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The Royal Australian and New Zealand College of Radiologists®

Radiation Risk of Medical Imaging During Pregnancy

Mr Anthony Wallace
Prof Stacy Goergen
Date last modified: November 28, 2015


1. What is radiation?

Background radiation
Background radiation is always present in our environment. It is inescapable and provides each of us with a continuous, small radiation dose.
The sources of background radiation in our environment are cosmic rays from the universe, naturally occurring radioactive substances in the food and water we eat and drink, the air we breathe, in the ground, in building materials and so on.

The amount of background radiation varies in different parts of the world due to the radioactivity of the soil, latitude, height above sea level and lifestyle (predominantly indoors or outdoors). Some countries have background radiation that is 10 times greater than generally found in Australia.

Every year, everyone living in Australia receives background radiation that is equivalent to the radiation dose from 50–100 simple chest X-rays.

X-rays are part of the electromagnetic range of energy that includes radio waves, visible light and gamma rays. Visible light and X-radiation (X-rays) both travel in straight lines and cast a shadow when they interact with a solid object.

When the path of an X-ray is altered by coming into contact with an object, the X-ray deposits some of its energy to the object (including the human body). This deposit of energy is called a radiation ‘dose’.

X-rays, gamma rays and some other high-energy forms of radiation can deposit enough energy into the human body to change molecules or proteins, which are the building blocks of the body. These high-energy particles or rays are referred to as ionising radiation, or sometimes simply as ‘radiation’.

X-rays
X-rays have more energy than visible light and can therefore penetrate much deeper into and through objects. An X-ray beam is absorbed differently by different structures in the body. A dense structure, such as bone, absorbs a high percentage of the X-ray beam; whereas low-density structures, such as soft tissue, absorb a small amount. Metal objects in the body (such as a swallowed coin, a bullet or a surgical plate used to fix a fracture) will usually appear as white. Air in the lungs will be black, because it absorbs so little radiation. These differences in X-ray absorption can be used to create a picture of the different structures, organs or tissues in the body that allow a radiologist (specialist doctor) to diagnose and treat disease.

X-rays are electrically generated and are only present when the X-ray machine is switched on, just like a light bulb. Once the X-ray machine is switched off, there is no residual radiation coming from the X-ray machine. Having an examination using ionising radiation does not make people radioactive.

Nuclear medicine
Nuclear medicine is a medical speciality that diagnoses and monitors disease by giving patients small amounts of a radioactive material, in the form of a ‘radiopharmaceutical’.

A radiopharmaceutical is a combination of a radioactive part and a pharmaceutical (or drug) part.

After a nuclear medicine test, where a patient will swallow, breathe in or receive an injection of a radiopharmaceutical, they will be very slightly radioactive for a short time. This small amount of radiation is picked up by a special camera called a gamma camera (see item on Nuclear Medicine). The pictures produced by the gamma camera are interpreted by a nuclear medicine specialist (specialist doctor). The amount of radioactivity that remains in the body and the length of time a patient remains slightly radioactive depends on the test carried out (for more information see nuclear medicine).


2. What tests/procedures are associated with radiation?

Table 1: Types of medical imaging
Types of radiology and nuclear medicine tests, and the use of ionising radiation.


Procedure

Uses ionising radiation?

Radiology

CT – computed tomography

Yes

Fluoroscopy

Yes

General X-ray

Yes

Mammography

Yes

MRI – magnetic resonance imaging

No

Ultrasound

No

Nuclear medicine

General nuclear medicine

Yes

PET CT – positron emission tomography computed tomography

Yes

Therapy

Yes

Reference:
Wagner L, Lester R, Saldana L. Exposure of the pregnant patient to diagnostic radiations, 2nd ed Medical Physics Publishing, Madison, 1997


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3. What are the risks of having diagnostic imaging during pregnancy for the mother?

A pregnant woman is no more sensitive to radiation than a woman who is not pregnant. The risk associated with a diagnostic imaging procedure to a pregnant woman will be the same for any woman of the same age who is not pregnant (see nuclear medicine).

