AAPM
Tutorial Science Michael
and V. Broadbent,
The
risks,
real
perceived tial
Perception PhD
and
on
#{149} Lincoln
supposed,
by patients,
impact
B. Hubbard,
of the
their
medical
of Radiation
use
families,
practice.
PhD
of radiation
and
the
Attempts
in medical
general
have
will
been
accept
for
radiologist tively
lowered
a given
must
over
benefit,
be well
communicate
the in part,
informed
this
years.
risk
The
about
a substan-
radiation
risk
what
ef-
the
that
the
limpublic
is permissible.
effects
that
as
causes considerable from this. Radiation in these limits have at the time. Dose
perceived
radiation so
have
low-level
determines
information
practice
public
to quantify
fects involve much conjecture and supposition. This uncertainty in results. Conflicting perceptions follow exposure limits have been established, and changes been made on the basis of the best available judgment itations
Risk1
and
public
The
must
will
effec-
be
well
in-
formed. INTRODUCTION Humans are exposed to varying amounts Figure 1 illustrates the current estimates equivalent. Natural sources of radiation als
naturally
alent from the greatest mainly
occurring
in the
of natural and manufactured radiation. of the components of the effective dose are cosmic radiation and radioactive materi-
environment
and
radon has become a matter fraction of radiation dose
used
in medical
irradiation
in the
of concern equivalent
of patients,
also
However, the estimated value has declined from mrem) per year over the past decade. This is due average dose received from medical radiography.
Abbreviations: ESEG
BEIR entrance
=
diological
Protection,
missible dose, sion, UNSCEAR Index
terms:
measurement
I
From
C RSNA,
Fields, Received
Biological
=
exposure
Radiations,
and
1992;
12:381-392
=
Food
Radiation, and
Commission
to patients
Drug on
and
contributes
CDRH
Radiation and Effects
personnel
effective
dose
equiv-
contributes sources,
a significant
fraction.
1 mSv (100 mrem) to 0.68 mSv (68 primarily to the decrease in the
=
Center
Administration,
Protection on the
The
for Devices
ICRP
Units
and
Measurement, ofAtomic
Radiations,
and
Radiological
International
Commission
Measurements, NRC Radiation
injurious
#{149}
=
MPD =
effects
Nuclear
=
maximum
Regulatory
Health. on
Raper-
Commis-
Radiations,
#{149}
radiologists
Hubbard, 28,
of Ionizing
Council on Radiation Scientific Committee
exposure
Radiology
March
FDA
International
NCRP = National = United Nations
Griffiths,
Effects guide,
ICRU
#{149}
RadioGraphics
sembly.
skin
body.
as this natural source to humans. Man-made
1991;
and
Broadbent,
accepted
April
2367
Oak
18. Address
Hill
Dr, Lisle,
reprint
IL 60532.
requests
to
From
the
1990
RSNA
scientific
as-
M.V.B.
1992
381
Consumer
Products
(3.0%) (11.0%)
‘?
Figure 1#{149} Chart trates contributions
illusfrom
various
sources
radiation
to the average dose equivalent United
nal (11.0%)
effective in the
States
Radon
(55.0%)
restial (8.0%) (8.0%)
(1).
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ja
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r
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STS.
MADISON
v-
!MD.
mission,
DEVELOPMENT
thy
:
Figure
..
:
made
a,.,
;
.‘
AND
Ezpn
.
ttstimony
advertisement
M(d
H. GRUBBE
for his x-ray
etc., by the X-Ray
Tuberculosis,
clinic
a s,
in 1899.
M. D.,
(Reprinted,
with
per-
2.)
Conrad R#{246}ntgen discovered with a Hittorf-Crookes tube.
abdominal
erythemas
from
carrying
small
samples
of radioactive
materials
their vest pockets (5). Early radiation injuries were primarily confined to the skin (Fig 3) and were sidered, at worst, annoying. This attitude soon changed with the development progression of more serious injury. Mihran Knikor Kassabian thoroughly docu-
mented exposure,
the course of the radiation injury to his hands, following tremendously that led to cancer and his eventual death (Fig 4). At about the same
experiments
U
RadioGrapbics
sthcit
re4cytd
x rays on November 8, 1895, while experiAfter several weeks of intensive investigation, he reported his results on December 28, 1895. This discovery helped motivate Antome Henri Becqucrel early in 1896 to investigate the fluorescence produced by uranium. (Materials on the early history ofradiology have previously appeared in RadloGraphics [2,3].) The first reports ofradiation injuries followed almost immediately. Emil H. Grubbe, an early and enthusiastic user ofx rays (Fig 2), demonstrated radiation ei-ythema on his hands to a group of physicians within 2 months of R#{246}ntgen’s discovery. By April 1896, John Daniel at Vanderbilt University observed and reported epilation on the scalp of his dean, who had volunteered to undergo radiography of the brain. Not surprisingly, early radiographic techniques lacked many of the safety features we are accustomed to today (4) . The acute hazards from radioactive materials also became obvious. Both Becquerel and Pierre Curie in-
curred
382
P.M.
