Diagnostic imaging: radiation exposure and safety

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MedicineToday PEER REVIEWED
SPORTS MEDICINE
Diagnostic
imaging:
radiation exposure
and safety
considerations
TOM CROSS MB BS, FACSP, DCH
MedicineToday 2012; 13(9): 72-75
D
octors have a number of investigations available to them
to help diagnose suspected injuries in sports medicine.
Many of these involve ionising radiation (conventional
radiography, CT scanning, bone scanning), whereas
others do not (MRI, ultrasound). One of the dictums of medical
practice is Primum non nocere (First, do no harm). Exposure to
diagnostic ionising radiation, like other medical tests and procedures, is associated with a risk to the patient. In this case, there is
a potential risk, albeit small, of a radiation-induced cancer
and/or a genetic disorder in one’s offspring.
Dr Cross is a Consultant Sports Physician in private practice in Sydney, NSW.
SERIES EDITOR: Dr Ken Crichton, MB BS(Hons), FRCSP, Director of Sports
Medicine, North Sydney Orthopaedic and Sports Medicine Centre, and
Consultant Sports Physician
at the Children’s
Institute
Sports
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Medicine, Sydney, NSW.
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The use of ionising radiation in medicine is the single largest
man-made source of population radiation exposure.1-3 Exposure
to diagnostic ionising radiation continues to rise significantly,
year after year. This is due to the increasing availability and
use of medical imaging procedures in modern healthcare systems as well as the development of some high-dose techniques.
For example, Americans were exposed to more than six times as
much ionising radiation from diagnostic medical procedures in
2006 than they were in the early 1980s.4
It must be appreciated that radiation is constantly present
in our environment. Sources of this ‘background radiation’
include cosmic rays from the universe, and naturally occurring
radioactive substances in the food and water we eat and drink
and the air we breathe, and in the ground and soils.
HOW ARE DOSES OF DIAGNOSTIC IONISING
RADIATION MEASURED?
The more common sports medicine tests associated with
ionising radiation involve x-ray radiation (from conventional
radiography or CT scanning) or gamma radiation (emitted by
radiopharmaceuticals, most commonly technetium-99m [99mTc]
in bone scanning) in nuclear medicine imaging. X-rays and
gamma rays ionise atoms and molecules in human tissues
through the deposition of energy. DNA strand breakages from
this ionisation process may be the first step in a series of events
that lead to a biological effect (cancer) and/or genetic effect.5
Biological exposure to ionising radiation is expressed in terms
of the ‘effective dose’, which takes into account the amount of
radiation absorbed by each irradiated organ and its relative
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Despite the overall health benefits to patients
from advances in medical imaging, the small
and theoretical risk of a detriment to health
from exposure to diagnostic ionising radiation
should be appreciated.
EFFECTIVE DOSES AND RISK ESTIMATES FOR SOME COMMON INVESTIGATIONS IN SPORTS MEDICINE*
The effective doses and risk estimates shown in the tables below were calculated for a theoretical male patient (20 to 29 years of age, 80 kg),
and are based on the machines and imaging protocols in use at a particular radiology practice in Sydney in 2003.10 It should be appreciated
that the effective dose can vary significantly between radiological practices because of differences in machinery and imaging protocols.1-4,7,12
Effective doses and risk estimates were estimated using mathematical modelling developed by the National Radiological Protection Board
(NRPB) and International Commission on Radiological Protection (ICRP).5,6,10,11
The risk estimate is defined as the risk incurred by this theoretical patient that he will develop a fatal cancer earlier in life than he would
otherwise have developed had he not been exposed to a particular effective dose of ionising radiation.
TABLE 1 . CONVENTIONAL RADIOGRAPHY
TABLE 2 . CT SCANNING PROCEDURES
Examination
Effective dose per
examination series (mSv)
Risk estimate
(fatal cancer)
Examination
Chest
0.067
1 in 250,000
Brain
2.3
1 in 7000
Ribs
0.720
1 in 23,000
Facial bones
1.0
1 in 16,000
Sternum
1.270
1 in 13,000
Chest
4.1
1 in 4000
Face/nose/orbit
0.030
1 in 550,000
Abdomen
7.6
1 in 2200
Cervical spine
0.034
1 in 480,000
Pelvis
4.5
1 in 3600
0.063 (with oblique views)
1 in 260,000
Cervical spine
4.4
1 in 3700
Thoracic spine
0.730
1 in 22,000
Thoracolumbar spine
11.7
1 in 1400
Lumbar spine
1.630
1 in 10,000
Lumbar spine
5.2
1 in 3200
Effective dose per
examination (mSv)
Risk estimate
(fatal cancer)
1.960 (with oblique views)
1 in 8000
Leg length
1.0
1 in 16,000
Pelvis
0.860
1 in 19,000
Shoulder
2.0
1 in 8200
Shoulder
0.040
1 in 410,000
Elbow
0.5
1 in 33,000
Elbow/forearm
0.003
1 in 5,460,000
Wrist
0.5
1 in 33,000
Hand/wrist
0.003
1 in 5,460,000
Knee
0.5
1 in 33,000
Knee
0.020
1 in 820,000
Foot and ankle
0.5
1 in 33,000
Leg
0.004
1 in 410,000
Foot and ankle
0.004
1 in 410,000
BONE SCANNING
For a bone scan, the effective dose for the same theoretical patient was calculated to be 4.6 mSv (based on 800 MBq of 99mTc radioisotope
injected intravenously).12 This effective dose confers a risk estimate of inducing a fatal cancer of one in 3500.10,12
* Effective doses and risk estimates are based on the methodology briefly described above. For a more thorough explanation, the reader is referred to reference 10. Methodology is
based on practice in 2003 – the transferability of data to 2012 is discussed in the text (see page 74).