Women of reproductive age who are referred by their doctor for computed tomography (CT) scanning, nuclear medicine, angiography and some simple X-ray examinations should let their doctor and the radiology practice know if there is any chance they may be pregnant. This may affect decisions about which test to do and whether the test should be delayed until after a pregnancy test has been carried out.

There are two main types of risk when anyone, including a pregnant woman, is exposed to ionising radiation:
1. Short-term risk
Examples of short-term (or deterministic) damage include burns to the skin and/or hair loss on areas of the body that have been exposed to a very large dose of X-rays. Short-term damage is usually the result of very long and complex CT or interventional fluoroscopic procedures that are carried out on seriously ill patients. There is a substantial radiation dose threshold that must be exceeded before short-term damage occurs. It does not normally occur in diagnostic imaging.

2. Long-term risk
This is also known as ‘stochastic’ risk. There is a very small increase in the possibility of cancer or genetic damage being caused as a result of medical ionising radiation.

The amount of risk associated with ionising radiation can be a difficult concept to imagine, because it is so small. Risk is expressed as chances per million, much the same way as you would think about the chance of winning the first prize in a lottery, the chance of dying in a car crash on the way to work or the chance of developing lung cancer if you smoke five cigarettes.

It is very important to understand that risk cannot be calculated for any individual person undergoing a procedure involving radiation. Risk estimates refer to large populations of people and are ‘average’ risks across that whole population. We all have different sensitivity to the effects of radiation as a result of our genetic or inherited characteristics. For the most part, we cannot measure or calculate the effect that our genes have on our personal sensitivity to the effects of radiation.

It is also important to realise that much of our knowledge about the potential small increase in the risk of cancer that might be associated with exposure to medical sources of radiation, such as CT scanning and nuclear medicine, is based on data collected from survivors of the Japanese atomic bombs in 1945 during the final stages of the Second World War, as well as from people who have been exposed to high doses of radiation from fluoroscopy. From this data, estimates have been made of the potential risk associated with the far lower levels of radiation associated with medical tests. These remain only estimates. To date, no study has been carried out that follows large numbers of people who have been exposed to CT scanning to see if they actually have a real increased risk of cancer.

Ref: radiation risk of medical imaging for adults and children


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4. What are the risks for the foetus of the mother having diagnostic imaging during pregnancy?

It is very important to realise that almost all imaging tests expose the foetus to such low levels of radiation that they are not a cause for concern. However, it is good practice if possible to avoid those tests and procedures that directly expose the uterus or abdomen to radiation if a woman could be or is pregnant. As with all medical imaging, the risk to mother and foetus should be outweighed by the benefit of the test or procedure.

Please note that these comments DO NOT apply to radiation therapy for cancer. Radiation used for cancer treatment involves radiation doses that are thousands of times higher than those delivered by diagnostic tests.

In general terms, the risk to the unborn foetus from ionising radiation used for medical diagnosis (X-rays, CT, nuclear medicine and angiography, for example) is dependent on:

  • the part of the mother’s body exposed to the radiation
  • the stage of pregnancy
  • the radiation dose received

If you are pregnant and you are scheduled to have any kind of medical imaging, you must tell your doctor and the practice or hospital where you are having the test.

It is understandable that most women who have had exposure to medical sources of radiation when they are pregnant or who have found out they are pregnant soon after a test or procedure involving ionising radiation are worried about whether this will affect their foetus. If you have any concerns at all regarding exposure to medical sources of radiation, please discuss these with your doctor.

1. Short-term risk
Because the foetus is rapidly growing and developing, short-term risks (or deterministic risks) are different for a foetus (or embryo, as the foetus is known in the first 8 weeks after conception) compared with a child or adult. They include death, slowing of normal growth, abnormal growth (resulting in deformities) and being intellectually or emotionally underdeveloped. The International Commission on Radiological Protection (ICRP) has stated that deterministic risks such as these would not be expected to occur in an embryo or foetus that had been exposed to less than 100 mGy of radiation.

Table 2 provides an indication of the expected dose of radiation to a foetus or embryo associated with various examination types. The foetal dose estimates provided apply only to the early stages of pregnancy when the foetus is small. It can be seen that even for the highest dose examinations at the bottom of the table, the expected dose to the foetus or embryo for a single examination is well below the 100 mGy threshold.