c.
tven
t::
4’.
‘#{149}
ijiisps.4
may
.
Laboratory
w
ksda.gra;th
mad.
, IU
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.
.\
STATE
k’ 4
-
tive
organs
U
Broadbent
demonstrated of animals
and
the
radiosensitivity
of the
blood-forming
and
in conand
high time,
reproduc-
(6).
Hubbard
Volume
12
Number
2
gure 3. Photograph of Spanish-American -#{149}r soldier shows radiation injury to the skin following an x-ray examination in 1898. (Rewith
a.
permission,
from
reference
2.)
b.
Figure 4. Mthran tailed photographic
Krihor Kassabian, records showing
hands appear tanned and aged due the x-ray beam. (b) In 1908, cancer gressive amputations. Dr Kassabian sion, from reference 3.)
Several ports
international
assessing
the
and
MD,
the progress
bodies
wrote
ofhis
have
of particular
textbooks
chronic
and
condition.
kept
de-
(a) In 1903,
his large doses received from calibrating to a succession of surgeries and proin 1910. (Reproduced, with perrnis-
to the extremely appeared, leading died ofmetastases
national
significance
of Philadelphia
been
levels
established
of radiation
that dose.
publish
Many
re-
of these
organizations also recommend permissible dose limits. The International Commission on Radiological Protection (ICRP) provides reports and recommendations on radiation effects and radiation protection standards. The International Commission on Radiation Units and Measurements (ICRU) provides definitions of fundamental radiologic quantities and units, as well as procedures for their measurement. The United Nations Scientific Committee on the Effects ofAtomic Radiation (UNSCEAR) reports on the effects of irradiation and proposes dose-effect models. In the United States,
the
National
Council
on
Radiation
Protection
and
Measurement
(NCRP)
pro-
vides recommendations on radiation quantities, units, measurement, and protection. The Committee on the Biological Effects of Ionizing Radiation (BEIR), which part of the National Research Council of the National Academy of Sciences, reports on the health consequences of radiation exposures. These organizations provide recommendations
based
on
the
best
available
sidered standards of good practice. Regulatory levels often use these recommendations as the The first attempts to establish safe radiologic
scientific
agencies baseline practices
evidence,
and
they
arc
is
con-
at the national and state for legal regulation. used the concept of toler-
ance dose. This concept implies a threshold dose below which no injury occurs. William H. Rollins is credited with publishing the first tolerance dose in 1902 (6). When stochastic radiation effects became known, tolerance dose gave way to the concept of maximum permissible dose (MPD). The MPDs are set at a level such that
March
1992
Broadbent
and
Hubbard
U
RadioGraphics
U
383
Tolerance 10
1
ii
Controlled
I
ALARA
0.4
a
0.1
0. 1
Noncontrolled
I Alara 0.02 0.01 1900
t
t
1920
1940
I
I
1960
I
1980
2000
Year
Figure
depicts the change in effective dose equivalent limits over time. Data are given in millisieverts per week; in most cases, the values represent an average over longer periods. Initial value of 10 mSv (1 rem) per week was reduced to 3 mSv (300 m.rem) perweek in 1949. By 1958, the concept had changed to MPD, and the occupational MPD (controlled) was reduced to 1 mSv (100 mrem) per week. The other (noncontrolled) category was limited to 0. 1 mSv. AL4RA = as low as reasonably achievable.
it is unlikely
that
an
individual
5.
will
Graph
incur
any
nonstochastic
injury
during
his
or her
lifetime and that the probability of stochastic injury is “acceptable.” The concept of MPD has evolved over the past 65 years. Its numerical value has declined over the same period (Fig 5). By 1958, separate values ofMPD were assigned to occupationally exposed individuals (radiation workers) and all others. The occupational MPD was reduced to 1 mSv (100 mrem) per week. The other category was limited to 0.1 mSv
(10
mrem)
per
week.
