NCRP, 2009). These two reports are comprehen-
sive, and each reviewed hundreds of different data sources and references.
They provide the most accurate overview of the magnitude of the U.S.
population’s exposure to radiation and the distribution of exposure among
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418 BREAST CANCER AND THE ENVIRONMENT
the various sources of radiation. Data from these two reports have been
summarized to highlight current exposures, sources that deliver high radia-
tion exposures, and sources that have changed over time.
Radiation from medical imaging is a large and growing source of
radiation exposure to the U.S. population. I have led several analyses
that describe medical imaging and associated radiation exposure within
a large cohort of patients (approximately 2.5 million) enrolled across six
integrated health plans in the United States (Burger et al., 2010a,b). These
health plans participate in the Health Maintenance Organization (HMO)
Research Network, and the National Cancer Institute (NCI)-funded Cancer
Research Network (CRN), and as such, they have common data elements to
permit assessing medical imaging within the HMO setting. The work was
used to supplement the NCRP reports and to further characterize the U.S.
population’s exposure to radiation from medical imaging. We had more
detailed data on medical imaging at the patient (rather than population)
level, allowing more accurate estimates of individual doses and the propor-
tion of patients who exceed certain threshold doses.
Sources of Radiation
Radiation exposure is described within five broad categories: (1) ubiq-
uitous background radiation, sometimes described as natural sources of
radiation; (2) radiation from medical procedures; (3) radiation from con-
sumer products or activities involving radiation sources; (4) radiation from
industrial, security, medical, education and research activities; and (5) occu-
pational exposure (see Table F-2).
Ubiquitous Background Radiation
There are four primary sources of radiation exposure that fall in this
category: (1) external exposure from the sun and cosmic rays (space radi-
ation); (2) external exposure emitted from the earth (terrestrial radia-
tion, primarily from potassium, uranium, and thorium in surface soil and
rocks); (3) internal exposure from inhaled radon (released from the earth);
and (4) internal exposure from radionuclides in the body (particularly
potassium). Exposure to radon is the largest contributor to this category,
accounting for nearly 75 percent of ubiquitous background radiation.
Radon gas comes from the breakdown of uranium in soil, rock, and water,
and it is trapped in houses and inhaled.
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APPENDIX F
TABLE F-2 Summary of U.S. Population’s Annual Exposure to Ionizing
Radiation from Different Sources, 2006–2010
Radiation Source EUS (mSv) EExp (mSv)
Ubiquitous background radiation 3.11 —
Radon and thoron (inhalation) 2.28
Space 0.33
Ingestion 0.29
Terrestrial 0.21
Medical radiation 3.00 7.8
Computed tomography 1.47 5.1
Nuclear medicine 0.77 0.61
Interventional radiology 0.43 1.14
Conventional X-rays* 0.33 0.68
Consumer 0.13
Industrial, security, research, education 0.003
Occupational 0.005 1.1
Medical 0.8
Aviation 3.1
Nuclear power 1.9
Industry and commerce 0.8
Education and research 0.7
Government, Department of Energy, military 0.6
TOTAL 6.2
NOTE: These numbers have been adapted from NCRP (2009) and from our research of ra-
diation exposure across six integrated health care systems that participate in the NCI funded
Cancer Research Network (calculations of EExp, medical imaging) (Burger et al., 2010a,b).
*Includes mammography.
SOURCES: NCRP (2009, p. 242–243; used with permission of the National Council on Ra-
diation Protection and Measurements, http://NCRPpublications.org); Burger et al. (2010a,b).
Medical Exposure to Radiation
Radiation used for medical imaging Most medical imaging tests—including
radiography, fluoroscopy, angiography, and CT (all utilize X-rays), and
nuclear medicine (utilizes gamma rays)—expose patients to ionizing radia-
tion. In any given year, between 30 and 40 percent of the U.S. population
will undergo one or more imaging tests that deliver radiation, varying
by age and sex, and this has increased only slightly over time (Burger et
al., 2010a,b). Numerically, conventional radiographs contribute the great-
est number of imaging tests (58 percent), followed by CT (12 percent),
and fluoroscopy, angiography, and nuclear medicine (2–5 percent each).
However, because the doses are so much higher for CT, it contributes the
majority (nearly 70 percent) of patients’ exposure to radiation from medi-
cal imaging (NCRP, 2009; Burger et al., 2010a,b). Currently, the average
exposure to radiation from medical imaging is around 3 mSv per year
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434 BREAST CANCER AND THE ENVIRONMENT
that have estimated the number of future cancers that would expected to
result from ionizing radiation associated with current levels of medical
imaging.
These analyses have estimated future cancers that would result from
exposure to conventional radiographs (Berrington de Gonzalez and Darby,
2004), CT (Berrington de Gonzalez et al., 2009), and myocardial perfu-
sion scanning (the most common type of nuclear medicine examination)
(Berrington de Gonzalez et al., 2010). Combined, these exposures would
account for approximately 80 percent of the U.S. population’s cumulative
annual exposure to radiation from medical imaging.