Acknowledgement: Tables 1 and 2 reproduced from reference 10 with permission of Wolters Kluwer Health. Cross TM, Smart RC, Thomson JEM. Exposure to diagnostic ionizing
radiation in sports medicine: assessing and monitoring the risk. Clinical Journal of Sport Medicine 2003; 13(3): 164-170.
radiosensitivity.6 The unit of effective dose is the sievert (Sv), often
expressed in millisieverts (mSv). As a general rule, the more radio sensitive tissues are located in the trunk region (gonads, lung,
breast, gut, bone marrow and thyroid), and therefore conventional radiography and
CT scanning
trunk 1:43
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much greater effective dose than these tests done of the extremities.6
The effective dose associated with most diagnostic imaging
modalities is in the range of 0.03 to 20 mSv.7 This dose range
may be compared with the annual dose of background radiation, which is about 1.5 mSv in Australia,8 or with the doses
received by the survivors of the two atomic bombs of 1945,
which were in the range of 5 mSv to more than 2000 mSv.9
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73
SPORTS MEDICINE CONTINUED
adjustments are made by the radiographer/nuclear medicine
technician performing the test.
It should be appreciated that the methodology discussed in
the box is based on practices in 2003. Overall the doses are
transferable to 2012, but there have been advances in CT technology that have resulted in reductions in ionising radiation
doses and most bone scans performed in 2012 are CT-SPECT
studies, which carry a higher effective dose than bone scans
performed in 2003.
Figure. A patient being aligned for a CT scan.
The box on page 73 shows the effective dose for various common investigations performed in sports medicine, which have
been estimated for a theoretical patient: Athlete X. Athlete X is
an 80 kg male athlete, aged between 20 and 29 years, who plays a
contact sport. Such a patient is common in sports medicine
practice. Effective dose estimates are shown for conventional
radiography (Table 1), CT scans (Table 2) and bone scanning.10
Analysis of the effective doses received by Athlete X in Tables
1 and 2 demonstrates some important points:
• CT scanning (particularly in the trunk region) and bone
scanning have a significantly higher effective dose than
conventional radiography. Although CT scans account for
only 11% of the radiological examinations in the USA, CT
delivers about 67% of the medical effective dose.13
• CT scanning and conventional radiography of the extremities
(distant from radiosensitive tissues) are associated with
significantly lower effective dose values than investigations
in the trunk region.
For a bone scan, the effective dose depends on the activity of
the radiopharmaceutical injected intravenously and is independent of the anatomical region studied in the bone scan.
These estimates of effective dose for radiography, CT and
bone scans are roughly
transferable
adult female
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and paediatric patients, provided that appropriate technical
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At the very low levels of radiation used in diagnostic procedures,
radiation-induced injury is expressed in terms of the probability
of biological and/or genetic effects.14 Since the first excess cancers were observed following the atomic bombs of 1945, scientists have worked to establish the relation between dose of
radiation and the risk of that exposure.9,15,16 The ICRP has
reviewed the research and concluded that all radiation exposure,
even at an extremely low level, carries a risk.6,11
Using accepted mathematical modelling, risk estimates for
Athlete X have been calculated for common sports medicine
investigations (see the box on page 73).10 The term ‘risk estimate’ is defined as the risk incurred by the theoretical patient
(Athlete X) that he will develop a fatal cancer earlier in life than
he would otherwise have developed had he not been exposed to
that particular dose of ionising radiation.10
These risk estimates are roughly transferable to adult female
patients. However, for paediatric patients, the risk estimates
are higher than for adults. This is because young people’s tissues
are more radiosensitive and also because their longer expected
life ahead means that they carry the risk for a longer period of
time. The ICRP estimates the relative risk to be 1.8 times higher
for a child exposed to a particular effective dose of ionising
radiation than for a 30-year-old adult.9
The risk estimates discussed in the box should be put in perspective by considering the high cancer burden in Australia.
According to the Australian Institute of Health and Welfare,
cancer accounted for 29% of all deaths in 2007.17 Therefore, in a
random sample of 2200 people it is to be expected that 638 individuals will die of cancer. A CT scan of the abdomen, associated
with a risk of fatal cancer of one in 2200, will theoretically
increase that number from 638 to 639 cancer deaths.