(For
The occupational limit effective dose equivalent
a discussion
of radiation
of 1958 could be exceeded, was limited by the following H
< 50(Age
-
18)
units,
see
provided equation:
the
that
Appendix.)
the
cumulative
mSv,
where
H = cumulative dose equivalent. For example, a 50-year-old worker could have received 1,600 mSv. The NCRP recommended several important changes in 1987 (7). These include the introduction of a third category: nonradiation workers who are frequently cxposed, usually as part of their employment (eg, a secretary who occupies an office next to a radiography room). The limit for such an individual is set at 1 mSv (100 mrcm) per year. The NCRP recommended that the limit for occupational exposure and for exposure of the general public remain at 50 and 5 mSv per year, respectively. It further recommended that the cumulative occupational exposure be limited
to
H For
example,
tive
dose
the of 500
Currently, tions
the
may
the be
same
worker
mSv.
would
be limited
to a maximum
cumula-
mSv.
values
proposed
BEIRVrcport Whatever
50-year-old
< 10(Age)
of the
MPD
arc
on
basis
of recent
the
being
actively
reviewed,
conclusions
and
of the
further
UNSCEAR
reduc(8)
and
(9). numerical
limits
are
set
for
radiation
exposure,
they
tend
to be
per-
ceived as if they were threshold doses. According to this perception, exceeding a dose limit will cause permanent and severe radiation injury. This perception is reinforced by regulatory bodies. For example, the Nuclear Regulatory Commission (NRC) fined the owners of a nuclear power plant $ 100,000 for allowing an individual to receive about 70% more than the yearly dose limit in a single incident (NRC Region III, oral communication, 1990).
384
U
RadioGraphics
U
Broadbent
and
Hubbard
Volume
12
Number
2
As stochastic reduce
radiation
dose
injuries
to a value
in situations
are
as low
in which
presumed
to have
as is economically
feasible;
exposure
limits
are
well
below
no threshold, this
regulatory
report on the BEIR V report dose equivalent
the dosimetry for survivors cates that victims received
bombings equivalents
“
estimated
This
concept
achievable.” It is anthe risks of radiation
BEIR V REPORT AND ITS
“Health Effects ofExposure (9), many standard-setting limits. Reassessment of
of Hiroshima lower than
EFFECTS
and Nagasaki mdithose previously
As a result, the lifetime risk of cancer attributable to a given dose of now appears to be greater than previously estimated. to revising the estimates of physical dose, the BEIR V Committee reexchoice of an appropriate dose-response model. They concluded that the
radiation
In addition amined the
dose-dependent
in the
to even
(8).
gamma
nificant rem]).
of the atomic effective dose
tries
be done
limits.
of practical dose minimization is called “as low as reasonably other expression of the desire to achieve a balance between exposure and the benefits of its use. With the publication in 1990 ofthe latest to Low Levels of Ionizing Radiation, the bodies have begun to reevaluate effective
one
should
excess
departure Even though lesser
risk,
mortality
from the
for all cancers
the linear dose data for leukemia
linear-quadratic
other
than
model at low were found
dose-response
leukemia
showed
levels (ic, below to be compatible
model,
the
BEIR
no sig-
4 Sv [400 with those
V Committee
chose to use a linear dose-response curve for all cancers. The carcinogenic effects of radiation reported for atomic bombing survivors are similar to findings reported for other irradiated human populations. In most ofthese populations, effects have been observed only at relatively high doses and usually at high-dose rates. Ifan individual were exposed to the same total dose with the exposure occurring over weeks or months, the lifetime risk may be reduced by a factor of two or more. Lifetime radiation risk varies considerably with the individual’s age at the time of
exposure. The childhood risk is estimated to be about twice The cancer risk estimates derived by the BEIR V Committee greater
for solid
cancers and four times greater for leukemia in the BEIR III report (10). These differences are
presented the linear-quadratic atomic
bomb
tistical
uncertainties
to the pure dosimetry
in the
which effects have actually certainly introduces more The BEIR V Committee in considering
the
linear
mentioned
safety
dose-response above.
available
arc
and
said
“Given
the
three
to the revised that
the
from
the
doses
been observed to the lower doses of diagnostic uncertainties into the estimates. concluded that genetic risks were not of great of medical
radiation
exposures.