These previously published results have been updated by Dr. Berrington
de Gonzalez for this report to focus specifically on future breast cancers,
taking into account the increase in imaging rates since these papers were
published, and including cancer risks associated with the additional imag-
ing examinations not covered in these prior reports. These calculations were
based on estimates of the number of imaging tests conducted annually, the
age distribution of patients undergoing those tests, radiation delivered by
those tests, organ-specific doses, and models of carcinogenesis, including
the biological effectiveness of the radiation, underlying cancer risks of the
population, lag time between exposure and cancer induction, and mortality
in the population undergoing imaging. The major sources of information
used in each of these analyses are clearly outlined in the primary manu-
scripts. For CT, which is the largest contributor to radiation dose, each
of the major assumptions used to generate the estimates was varied in the
model to generate an estimate of the uncertainty limits. Each of the papers
is summarized below.
Cancers Associated with Radiography (Berrington de Gonzalez and
Darby, 2004)
Dr. Berrington de Gonzalez’s original analysis estimated the risk of
cancer from conventional X-rays in the United Kingdom and 13 other
developed countries. The authors combined data on the frequency of X-rays
(including mammography), estimated organ-specific radiation doses from
each type of X-ray, and used cancer risk models for each organ system that
were based on the Japanese atomic bomb survivor data, and combined
these with population-based cancer incidence rates. The authors estimate
that in the United Kingdom about 0.6 percent of the cumulative risk of
cancer to age 75 years could be attributable to diagnostic X-rays. This
percentage is equivalent to about 700 cases of cancer per year. In the 13
other countries, estimates of the attributable risk ranged from 0.6 percent
to 1.8 percent; whereas in Japan, which had the highest estimated annual
exposure level in the world, it was more than 3 percent.
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APPENDIX F
The model developed for the 2004 paper was updated for this Insti-
tute of Medicine summary to estimate breast cancers that would result
from conventional radiographs conducted in the United States in 2006.
This involved updating assumptions regarding the number of conventional
radiographs conducted in the United States in 2006 (Mettler et al., 2009)
(overall increase from 960 to 1,080 tests /1,000 patients/year), and adapting
the results for the 14 countries included in the original publication to cancer
incidence rates in the United States. Based on these analyses, Dr. Berrington
de Gonzales has estimated that 250 breast cancers would result from
conventional X-ray use in the United States in 2006, and that the number
would be likely to increase to 280 with imaging rates observed in 2010.
Cancers Associated with CT (Berrington de Gonzalez et al., 2009)
Berrington de Gonzalez et al. (2009) estimated the frequency of dif-
ferent types of CT scans performed in the United States in 2007 using a
combination of data sources, primarily Medicare claims data and the IMV
Medical Information Division survey of CT scan use in 2,451 U.S. facilities
in 2007. Radiation-related cancer risks depend on sex and age at exposure,
and the authors estimated the age and sex distribution for each CT scan
type using a large national commercial insurance database. These estimates
were scaled to be applicable to the age–sex distribution of the U.S. popula-
tion and combined with the national frequency estimates. The authors used
risk models based on the National Research Council’s BEIR VII report
(NRC, 2006) with minor modifications, and they developed additional
models for anatomic sites that were not covered in the BEIR VII report.
An important assumption in the estimation of lifetime radiation-related
cancer risk is the life expectancy of persons receiving CT scans. There is
a lag between radiation exposure and cancer diagnosis (which can be as
short as 2 years for leukemia and 10 years for brain cancer [Preston et al.,
2007]), and patients who are unlikely to survive that long are unlikely to
develop imaging-related cancer. To address the problem of survival, the
authors used the commercial insurance data set to estimate the proportion
of scans performed in patients who were unlikely to survive long enough
to develop cancer. Scans performed in cancer patients were also omitted,
as consideration of second cancers was outside the scope of their analysis.
The estimated mean number of radiation-related incident cancers, with
95 percent uncertainty limits (UL), were calculated using Monte Carlo
simulation. Uncertainty intervals were estimated to account for statistical
uncertainties in the risk parameters, uncertainties in the dose rate reduction
effectiveness factor, and differences in application of risks from the Japanese
to the U.S. population.
Overall, the authors estimated that approximately 29,000 future can-
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436 BREAST CANCER AND THE ENVIRONMENT
cers (95% UL, 15,000–45,000) could be related to CT scans performed
in the United States in 2007. The largest contributions were from scans
of the abdomen and pelvis (n = 14,000) (95% UL, 6,900–25,000), chest
(n = 4,100) (95% UL, 1,900–8,100), and head (n = 4,000) (95% UL,
1,100–8,700), as well as from chest CT angiography (n = 2,700) (95% UL,
1,300–5,000). Approximately one-third of the projected cancers were from
scans performed on patients between the ages of 35 and 54 years, whereas
only 15 percent were from scans performed on patients under the age of
18 years. This is important, as it is widely believed that cancer risks are
only of concern in children; however, because imaging with CT increases
so much with increasing age, cancers that result from radiation from the
more frequent medical imaging that occurs in middle age are actually more
numerous. Further, Berrrington de Gonzalez’s models assumed that the risks
of cancer per exposure decline with advancing age. However, several recent
analyses have suggested that risks might even increase with exposures at
older ages (Preston et al., 2007; Shuryak et al., 2010), making the estimates
presented here highly conservative (i.e., the true risks may be higher than
these analyses suggest).
