Radiation Risk of Medical Imaging for Adults and Children

Mr Anthony Wallace*
                            Dr Timothy Cain *

What are the generally accepted indications for a medical imaging procedure?

An appropriate imaging procedure is justified if it changes patient management or treatment, or confirms or excludes the presence of disease. The type of imaging procedure, as to whether ionising radiation is involved, will be determined by the presenting clinical problem. For further information about choices of procedure see: Diagnostic Imaging Pathways.

What are the absolute contraindications for ionising radiation?

There are no absolute contraindications to exposure to ionising radiation through traditional medical imaging.

Very large patients might exceed the weight limits for angiography and CT scanning tables, and therefore not allow the patient to be examined. These limits (approximately 130 kg) vary between manufacturers, so if you think that this might be a consideration for your patient, it is prudent to check with the facility where you are planning to refer them for the examination.

What are the relative contraindications for ionising radiation?

When there is another imaging test available that does not use ionising radiation, but answers the clinical question accurately. This should be particularly considered in high-risk groups, such as children, patients who have a genetic predisposition to radiation risks (see below) or those patients who have had excessive previous ionising imaging procedures.

Which tests give the most ionising radiation and which ones the least?

X-rays and nuclear medicine studies all expose patients to ionising radiation. The radiation dose to the patient is lowest in simple X-ray examinations (such as a chest or arm or leg radiograph).
MRI and ultrasound do not use ionising radiation.

Simple and short fluoroscopy examinations, such as a paediatric barium meal, a CT examination of the wrists or ankles and a nuclear medicine examination for gastro-oesophageal reflux, also involve a relatively low dose of radiation to the patient.

However, CT examinations of the chest, abdomen and pelvis, complex fluoroscopy (such as interventional radiology or interventional cardiology procedures), PET scans, and some other nuclear medicine procedures can result in a radiation dose equivalent to at least one or more years of background radiation.

What are the risks associated with radiation from diagnostic X-ray imaging and nuclear medicine procedures?

There are two main types of risk in exposure to ionising radiation:

1. Stochastic
There is a very slight increase in the possibility of cancer being caused as a result of the radiation exposure. This is a complex concept to understand. Risk is expressed as chances per million (or some other number), much the same way as the chance of winning first prize in a lottery or dying in an aeroplane accident. Although this risk is expressed as a number, individuals either suffer the consequences or not; one person in a million getting cancer means that one person gets cancer and 999,999 do not. The whole population does not get a tiny bit of cancer.
To a much lesser extent, there is a possibility of genetic damage that might affect future generations through changes in genes or chromosomes resulting in a new trait or characteristic that can be inherited, for example physical or intellectual disability.

The risk of harmful effects from having X-ray, CT or nuclear medicine studies only becomes significant in most people after substantial numbers of high-dose radiology procedures. However, children and females are slightly more sensitive to the effects of ionising radiation. There are some people with a genetic predisposition to the long-term effects of ionising radiation due to ineffective protein (DNA) repair mechanisms.

The risk of harmful stochastic effects of ionising radiation is calculated using sophisticated and complex population statistics. The normal lifetime risk of death from cancer in the Australian population in 2008 was approximately 25–30%.

The table on the following page shows the additional percentage risk from radiation exposure from a single exposure to some typical X-ray procedures. For example, if a person had a risk of cancer of 30% before a single chest X-ray, it would be 30.00013% after the chest X-ray.

Procedure Typical effective dose (mSv) Additional % stochastic risk*

Chest X-ray – posterior–anterior (from the back to the front)

0.025 0.00013%

Lumbar spine X-ray (single view image of the lower spine for a bad back)

0.7 0.0035%

Pelvis X-ray anterior–posterior (from the front to the back)

0.7 0.0035%

CT of the chest

8 0.04%

CT of the abdomen

10 0.05%

CT of the pelvis

10 0.05%

Mammogram

1.3 0.0065%

Typical annual natural background radiation dose

1.5 0.0075%

Ultrasound

0 0

Magnetic resonance imaging (MRI)

0 0

*Over and above the normal lifetime incidence of death from cancer in the population.