pressure
of BEIR
V, the
At present,
council
might
times
in mind
Extrapolation
cer is more significant. The committee believes that adequate genic hazard will adequately handle the risk ofgenetic damage. The publication of the BEIR V report has produced reaction, community and in the media. In January 1990, Warren Sinclair, NCRP
about
than the risk estimates due to a change from
be borne
large.
of adults.
are
model
It should
data
that
at
imaging importance
the
control
sta-
risk
of the
of can-
carcino-
both in the scientific president of the very
well
feel
that
now
is the time to reduce maximum occupational exposure limits from 5 rem per year to something less.” After the press conference accompanying the release ofthe report, Arthur Upton, the BEIR V Committee chairman, was quoted in the Washington Post: “The average citizen should not view this [report] as a source ofgrcat concern” and “X-rays and other medical treatments involving radiation clearly remain appropriate
when the benefits port and its many
outweigh caveats,
the risks.” Nevertheless, headlines reported: “Low (Washington Post. December (New York Times. December
despite
the wording
of the re-
Radiation Causes More Deaths than Assumed” 20, 1989) and “Higher Cancer Risk Found in Radiation” 20, 1989). Newsweek (January 1, 1990) began its account with: “When your dental technician dashes out of the room before zapping your molars with x-rays, she knows what she’s doing; radiation is dangerous.” Science (January 5, 1990) stated in its lead paragraph that: “the 421
page
report
.
.
that the dangers quotes as reported
March
1992
.
pulverizes
an
argument
made
Level
by a group
oflow-level radiation were being by Goldsmith SJ in Radiology
of experts
exaggerated” Today, 1990;
Broadbent
ten
years
ago
(all preceding 1:22-23).
and
Hubbard
U
RadioGraphics
U
385
METHODS OF REDUCING RADIATION EXPOSURE
Many
early
radiologic
techniques harmful
6, 7) (1 1). Once the effort to reduce radiation (Figs
graphic
intensifying
exposure
time
arc considered unnecessarily effects of radiation were
exposure
screens were (Fig 8). A beneficial
was begun developed side effect
hazardous recognized,
by all concerned. to improve image was the reduction
6.
today a continuing
For example,
radio-
quality by reducing in patient dose.
7.
6, 7. (6) WilliamJ. Morton tests the penetrating power ofthe x-ray tube by observing his own hand with a hand-held fluoroscope. (Reproduced, with permission, from reference 3.) (7) Photograph obtained in 1899 of a patient being examined with a handheld fluoroscopic screen. The tube is somewhat shielded with hard rubber. The primary use of the rubber shield was to improve the image by reducing scattered radiation. It also reduced the dose to the patient and fluoroscopist (1 1). (Courtesy of Nancy Knight, Amencan College of Radiology, Reston, Va.) Figures
Figure
8.
Advertisement
for screens published the Americanjournal Roentgenology (vol 1913.
in of 1) in
Snook Lagless
Intensifying
SCREENS For
Rapid T
HESE
Radiography
SCREENS
have
no lag,
no peer, are fast, durable cost little, and are the greatest development of intensifying
make
the
exposures
no grain,
and washable; big stride in the screens ; they
instantaneous.
PRICES SCREEN
SIZE
8’ x 10’ 10’ x 12’ 11’ x 14’ 14’ x 17’
Snook
Roentgen 1210 Race
386
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Hubbard
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(P1st.
HoId.r)
6.00 10.00 11.00 13.00
Philadelphia,
Co. Pa.
Volume
12
Number
2
Many
subsequent
developments
in screen-film
technology,
ofrarc-earth
phosphors, were targeted primarily None of the original x-ray tubes had any means Lead glass balls and collimators were introduced tury. These diagnostic
devices provided some x-ray tubes are electrically age to less than 1 mGy (100 mrad) conditions. This value is comparable
produced
by the patient
in tube in the
shielding room.
interest.
would
during not
Collimation
Collimation
measure of protection to shielded (shockproof) per hour at 1 m from the to the average intensity
a radiographic
substantially
is required reduces
the
such
patient’s
introduction
the the
integral
total useful dose
shielding. 20th cen-
the patient. Modern and limit radiation leakfocal spot under most of scattered radiation
examination.