The breakdown by cancer site showed that lung cancer was the most
common projected radiation-related cancer (n = 6,200) (95% UL, 2,300–
13,000), followed by colon cancer (n = 3,500) (95% UL, 1,000–6,800)
and leukemia (n = 2,800) (95% UL, 800–4,800) (Figure F-6). The cancer
sites with the highest risks were common cancers with a high frequency of
exposure to that organ (e.g., colon from CT of the abdomen and pelvis and
lung from CT of the chest) or higher radiosensitivity (e.g., red bone marrow
and leukemia). The projected radiation-related cancers would be spread out
over many decades. However, if CT scan use remains at the current level or
increases further, then the results of the Berrington de Gonzalez analyses
suggest that eventually 29,000 (95% UL, 15,000–45,000) cancers every
year could be related to past CT scan use. This is equivalent to approxi-
mately 2 percent (1–3 percent) of the 1.4 million cancers that are diagnosed
annually in the United States being attributable to the prior use of CT.
Using very slight modification of these methods, Dr. Berrington de
Gonzalez estimates that 1,800 future breast cancers could result from CT
utilization in 2007.
Cancers Associated with Cardiac Nuclear Medicine Imaging (Berrington
de Gonzalez et al., 2010)
Nuclear cardiac perfusion tests are used in the assessment of coronary
artery disease, and they represent the second largest source of medical
radiation exposure in the United States. It is estimated that 85 percent of
radiation exposure from nuclear medicine procedures is accounted for from
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APPENDIX F
Number of future cancers
FIGURE F-6 Projected number of future cancers (mean and 95% uncertainty lim-
its) that could be related to CT scan use in the United States in 2007, according to
cancer type.
SOURCE: Berrington de Gonzalez et al. (2009, p. E1). Reprinted with permis-
sion from Berrington de Gonzalez, A., M. Mahesh, K. P. Kim, M. Bhargavan,
R. Lewis, F. Mettler, and C. Land. 2009. Projected cancer risks from computed
tomographic scans performed in the United States in 2007. Arch Intern Med
169(22):2071–2077. Figure F-6 Future cancers.eps
bitmap, type so small, suggest redraw
these types of examinations. As described above, Berrington de Gonzalez
and colleagues (2010) estimated future cancer risks that would result from
the cardiac perfusion scans conducted in 2007. Combination of cancer
risk estimates with data on frequency of use suggested that the 9.1 mil-
lion annual cardiac perfusion tests in the United States could result in
8,100 (3,700–15,200) additional future cancers, assuming use of median
radionuclide activity. If the tests were performed at the lowest or highest
recommended activity levels, risks would be reduced or increased by about
20 percent. If the levels of use remain similar in the future, the estimates
suggest that, eventually, approximately 0.6 percent (0.3–1.1 percent) of the
1.4 million cancers diagnosed annually in the United States could be related
to these studies.
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438 BREAST CANCER AND THE ENVIRONMENT
TABLE F-5 Estimated Number of Future Breast Cancers That Might
Occur Related to a Single Year of Medical Radiation Exposure
Source of Medical Radiation Estimated Number of Cancers
CT scans 1,800
Conventional radiography 280
Nuclear medicine 150
Other 560
All sources 2,800 (UL, 2,000–3,500)
Using very slight modification of these methods, Dr. Berrington de
Gonzalez estimates that 150 future breast cancers could result from nuclear
medicine utilization in 2007.
Summary of Future Breast Cancers from Medical Imaging
Berrington de Gonzalez estimates that 2,230 future breast cancers
would result from radiation from conventional radiography (280), cardiac
perfusion imaging (150), and CT (1,800), as described above. These cancers
would occur over women’s lifetimes, but result from these medical imaging
exposures from a single year. These estimates account for about 80 percent
of collective medical radiation dose to the U.S. population as interventional
radiology is not included (16 percent of collective U.S. dose) and the other
15 percent of nuclear medicine (4 percent of collective U.S. dose). Thus she
estimates an additional 560 breast cancers would result from the remain-
ing radiation exposure from angiography and interventional radiology
procedures, and from the nuclear medicine procedures not counted in this
estimate. This yields a total estimated cancer burden of approximately
2,800 future breast cancers (95% UL, 2,000–3,500) that would result from
1 year of medical radiation exposure among the entire U.S. female popula-
tion (Table F-5).
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