2. Long-term risk
This is also known as stochastic risk. As explained above, currently it is only possible to estimate potential risks. These are the theoretical risks of cancer some time after birth (as a child or adult) and the risks of hereditary diseases occurring in the descendants of someone who was exposed to radiation as a foetus.

The scientific information available about the effect of small amounts of radiation on the fertilised egg before it implants in the uterus (in other words, within hours to days of conception) and in the first 3–4 weeks after conception is limited. For most diagnostic radiation exposures of women in the first 3–4 weeks after conception when the pregnancy is unrecognised, the risks of childhood cancer will be very small (and probably much smaller than if the exposure had occurred later in pregnancy).

Approximately 1 in every 500 children develops cancer during childhood (even in the absence of any exposure to radiation as a foetus). For the examinations in the lowest dose range in Table 2, the estimated potential additional risk (over and above the normal lifetime risk of developing a cancer) for the foetus is approximately 1 in 1,000,000 and for the highest dose examinations at the bottom of the table, the additional risk (over and above the normal lifetime risk of developing a cancer) is thought to be approximately 1 in 1000 to 1 in 200. This means that for a foetus exposed to one of the highest doses of radiation after the first 4 weeks, the risk of childhood cancer might be increased to 3 in 500.

This is still considered to be a very low risk. If a higher dose examination is needed, because the risk of not doing the test is very significant for the mother, then this is appropriate. If a woman has one of the higher dose tests before she knows she is pregnant, the increased risk to the foetus is still considered to be small. This would not be a reason on its own to consider pregnancy termination, because of fears about cancer risks for the baby or child.

Table 2: Typical foetal doses and risks of childhood cancer for common radiology tests and procedures*
Grouped according to foetal dose, from lowest to highest


Examination type

Typical foetal dose (mGy)

Risk of childhood cancer per examination

Group 0:
Ultrasound

Magnetic resonance imaging (MRI)

0

0

Group 1:
X-ray skull

X-ray chest

X-ray thoracic spine

Mammogram

Head or neck CT scan

0.001–0.01

< 1 in 1,000,000

CT pulmonary angiogram

Lung ventilation scan

0.01–0.1

1 in 1,000,000
to
1 in 100,000

Group 2:
X-ray of abdomen, pelvis or hip or barium meal

CT scan of the chest and upper abdomen

Nuclear medicine scans using technetium-99m including: thyroid scan, lung perfusion scan, renal scan (MAG3, DMSA) or white cell scan

0.1–1.0

1 in 100,000
to
1 in 10,000

Group 3:
Lumbar spine X-ray

Barium enema

Intravenous pyelogram or urogram

CT abdomen or CT lumbar spine

Nuclear medicine scans using technetium-99m

Bone scan

Cardiac blood pool scan

Myocardial scan

Renal scan

Thallium-201 myocardial scan

1.0–10

1 in 10,000
to
1 in 1,000

CT of pelvis, or pelvis plus abdomen

PET-CT

Technetium-99m myocardial SPECT
(rest – exercise protocol)

10–50

1 in 1,000
to
1 in 200
Natural childhood cancer risk:
1 in 500

*Advice from the (UK): Health Protection Agency, the Royal College of Radiologists and the College of Radiographers


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5. What are the benefits for the mother?

Before referring a pregnant woman for an imaging test or procedure, the woman’s doctor will consult with her and make an assessment that the benefits outweigh any possible risks. The radiologist or nuclear medicine specialist carrying out the test or procedure will also assess if it is the most appropriate test or procedure, taking into account the information the referring doctor has written on the request form together with the patient’s medical history (see Radiation Risk of Medical Imaging for Adults and Children)


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6. What if a woman has had a test involving radiation and then finds out she is pregnant?

If a woman undergoes a radiology or nuclear medicine procedure and then finds out she is pregnant, it is important the woman talks to her doctor about theoretical risks. This will avoid needless worry or inappropriate consideration of pregnancy termination as a result of fears about the wellbeing of the foetus because of exposure to ionising radiation.