reduce to restrict
as the
toward dose reduction. of radiation or electrical in the first years of the
Thus,
further
increases
secondary
radiation
x-ray
to the
and
beam also
reduces
level area the
of
pro-
duction of scattered radiation. Most modern radiography equipment is required to have automatic collimation, which limits the beam size to that of the image receptor (Fig 9). If the examination permits, a reduction in field size will both improve image quality and reduce radiation dose. The useful x-ray beam itself must be managed to minimize patient dose. Filters are placed between the focal spot and the patient to attenuate soft x rays that have no chance of penetrating the image receptor. Table 1 summarizes the minimum filtration recommended by the NCRP (12). Once the beam has passed through the patient, it is desirable to convert as much of it as possible into a useful image. Thus, radiographic tabletops and cassette faces must not be too attenuating. These aspects
are regulated in the United States by the Center for Devices and Radiological (CDRH), an agency of the Food and Drug Administration (FDA) (13). Some regulations Shielding fluoroscopes primary beam
arc shown in Table 2. is also required to reduce are interlocked is intercepted
Table 1 NCRP Recommendations
radiation
dose
to the
staff.
be produced only and its associated
FDA
Regulations
Equipment Minimum (mm
Kilovoltage
BelowSO 50-70 Above70
0.5 1.5 2.5 12.
Aluminum
Filter Al)
Component
Equivalent
Stationary tabletop Movable tabletop Cradle-type tabletop Cassette front Film
Figure
1.5 2.0 1.0 1.0
changer
9.
cal Systems,
(mm) 1.0
13.
Photograph
of a modem
tic x-ray tube housing with matic collimator. (Courtesy
1992
example,
for Radiography Components
Source-Reference
March
For
when the entire shielding.
Table 2 for Minimum
Filtration
Source-Reference
the
so that radiation can by the image intensifier
Health of their
Shelton,
Broadbent
an attached of Philips
diagnosautoMcdi-
Conn.)
and
Hubbard
U
RadioGrapbics
U
387
Most fluoroscopes provide additional of Bucky slot covers and lead drapes.
Pb) should
be used
noscopy.
gonadal
RADIATION TO STAFF PATIENTS
shielding
Few formal
provide
(minimum
Adequate to less than
regulations
to the
during
protection
thickness,
that
limit
fluoroscopy The 5-minute
passage
in a room
better 0.5
radiation thickness,
mm
Pb)
of time.
dose
rate
or dose
simple
should
also
skin
delivered
exposure
or fluoaprons.
be
Pa-
employed
to individuals
outside
to patients.
is limited to 100 mGy (10 rad) fluoroscopic timer is present
Entrance
in the form 0.25 mm
radiography
than
room shielding limits the dose the relevant dose equivalent.
exist
dose delivered during normal circumstances. roscopist
present
garments
when appropriate. procedure room
DOSE AND
by all personnel
Wraparound
tient
shielding from scattered Lead aprons (minimum
guides
a
The
per minute under to alert the fluo(ESEGs)
have
been
defined for 10 common radiographic projections (14). These arc shown in Table 3. Adherence to these guidelines is mandatory for federal facilities and is recommended by the FDA for other facilities. The American College ofRadiology specifies a maximum mean gI andular dose of 4 mSv (400 mrem) pen average mammogram.
The Joint patient
Commission
doses
on Accreditation
of Healthcare
Organizations
requires
that
by a physicist for commonly employed examinations performed by each radiographic unit (15). It further requires that these evaluations be compared with the ESEG values or with national averages, such as the Nationwide Evaluation ofX-Ray Trends (NEXT) (16). The NCRP has published guidelines for calculating fetal and childhood doses from x rays (17,18). Dosimetry from nuclear medicine procedures can be calculated by using
the
report
be determined
Medical
Internal
provides
(20).
Some
viewed
specific
this
Once
from
report
are
the easy
reduction
features.
matic
presented
increased
service. Before ally be adjusted
(Webb
in Table
to reduce
the the
1974
cost
and
number
automatic collimation, in the field in a few
Values
for
Anatomic
Region, View
Chest, Skull,
ESEG
PA lateral
Abdomen, AP Cervical spine, Thoracic
spine,
(mR)*
30 (0.26) 300 (2.62) 750 (6.55)
AP AP
Retrograde
pyelognam,
Lumbosacral
spine,
AP AP
Full spine, AP Feet, dorsal plantar Dental (bite wing)
help
and
250
(2.18)
900 900
(7.86) (7.86)
1,000 300
(8.73) (2.62)
270 700
(2.36) (6.11)
Note.-AP = anteropostenior, PA = posteroanterior. From reference 14. *Numbe in parentheses represent air kerma values in milligrays.
