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Managing radiation exposure in medical imaging

Plain radiographs such as CT scans can expose a patient to ionising radiation which, in high doses, can be harmful

A typical CT scan session. Although our cells have in-built mechanisms to repair genetic damage, cells with damaged genetic material may die or become cancerous if the damage is too severe or if the damage is incorrectly repaired.

HOW much radiation will I be exposed to if I undergo this scan? - is a seemingly innocuous question that is often posed to staff working in a radiology centre or department.

My approach to finding the answer to this question, and most other questions for that matter, is to type "question being asked" in the Internet browser of any computer or mobile device with access to the Internet.

After checking that the search result is from a reputable source, I can then confidently relay the answer to the patient who asked me the question: "The approximate effective radiation dose for your imaging procedure, a chest X-ray, is 0.02 millisieverts."

Unfortunately, most patients do not have any particular interest in radiation physics and if I were to truly give such an answer to the patient, I would probably receive one of two possible replies, "milliWHAT?" or "so will that increase my risk of getting cancer?"

In this day and age of easy access to information, doctors sometimes assume that patients are well aware of the risks of ionising radiation when they are referred for a scan that requires exposure to X-rays.

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Ionising radiation

These include plain radiographs (X-rays), fluoroscopic examinations, mammograms, bone mineral density scans and Computed Tomography (CT) scans. Alternative imaging modalities such as Magnetic Resonance Imaging (MRI) and ultrasound scans do not require exposure of the patient to ionising radiation but these imaging modalities have their limitations and sometimes the best form of imaging for a particular patient or condition may require exposure to X-rays.

So what is a millisievert (mSv) and how exactly is radiation exposure measured? In general, we can measure radiation by absorbed dose, equivalent dose or effective dose.

The effective dose, measured in mSv, takes into account the type of radiation a person is exposed to and the sensitivity of different organs and tissues in the body to radiation exposure.

As such, different procedures with the same effective dose will result in the same radiation risk.

For example, undergoing a CT scan of the head (effective dose of 2 mSv) will result in the same risk as undergoing 100 chest X-rays (each chest X-ray has an effective dose of 0.02 mSv).

So what is the problem with being exposed to ionising radiation or, more specifically, X-rays? Ionising radiation has enough energy to affect the atoms in living cells and damage our genetic material (DNA).

Although our cells have in-built mechanisms to repair genetic damage, cells with damaged genetic material may die or become cancerous if the damage is too severe or if the damage is incorrectly repaired.

Exposure to very high levels of radiation, such as in an atomic bomb blast, will cause acute health effects such as skin burns and acute radiation syndrome (radiation sickness).

Such catastrophic events are thankfully rare. What we are more concerned with in medical imaging is exposure to low levels of ionising radiation that do not cause immediate health effects but can increase the risk of cancer over a lifetime.

Studies of large numbers of people who have been exposed to radiation show that radiation exposure increases the risk of getting cancer; the higher the radiation dose, the higher the risk.

The average person will have a 5 per cent increased risk of fatal cancer after a whole body dose exposure of 1 Sv (equivalent to 1,000 mSv).

This risk is higher in children and adolescents but lower in individuals over the age of 60.

To put things in perspective, the average effective dose of one of the more commonly performed CT examinations, a CT scan of the abdomen and pelvis varies from 6 -10 mSv. If we were to consider a large population where every person had one such scan with an effective dose at the higher end of the average range (10 mSv), the theoretical increased risk of fatal cancer from radiation exposure would be 1 in 2,000 individuals, or 0.05 per cent.

This risk is very small when compared to the risk of the average person developing cancer which is about one in five, or 20 per cent.

Moreover, radiation safety experts generally agree that exposure to radiation doses of up to 50 mSv a year or 100 mSv over a lifetime usually result in no harmful health effects at all.

These "cut-off" values are over and above the radiation we receive from our surrounding environment, which is termed background radiation.

Background radiation

There is however considerable controversy over the risk of low dose radiation exposure because most of the risk data that we have is based on high radiation doses received over a short period of time.

The risk of low dose radiation exposure is extrapolated from high dose exposure data and critics argue that this extrapolation is flawed.

Every living being on earth is exposed to a certain level of background radiation from ground sources, cosmic radiation and even from food and water.

Frequent air travellers are exposed to an even higher level of background radiation due to greater exposure to cosmic radiation. While medical imaging involving the use of ionising radiation may contribute significantly to an individual's radiation exposure, there is usually an underlying clinical question that needs to be answered, justifying the increased radiation exposure.

From a more holistic point of view, it is important to balance the risk of increased radiation exposure with the risk of an undiagnosed medical condition; the medical benefit from an imaging examination usually far outweighs any radiation risk.

While the theoretical risk of developing fatal cancer from exposure to ionising radiation due to medical imaging is small, scanner manufacturers, radiographers and doctors are continuously striving to achieve better images with lower doses.

Improvements in imaging technology in newer scanners allow diagnostic quality images to be obtained despite patients being scanned with significantly lower doses.

When referred for a scan, patients can also play an active role in reducing their potential radiation exposure by having discussions with their doctors on the purpose of the scan and if there are any suitable alternatives that do not require radiation exposure.

This series is produced on alternate Saturdays in collaboration with MWH Centre for Medical Imaging.

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