Delving into the radiation exposure from common ED imaging modalities
Dr Emma Bellenger
DR David McCreary
As is tradition, a poem to get us in the mood.
We send our patients
off to scan
‘cause that’s what we do
as part of our plan
But what is the risk
of doing a ct,
There’s radiation exposure
that damages DNA,
and may cause cells to mutate
in a carcinogenic way
But how exactly
do we convey
this risk of cancer to our patients
in a meaningful way
Well, you have more of a chance
of getting hit by a car
but with accidental drowning
the risk is on par
That’s not to say
scans are all bad
they help us diagnose
for which we are glad
So stop and think
scans are not risk free
weigh up the risk vs harm
and use them cautiously.
By Emma Bellenger (The ED poet)
Every day I send patients for X-rays or CT scans to help with the diagnosis of various conditions. I often mention the “risk of radiation” to my patients, but truth be told is I’ve never really known how much risk there actually is.
I’ve managed to elude needing to know this for 5 years now, but recently I was caught out when a patient asked me, “so how much risk are we talking, doc?” At which point I blushed, shrugged and told them “pretty low risk” before slowing backing out of the room.
So if you, like me, are ready to add some facts to that “at risk” spiel, then this is the blog for you. I have aimed to not get blogged down in the numbers but instead represent the risk in a way that is relatable and meaningful to us all.
Why is ionising radiation harmful?
🚨 Trigger warning – primary exam knowledge 🚨
Ionising radiation → ionised atoms → free radicals → DNA damage → mutation → cancer
As the name suggests, ionizing radiation causes an atom to become ionised (unstable – missing an electron). This can lead to the production of free radicals and subsequent tissue damage through reactions with DNA. Dividing cells are most vulnerable to these processes and can result in cell death or mutation that can then lead to carcinogenesis.
What forms of imaging use ionising radiation?
Xray and CT are our main culprits within the ED. Angiography and fluoroscopy, too – but that’s not something we use frequently within the department.
Relevant negatives are USS and MRI which are both free of ionising radiation.
How much radiation?
In day-to-day life we are all exposed to a degree of background radiation. The amount is dependent on many different factors, but the average Australian is exposed to 1.5-2 mSv per annum. (source: arpansa.gov.au)
One potential way to explain to patients how much radiation a scan will expose them to, is by relating the amount of radiation to a year’s supply of background radiation. For example, a CT chest is equivalent to about 4 additional years of background radiation.
If you feel this would work for your spiel, the table below from the American College of Radiologists represents these figures nicely:
C’mon, give the people what they want, what’s the risk of all this radiation?
So now that we know how much radiation certain scans add to our background radiation exposure, we need to know how we can convey this in a meaningful way in terms of risk. And what we really mean by that is, what’s the risk of getting cancer?!
Well, turns out radiation is not as carcinogenic as many people think (or at least as I thought). The risk depends on many factors including the person’s age, gender and part of the body being exposed; however, most sources agree that a CT will increase a person’s risk of fatal cancer to between 1 in 1000 to 1 in 2000 people. When you compare this to the life-time risk of getting cancer anyway, 400 in 2000 people (or 1 in 5), this is a low risk (FDA.gov).
For those of you who are still sceptical about the risk, how many of you have ever thought you would die from accidental drowning? I’m guessing not many…and turns out that’s approximately the same likelihood as dying from cancer from imaging radiation exposure.
What about high-risk populations?
Pregnancy, infants and children make up a group where exposure to radiation is a particularly worrisome thought.
The figure below shows that young girls are of highest risk due to the nature of dividing cells, in particular breast tissue (1 in 100 risk of cancer from 1 CT scan). Comparatively, elderly people have extremely low risk of cancer, largely due to remaining life expectancy limiting their chances of developing cancer before they die.
The concern with pregnant ladies is the harm caused to the foetus. Detrimental tissue effects from foetal exposure to radiation have only occurred at doses greater than 100msv.
All imaging in pregnancy should be discussed with a senior clinician and may need input from your radiologist ± an obstetrician, but the graph and table below gives us a brief overview of the stats.
Source: RANZCR, Inside Radiology: Radiation Risk of Medical Imaging During Pregnancy
CTPA vs VQ scan for investigating PE in pregnancy?
It’s worth being aware of this for a couple of reasons:
- It’s a common clinical conundrum
- It’s a common exam conundrum
Let’s look at the estimated radiation doses and risk from CTPA vs VQ scan in pregnancy:
|Foetal radiation (1st trimester)||0.1 mSv||0.5 mSv|
|Maternal radiation||5-10 mSv||1.2-5 mSv|
|Risk of fatal maternal breast Ca||0.005-0.01%||<0.001%|
Given that the foetal radiation exposure is minimal in both scans, the decision comes down to maternal risk. For this reason, VQ scan is recommended over CTPA as it limits radiation to the breast tissue.
Imaging in the emergency department is an important part of our workup for many patients. As we have learned, the risk of cancer is relatively low, none the less it exists and thus scans should only be ordered when they are justified.
There are plenty of ways to express the radiation exposure to our patients – be it through comparing the radiation to yearly background radiation, chest X-ray equivalents (see table below), or through comparing the risk to other ways of dying (morbid, right?).
Summary of Radiation Exposure of Common ED Imaging Modalities Expressed in Chest X-Ray Equivalents
|Radiological examination||Radiation (mSv)||Chest X-ray equivalents|
|CT Pulmonary Angiogram||10||500|
So next time a patient asks you what is their risk of getting cancer from a scan, no more backing out of the room slowly… you can hit them with the facts; ask them how often they go swimming; and help them make an informed decision.
Picture source: https://ctscanmachines.blogspot.com/2018/06/ct-scan-funny.html
Emergency Registrar, Alfred Health
Dr Emma Bellenger is an Emergency Registrar at the Alfred Hospital with a passion for life long learning, quality of life and efficiency. Alongside her role in ED she dabbles in sports medicine working with various football teams.
Pre-pandemic Emma enjoyed doing hikes around the world, but of recent times has found solace in the simple things in life such as trail running, cuddling her cats and LEGO.