dose
have
may
require
complexity
RadioGraphics
U
Broadbent
and
Hubbard
NCRP
scmntigraphy dose
report,
1990).
are
re-
Excerpts
employed,
tnade-offs the
collimator Now, the
further with
piandatory
other
use
of radiographic failures
de-
of auto-
equipment.
and
the
difficulty
of
size indicators could usuadjustment of size indicators
Table 4 ICRP Committee 3 RecommendatIons for Reducing Radiation Dose Method: Define
radiologic rigorous
Improve
procedure referral criteria
availability
Minimize
of previous
radiographs
images
per procedure
Minimize
fluoroscopic exposure QA and QC programs Regularly assess repeat rates Shield sensitive organs when possible
Improve
Choose projections to minimize Method: radiologic equipment Thorough QC of processor
dose
Increase use of digital image processing Use lowest dose image recording device consistent with image quality Use pulsed fluoroscopy as appropriate Note.-QA unpublished
U
additional
patient
been
for
quality
388
An
undergoing
reduce
requirement
manual minutes.
Views
(19).
unpublished
of equipment
Radiographic
Common
tables
patients
4.
in cost
Table 3 ESEG
can
GAM,
radiation
the
increased
complexity
that
ICRP
example,
(MIRD)
for pediatric
an increase
For
collimation
This
the
methods
necessitates
sirable
Dose
guidance
recommendations
in a report
from
Radiation
dosimetric
=
control.
quality From report,
assurance, QC Webb GAM, 1990.
Volume
12
=
Number
2
on the automatic collimator usually requires the return tory. In our practice, we frequently hear that approximately and costs are associated with the automatic collimator.
A decrease
in the effective
dose
equivalent
limit
of the
collimator 50% of the
necessitates
to the facrepair calls
an increase
in radio-
logic room shielding. This may have many implications beyond the cost of the additional shielding. For example, reduction in the radiation limits by a factor of five for a 10-MV accelerator will require approximately 12 additional inches of concrete in
the walls. When radiation protection standards are unreasonable, the costs can become staggering. An extreme example of this situation is the recent case of the loss of two small radionuclide sources (250 tCi ofsulfur-35 and 250 tCi of phosphorus-32) (2 1). These quantities of radionuclides and their relative risk arc very small by clinical nuclear medicine standards. The sources were presumed to have been accidently discarded in the ordinary trash. By the time the error was discovered, the trash had been mixed with that ofa major city to a total of 1,600 tons and delivered to an electric generating station, which uses the trash as a fuel source. The pollution control statutes of the state in question prohibit the disposal of any radioactive waste by this generating facility. The state agency finally agreed to allow the waste to
be placed out first. $250,000
in a landfill The total (21).
As we have
(also
cost
become
forbidden),
for the
healthier
provided
search,
and
that
transportation,
safer
than
preceding
a vigorous and
search
landfill
generations,
was carried
exceeded
we have
be-
RISK
come more, rather than less, concerned about risk. We feel increasingly vulnerable to the risks ofmodern life and take the benefits for granted. There arc many aspects to the concerns experienced by the public. For example, accidents that take many lives may produce relatively little societal disturbance if they occur as part of a familiar and well-understood phenomenon (eg, Saturday night car accidents). However, a small accident in an unusual context, such as in a recombinant DNA laboratory, may have immense social consequences. This is especially true if the accident is perceived as a harbinger offurther, possibly catastrophic, mishaps (22). The way in which data arc presented substantially affects how they arc interpreted. Ifyou want to impress people with the gravity of a situation, you can employ
the strategy them
occurring.