2. Deterministic (tissue effects)
Tissue damage does not normally occur in diagnostic imaging, and is mainly the result of long and complex therapeutic CT or complex fluoroscopic diagnostic imaging procedures. Examples of tissue damage include skin redness (erythema), atrophy or necrosis. Temporary or permanent hair loss can occur as part of damage to skin, but once again this only occurs well in excess of the range of doses typically used in diagnostic imaging.

Why are these risks different for males and females?

Different body tissues have different sensitivity to radiation. Skin and bone are not very sensitive, but breast tissue, bone marrow, and the lining of the stomach and intestine are more sensitive to the effects of ionising radiation. On average, females are more sensitive (´1.5) to the stochastic effects of ionising radiation than males.

Why are these risks different for children and pregnant women?

The principal risk for children exposed to X-rays is that at the time of exposure, their growth means more cells are dividing, providing a greater risk of radiation disrupting cell development. Children also have a longer life expectancy, giving a longer time for the effects of any radiation damage, if present, to have an effect on long-term health.

Recent research1 has confirmed a slight increase in cancer risk in children who have had previous CT examinations. This risk is not significantly different from previous calculated risk estimates.

Recommendations for the use of CT in children include low dose examination using modern CT equipment, consideration of other non-ionising imaging methods (ultrasound and MRI), and adequate consultation between the specialist, referring doctor and patient/family1.

Special care needs to be taken with women of reproductive age. In pregnant women, the foetus (unborn baby) is highly sensitive to radiation, particularly in the early stages of the pregnancy from weeks 4 to 8. The pregnancy status of the patient should be discussed before referral and confirmed before the investigation is undertaken.

Radiological referral guidelines have been established in order to reduce any potential radiation exposure to the unborn foetus. See: Radiation Risk of Medical Imaging during Pregnancy.

Are there other reasons the risk of harm from ionising radiation is different for different individuals?

As well as the differences in risk associated with age, sex and different body tissues, we know that the time between radiation exposures can influence how the body responds to the effects of ionising radiation. The body has repair mechanisms that can ‘fix’ some damage caused by ionising radiation. Some individuals have a slightly poorer capacity to fix this damage than others, but we are still learning about this and are not able to easily identify which people are at a greater risk. An example of this increased genetic tendency to develop radiation-induced cancer is the BRCA1 and BRCA2 gene mutation, which predisposes people who carry the gene to develop breast cancer.
There could be other factors that make some people more sensitive to the effects of radiation that are still not known about, and that is why it is important that each X-ray, CT scan or nuclear medicine test can be justified as an important investigation with a benefit to the patient.

How do we know what the risks really are? How accurate are these estimates of risk?

Estimating radiation risk is complex and problematic. Most of our knowledge has come from observations of the effects of relatively high doses of ionising radiation on the survivors of the atomic bombings of Hiroshima and Nagasaki. Studies have also been carried out on other groups of people, including those working in the nuclear industry, groups accidentally exposed to a known ionising radiation dose in various incidents and patients treated with radiation for non-malignant conditions.

As the risk of harm from ionising radiation is so small, it takes very large population samples to be able to provide accurate statistical calculations of risk. At present, the best estimate of risk we have is that we expect a 5% increase in death from solid tumours in a population that has been exposed to an equivalent dose of 500 years worth of background radiation. This is approximately equivalent to 100 CT abdomen scans or 50,000 chest X-rays.

It is important to understand that these statistical assessments can only be applied to populations, and should not be applied to individuals. Each individual’s sensitivity to ionising radiation will be different depending on their age, sex and other factors discussed above.

