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).

-I :

.

t -

si;,

Hot

‘ .

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.

518.

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of its hind in the country #{149}A’

htst

and

all

conditinn

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or

thetnat1sEddCanc#{231},

most

X.Ra

cumpkt ikmontrahk

and Electro-Tbeap*k XRay. We aho

hs thc

Elvctnaty

Fara*lic

as

Lti*,

reference

&zema.

ja

Me

r

Dr Grubbe’s from

Wilhelm menting

..

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

‘,.

.

.\

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

U

RadioGraphics

U

Broadbent

and

Hubbard

KASSETTE

8.00 12.00 15.40 23.80

Manufacturing Street,

(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

on

populations ionizing

11.

12.

of exposure to low levels of radiations. Washington, DC: National Academy of Sciences/National Research Council, 1980. Grigg ERN. The trail of the invisible light: from X-strahlen to radio(bio)logy. Springfield, Ill: Thomas, 1965; 66. National Council on Radiation Protection and Measurements. Medical x-ray

and gamma-ray

protection

for energies

up to 50 MeV (equipment design, performance, and use). NCRP report no. 102. Bethesda, Md: National Council on Radiation Protection and Measurements, 1989.

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and

Hubbard

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13.

14.

The federal performance standards for diagnostic x-ray systems and their major components. 2 1 CFR, parts 1020.301020.33, 1988. Radiation protection guidance to fed-

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report

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Dixon RI, Tolbert DD. the newJCAH standards

19.

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Council

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Broadbent

and

Hubbard

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Radiation

Measurements,

radionu-

22. 23.

1,5-7. Slovic 1987; Slovic

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and

1981.

clides and organs. MIRD pamphlet 1 1. New York: Society of Nuclear

Adm

Assurance in Di(H.7). Average patient exposure guides 1988. CRCPD publication no. 88-5. Frankfort, Ky:

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Md: National Protection

Snyder WS, Ford MR, Warner son SB. “5,” absorbed dose

cumulated

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on Radiation

Measurements,

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no. 68. Bethesda,

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ionizing

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Phys 1981; 4 1:589-598. PaulosJA. Innumeracy: mathematical illiteracy and its consequences. New York: Hill & Wang, 1988; 125-126. ReisslandJ, Harries V. A scale for measuring risks. New Scientist 1979; 83: 809-813.

Volume

12

Number

2

Science and perception of radiation risk.

The risks, real and supposed, of the use of radiation in medical practice as perceived by patients, their families, and the general public have a subs...
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