of quoting
the absolute
argument
technique
number
about
works
of events,
rather
than
the probability
of
if the event is rare and the underlying base population is large. For example, the NRC estimates that one of its proposals might result in an increased risk of death from cancer of about five in 1 million. Opponents to this proposal contend that this risk translates into 12,000 additional cancer deaths a year (Chicago Tribune. October 29, 1990). An action increasing one’s annual chances ofdcath from 1 in 10,000 to 1.3 in 10,000 would probably be seen as much more risky if it were described as producing a 30% increase in annual mortality (23). The perception of risk is influenced by its context. Headlines such as “Holiday Carnage Kills 500 Over Four Day Weekend” overlook the fact that this is the typical number of deaths in any 4-day period (24). The fact that subtle differences in how risks arc presented can have marked effects suggests that individuals who inform others have considerable potential to manipulate perception. Ethical professionals have a responsibility to present information fairly, objectively, and understandably to the people with whom they arc attempting to communicate. Every activity involves some risk. Voluntary activities such as hang gliding, smoking, or traveling by car carry substantial risks whose levels would be unacceptable to industry. These voluntary risks are often compared with occupational risks to decide “acceptability,” but workers who incur both types ofrisks do not judge them on the same basis. There is a strong human tendency to want everything and to deny that trade-offs are usually necessary. Constructive
This
PERCEPTION
hazards
well
is impeded
by misunderstanding
of the
cept of acceptable risk. It is important to distinguish between acceptable cepted risk. Clearly, no human ill-health is acceptable if it is unnecessary. per year among 1 million workers is a very low fatality rate, which industry
March
1992
Broadbent
and
and One
does
Hubbard
con-
acdeath
not
U
RadioGraphics
U
389
achieve.
But
to friends
and
relatives
of the
victim,
the
cost
of that
one
death
is too
high. pared times
This dichotomy is manifest when it comes to estimating the price we are preto pay to save a life. The community is apparently prepared to pay at least 10 as much to save the life of a named person than to save a “statistical” life. For example, the money spent on an air-sea rescue search to try to save a known life
would
pay for many
miles
of crash
save several lives, but we do not Experts seem to have a different This often creates communication in terms
ofpnobability
and
barriers
on divided
know whose viewpoint problems.
magnitude
highways.
ofharm.
Most
people
Public opinions arc more affected by risk characteristics catastrophic potential, equity, dread, and so forth. Risk information may frighten and frustrate the public. cerned with radiation exposure scenarios. Selective reporting
contributes
proach
problems
to assessing
ionizing
verse them
and
potential
radiation
delivered
other
consequences of irradiation more frightening. Imagination
Perceived cause
of frequent and likely
frequencies
is dramatic
latter
would
rates.
rather
with Merely
qualitatively.
to controllability,
often those conpresent worst-case
All too
issues
to a phobic
associated
risks
relate
than
a rational
exposure
ap-
to low doses
mentioning
the
to be likely
events are occurrences
ofvarious
causes
or sensational,
such
or frequent
if it is easy
typically easier to recall are easier to imagine
of death as unusual
of
possible
ad-
could enhance their perceived likelihood, can blur the distinction between what
motely possible and what is probable. Individuals tend to judge an event on recall. Instances less frequent events,
treat
that
environmental
at low-dose
The
in advance (23). on risk than does the general public. Experts usually view risk quantitatively
than than
making is re-
to imagine instances unlikely
are greatly
overestimated
accidents,
homicide,
of ones.
when
the
botulism,
or
tornadoes. Frequencies of undramatic causes of death, such as asthma, emphysema, and diabetes, which take one life at a time and are common in nonfatal form, are greatly underestimated. News coverage of fatal events is biased in much the same way; this contributes to
the difficulties
of keeping
tant type of misconception to many hazards that we
proper
mental
accounts
of everyday
is the tendency to consider admit pose a serious threat
risks.
ourselves to others.
Another
personally People arc
imporimmune unrealisti-
optimistic when evaluating the chances that a wide variety of good and bad life events (eg, living past 80 years ofage, having a heart attack) will happen to them. Comparisons across types of hazards and comparisons with natural levels of risk may be useful for educating the public, but they should be presented with caution. Such a comparison must be seen as only one of several inputs into the making of a risk decision, not as the primary determinant. There are proved pitfalls when risks of diverse character are compared, especially when the intent of the comparison can cally
be seen
as that
of minimizing
useful are comparisons risk estimate, (b) occur ing to a given destination),
hard
to modify.
ization
ofopinion
while
safe,
others
The
difficulties
about
view
a risk by equating
it to a seemingly
trivial
one.