Procedure Risk of fatal cancer Equivalent to number of cigarettes smoked Equivalent to number of highway km driven*

Chest radiograph

1.3 per million 10 40–60

Skull X-ray series

6 per million 40 170–280

CT scan of the chest

400 per million 3000 11,500–19,000

Radiation exposure from 18-hour plane flight at 11,000 m

3 per million 20 80–140

*Based on Australian road trauma statistics. More road travel gives a greater exposure to the risk of a motor vehicle accident.
Adapted from information in Radiobiology for the Radiologist, Fourth Edition; Eric Hall 1994, J.B. Lippincott Company. The comparisons of relative risks are still valid as approximations, even though these figures were first estimated about 20 years ago.

Radiation doses from various sources:

Source of exposure Average radiation dose in millisievert (mSv*) Range

Average dose to Australians from background radiation per year

2 1–10

Arm or leg X-ray

0.005–0.05

Chest X-ray 2 views

0.06 0.04–0.2

CT scan (head)

2 1–15

CT scan (chest)

8 1–15

Ultrasound

0 0

Magnetic resonance imaging (MRI)

0 0

*The ionising radiation dosimetry unit of mSv (millisievert) is the common unit used to measure and combine both the radiation dose and the consequent risk delivered by an exposure. It is a unit that is independent of both source and radiation type, allowing easy comparison between different sources of exposure.

The radiation risk from X-ray procedures is, in most cases, extremely small and should be far outweighed by the benefits of the procedure. Every X-ray examination should be justified in terms of its risks versus expected benefits.

What is the risk of harm from ionising radiation?

The generic stochastic risk for the population for radiation-induced fatal cancers is approximately 5% per Sievert, and the risk for non-fatal cancers is 1% per Sievert.

A dose below 100 mSv to the foetus results in a small, but increasing, risk of detriment. Any foetal dose estimated to be above this value would require a formal risk assessment and patient counselling.

Are there alternative imaging tests, interventions or surgical procedures to ionising radiation?

Certain anatomical structures, clinical conditions and injuries might be better imaged with MRI or ultrasound. The choice of imaging modality should be made with consideration of the potential risks and benefits to the patient, taking into account the psychosocial and financial costs. This should be in consultation with the radiologist or nuclear medicine specialist when the risks are high or the benefits small.

Further Information:

You might find the following references useful for informing your discussions with patients regarding the risks and benefits of diagnostic imaging tests that involve radiation:
Australian Radiation Protection & Nuclear Safety Agency – fact sheets
http://www.arpansa.gov.au/radiationprotection/FactSheets/index.cfm
UK Health protection agency – understanding radiation
http://www.hpa.org.uk/webw/HPAweb&Page&HPAwebAutoListDate/Page/1153846673570?p=1153846673570
US Environmental Protection Agency – Risk Assessment
http://www.epa.gov/rpdweb00/assessment/
Idaho State University – Radiation Risk
http://www.physics.isu.edu/radinf/risk.htm
Georgia State University – Radiation Risk
http://hyperphysics.phy-astr.gsu.edu/HBASE/nuclear/radrisk.html
Australian Nuclear Science and Technology Organisation (ANSTO)
http://www.ansto.gov.au/NuclearFacts/AboutNuclearScience/NaturalBackgroundRadiation/index.htm

Useful websites:

Australian Radiation Protection & Nuclear Safety Agency
www.arpansa.gov.au
Diagnostic Imaging Pathways
http://www.imagingpathways.health.wa.gov.au/
Image Gently
http://www.pedrad.org/associations/5364/ig/
International Atomic Energy Agency
http://rpop.iaea.org/RPoP/RPoP/Content/index.htm
National Council on Radiation Protection & Measurements

Home


Health Physics Society
http://www.hps.org/
Royal Australian and New Zealand College of Radiologists.
www.ranzcr.edu.au

References:

1. Mathews et al, Cancer risk in 680000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians, BMJ 2013; 346: f2360

*The author has no conflict of interest with this topic.

Page last modified on 21/7/2017.

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