UNCERTAINTIES IN THE ESTIMATION OF RISK
Risk estimates are derived from epidemiological studies of
survivors of the two atomic bombs of 1945. Complex
mathematical modelling by the ICRP has estimated and
extrapolated the risk estimates to the very low levels of ionising
radiation associated with the diagnostic tests stated above, but
some uncertainties remain in this process.6,9,11,15 Indeed, a lot of
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WHAT IS THE RISK OF RADIATION-INDUCED INJURY?
scientists argue many of the DNA breakages caused by very low
levels of radiation are repairable and therefore a ‘threshold’ level
of ionising radiation exists, below which there is no risk.18
There are no convincing studies in the medical literature
that have proven or disproven that individuals exposed to diagnostic radiation from conventional radiography, CT scans or
bone scans have developed early fatal cancers or have an increased
incidence of birth defects in their offspring.11 No statistically
significant increase in genetic effects have been observed in the
children of the atomic bomb survivors of 1945.19 It is extremely
difficult to accurately demonstrate causality between low-dose
radiation and the risk of inheritable disease. This is because
the natural incidence of genetic anomalies in children is high
(one in 44 births). The ICRP estimates that 1 mSv of radiation
exposure may confer an increased risk of a genetic anomaly in
one in 77,000 births.11
CUMULATIVE EFFECTIVE DOSE AND CUMULATIVE RISK
It should be appreciated that radiation-induced effects are
believed by the ICRP to be cumulative – that is, the dose and
risk associated with each new exposure can be added to the dose
and risk from any previous exposure(s).11 The cumulative effective dose and cumulative risk for an individual may become
quite significant. Such an individual may be an elite athlete who
has a long career and suffers many injuries over a period of
years, or a patient with a chronic disease such as rheumatoid
arthritis or a chronic respiratory disease.10
DOSE REDUCTION STRATEGIES
The ICRP promotes two important dose reduction strategies
for minimising patients’ exposure to ionising radiation:6,11
• justification
• optimisation.
It is ethically right to restrict the use of diagnostic tests that
involve ionising radiation to those who will benefit from
them. It is incumbent on the treating doctor to balance expected
benefits and possible risks for every investigation that is ordered
in each patient’s particular case. It should be stated clearly that
if the result of performing an investigation on a patient will benefit his or her overall health (in the short and longer term)
despite the possible theoretical risk of the radiation exposure
discussed, then the investigation is justified.6,11 For example,
a whole body bone scan is justified to investigate a patient
presenting with multiple joint symptoms suggestive of an
inflammatory polyarthropathy or spondyloarthropathy. A CT
scan of the lumbar spine is not justified to investigate nonspecific mechanical low back pain.
When a patient presents to a radiology practice for an x-ray,
CT scan or bone Copyright
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radiation exposure (as much as is reasonably achievable) without
FURTHER RESOURCES
The Royal Australian and New Zealand College of Radiologists
provides information for health professionals and the general
public about various aspects of radiology, including radiation risk.
This is available online – see www.insideradiology.com.au
compromising the quality of the diagnostic images. This is the
principal of optimisation.6,11 Closer communication (either
written or spoken) between the referring doctor and the radiologist/radiographer may result in more limited imaging protocols being adopted and a reduction in the effective dose.1,2,11,20
The principles of justification and optimisation are particularly relevant to paediatric patients.1-4,9-17,21
Whenever possible, diagnostic imaging procedures that
do not use diagnostic radiation (MRI and ultrasound) should
be used if they can yield the same (or superior) information.
MRI is decreasing in cost and becoming increasingly available to
many patients.
CONCLUSION
The significant overall health benefits to our patients from
advances in medical imaging cannot be overstated. However,
the small and theoretical risk of a detriment to their health from
single or multiple exposures to diagnostic ionising radiation
should also be appreciated.
Doctors who care for patients who require sports medicine
diagnostic procedures involving ionising radiation (and indeed
other interventions that involve exposure to such radiation)
should have a working knowledge of the effective doses and risk
estimates associated with the more common tests. The concepts
of justification and optimisation should be appreciated, particularly when caring for paediatric patients. Investigations that do
not involve ionising radiation should be considered whenever
possible and affordable.
MT
ACKNOWLEDGEMENT
the author acknowledges the collaboration of Dr richard smart and Dr Julian
thomson, co-authors in earlier published research on this topic.
REFERENCES
References are included in the pdf version of this article available at
www.medicinetoday.com.au.
COMPETING INTERESTS: None.
MedicineToday
❘
september 2012, Volume 13, Number 9
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75
Medicine Today 2012; 13(9): 72-75
Diagnostic imaging:
radiation exposure and
safety considerations
TOM CROSS MB BS, FACSP, DCH
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Arztebl Int 2011; 108: 407-414.
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