More
of risks that (a) help convey the magnitude of a particular in the same decision contest (eg, risks from flying and drivand (c) have a similar outcome (25). Strong beliefs arc
of regarding
technologies.
them
Some
as catastrophes
life as a gamble view
contribute
technologies
in the making.
to the polar-
as extraordinarily
People’s
beliefs
change
slowly and are extraordinarily persistent in the face ofcontrary evidence. Initial impressions tend to structure the way in which substantive evidence is interpreted. New evidence appears reliable and informative if it is consistent with one’s initial belief; contrary evidence is dismissed as unreliable, erroneous, or unrepresentative. Each individual radiologist presumably understands the risks of radiation well
enough
to make
logic) ofa given task of accurately
an informed judgment as to the risks (radiologic procedure relative to the benefits ofperforming
this understanding to individual the general public is difficult. It is hoped that this series of articles through March 1992 issues ofRadioGrapbics) has been of help.
390
U
Ra4ioGrapbics
U
Broadbent
communicating
and nonradiothe procedure.
and Hubbard
The
patients and (May 1991
Volume
12
to
Number
2
Radiation
The
Quantities
and
International
System
Units
APPENDIX
of Units
(Syst#{233}meInternational
[SI]) defines
all units
of measureto radiation dosimetry
ment in terms of SI base units. A brief review of those units pertinent follows. The unit ofabsorbed dose (D) is the gray. One gray is defined as one absorbed per kilogram of matter (1 Gy = 1 J/kg). This unit replaces the
1 Gy The unit
of dose
and the quality
equivalent factor
for the irradiating
The
beam. 1 Sv
The unit
replaces
(1-4) is also
the the
equivalent
W1H1
“C
where w the weighting The committed effective
+
w2H2
sievert.
product
absorbed
The
unit
second.
ofactivity
This
very
quantity
accounts
nonuniform
unit
One
replaces
the
1 Bq
becquenel
for the
distribution
of
+...
for tissue
is the becquerel. small
dose
the rem:
This
(typically)
of the
i and H, the dose equivalent received dose equivalent is the dose equivalent integrated over subsequent to the intake of a radionuclide. The effective life of the radionuclide is also accounted for in this concept. factor
=
years tissue
This unit
sensitivity of different tissues, of the beam. It is defined as:
dose
radiation the quality
is the
rem.
ofeffective
differential dose, and
sievert
100
=
of energy
100 rad.
=
(Ii) is the sievert.
joule rad:
is defined
as one
by tissue the
disintegration
i.
50 in each
per
curie: =
2.7
x 10-”
Ci.
Nonstochastic and Stochastic Radiation Effects Radiation effects may be divided into nonstochastic on stochastic categories. Nonstochastic radiation injury increases in severity with increasing dose. There appears to be a threshold dose of several grays for these effects Radiation bums, organ atrophy, and fibrosis are typical nonstochastic injuries A stochastic radiation injury is a random event in which the severity of the effect is indepen-
dent
of the dose
chastic
injuries
individual
received.
Stochastic
events
are
have
solid tumors, leukemia, and may be significant, particularly
doses
no evidence
of a threshold.
The main
sto-
genetic when
effects. Stochastic injury from small these doses are received by large popu-
8.
United Nations Scientific Committee on the Effects ofAtomic Radiation. Sources, effects, and risks of ionizing radiations. UN publication no. E.88. 1X.7.647. New York: United Nations, 1988. BEIR V Committee. Health effects of exposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National Research Council, 1990; 5-6.
lations (1).
1.
2.
National tion and
Council on Measurements.
Radiation
ProtecIonizing nadi-
ation exposure of the population of the United States. NCRP report no. 93. Bethesda, Md: National Council on Radiation Protection and Measurements, 1987. Feldman A. A sketch of the technical
3.
history ofradiology from 1896 to 1920. RadioGraphics 1989; 9:1113-1 128. MurphyWA. Introduction to the history of musculoskeletal radiology. RadioGraphics 1990; 10:915-943.
4.
del RegatojA.
5.
6.
NewYork: Brodsky A, velopment in radiology. 1267-1275. Parker HM. mentation,
Health 7.
AAPM 1985; 1-10. Kathrem RL. Historical deof radiation safety practices RadioGraphics 1989; 9: Health physics, instruand radiation protection.
1980;
Bethesda,
38:957-996.
1992
Md: National Protection
and
10.
physicists.
National Council on Radiation Protection and Measurements. Recommendations on limits for exposure to ionizing radiation. NCRP report no. 91. diation 1987.
March
Phys
Radiological
9.
Council Measurements,
on Ra-
BEIR III Committee.
The effects
REFERENCES
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11.
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Volume
12
Number
2