It is entirely appropriate that the woman talks to her referring doctor about any worries or concerns. The information provided in this publication will for the vast majority of women be reassuring. The information is also designed to inform the discussion about potential risk between a woman and her doctor.


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7. What should a woman of reproductive age consider if referred for a test involving ionising radiation?

This depends on the kind of test the woman has been referred for. In Table 2, it can be seen that the tests have been grouped from lowest to highest in terms of the radiation dose that is given to the uterus (and if a woman happens to be pregnant, to the foetus in the uterus).

When having any kind of nuclear medicine test or a radiology test or procedure that involves exposing the abdomen or pelvic area to radiation, it is very important for a woman to know if she is pregnant.

If a woman is definitely not pregnant (in other words, not sexually active) it is safe to proceed with the test.

If a woman is definitely pregnant, she must tell her doctor and the practice where she is having the procedure. Medical staff will assess the potential risk of not having the test and balance this against the potential risk to the foetus from the radiation. It may be decided that the procedure can wait until after the baby is born or that another test not involving radiation exposure of the foetus is more suitable.

If a woman cannot be certain that she is not pregnant because she is sexually active, but her period is not overdue AND she has been referred for one of the lower dose procedures involving a dose of less than 10 mGy to the uterus (groups 1, 2 or 3 in Table 2), it is generally considered safe to proceed without having a pregnancy test.

If a woman’s period is late, she should follow the recommendation for probably or definitely pregnant patients until she has a pregnancy test and it is negative.

If a woman has been referred for one of the higher dose tests (more than a 10-mGy dose to the uterus), it is more important to be sure she is not pregnant. A woman may be sure because she is not sexually active. It is also very unlikely a woman is pregnant if she is in the first 10 days of the menstrual cycle (from the first day of the last period). If a woman is sexually active, in her reproductive years (in other words, has not gone through menopause) and in the last half of the menstrual cycle, it is best not to have a higher dose procedure without consultation between the radiology practice and the referring doctor. If the test is not a medical emergency, it is advisable to wait and schedule the test or procedure after the period starts up to 10 days from the beginning of menstrual bleeding. The chance of becoming pregnant during this time is extremely low.

The list of tests and procedures provided in Table 2 is not complete. A woman should check with her referring doctor or the practice or hospital where she is having the test or procedure about possible radiation exposure to the uterus if she thinks she could be pregnant.


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8. What do I need to consider if I am trying to become pregnant?

Preconception radiation exposure of either parent’s reproductive organs has not been shown to result in increased cancer or abnormalities of children.3

If you are trying to become pregnant, and you need to  have a medical imaging test involving radiation, you need to consider the timing  of this test in relation to your menstrual period see what are the radiology referral guidelines for women of reproductive age?


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9. What if a woman is breastfeeding and needs to have a nuclear medicine procedure

When a radiopharmaceutical is given as part of a nuclear medicine procedure, it spreads throughout the body. Some organs will collect a higher concentration of the radiopharmaceutical than others, but it will be distributed throughout the body. Some radiopharmaceuticals can enter the breast milk of nursing mothers and be consumed by an infant.

Usually, to avoid unnecessary exposure of an infant to the radiopharmaceuticals, breastfeeding will have to be suspended for a short period of time after the nuclear medicine procedure. The length of the period of time will depend on the type and amount of radiopharmaceutical used (see InsideRadiology item on Nuclear Medicine). If breastfeeding at the time of a nuclear medicine procedure, a woman needs to inform the imaging staff before the procedure and seek advice from the nuclear medicine specialist as to when breastfeeding can resume. The facility where the woman is having the test or procedure will normally be able to provide advice concerning the possibility of expressing and storing breast milk before the procedure is carried out.

Women due to undergo therapy with radioiodine may find it beneficial to stop breastfeeding 2–3 weeks before receiving therapy. This is because breast tissue that is producing milk will receive a higher dose of radiation than the breast tissue of a woman who is not breastfeeding. Again, specific advice needs be sought from the nuclear medicine specialist.


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10. Where can I find more information?