Thursday June 7 2012
Research suggests CT scans in youth 'may raise cancer risk'
“CT scans can triple the risk of children developing leukaemia and brain cancer,” The Independent has today reported. Computerised tomography, or CT, is a technique that uses advanced X-ray technology to build up a detailed picture of a patient’s insides. Like regular X-rays, CT exposes patients to radiation that could potentially increase the risk of cancer.
Today’s news is based on a 24-year study that investigated whether children and adolescents given CT scans had an increased risk of brain tumours and leukaemia in the years that followed. The research compared the risk among children who were exposed to high doses of radiation during the scans with that of children who were exposed to the lowest dose of radiation. They found that the risk of developing leukaemia or a brain tumour increased as radiation exposure went up. Children who were exposed to a dose of radiation equivalent to two to three CT scans had nearly three times the risk of developing a brain tumour in the next 10 years compared with children with the lowest exposure. Those who had been exposed to a dose equal to 5 to 10 scans had approximately three times the risk of developing leukaemia compared with those who received the lowest dose. However, it should be noted that the total number of cancer cases was low, and the overall risk of developing either a brain tumour or leukaemia remained well below 1%.
This study suggests that children who are exposed to higher levels of radiation during a CT scan may have an increased risk of developing certain types of cancer. This increase in risk is small in real terms, but should still be weighed up alongside the benefits of a scan. It’s important to note that these risks don’t apply to MRI scans, which are alternative scans that don’t use the ionising radiation of X-rays. However, MRI scans don’t always provide the same detail as CT scans and are therefore not always the most appropriate scanning method.
Where did the story come from?
The study was carried out by researchers from Newcastle University and other institutions throughout the UK, US and Canada. The research was funded by the UK Department of Health and the US National Cancer Institute.
The study was published in the peer-reviewed medical journal The Lancet.
The media reported the story accurately, and most news sources emphasised that the absolute risk of developing cancer remained low even after repeated CT scans. Absolute risk presents a person’s overall risk of developing a condition, rather than how much a person’s risk is raised by an event such as a CT scan. News coverage also reported that the benefits of scans generally outweigh the risks.
What kind of research was this?
This was a retrospective, cohort study of children and adolescents that examined the association between estimated radiation exposure during a computed tomography (CT) scan and the risk of developing a brain tumour or leukaemia.
CT scans are used to create images of the body that are much more detailed than those obtained by a typical X-ray. CT scans are usually used to diagnose a number of conditions, including cancers, and bleeding or swelling in the brain. They may be used after serious accidents to determine whether there are serious internal injuries. The amount of radiation absorbed during a CT scan depends on various factors including the body part scanned, the tissue or organ of interest, the age of the scanning technology used and the patient’s age and sex. The researchers took these factors into consideration in order to estimate the amount of radiation each patient was exposed to.
This was a large, long-running retrospective cohort study. A study of this type can establish an association between exposure to CT radiation and cancer risk, but can’t conclusively show that one causes the other.
What did the research involve?
The researchers examined the medical records of over 175,000 patients under 22 years old who had undergone a CT scan between 1985 and 2001. The researchers estimated the amount of radiation that the patients were exposed to during these scans and grouped the patients based on this estimated dose.
They then determined how many of the patients went on to develop a brain tumour or leukaemia (a type of blood cancer) and determined the risk of developing one of these cancers over an average of 10 years based on radiation dose. The researchers then compared the risk of developing one of these cancers in the higher radiation-dose groups with the risk in the lowest-dose group.
What were the basic results?
Among the 175,000 patients, 135 brain tumours and 74 cases of leukaemia were diagnosed during the follow-up period. The researchers found that the risk of both of these cancers increased with higher radiation doses.
Compared with patients exposed to the lowest dose of radiation:
- Patients exposed to a dose equivalent to two to three CT scans had over a three-fold increased risk of developing a brain tumour (relative risk [RR] 3.32, 95% confidence interval [CI] 1.84 to 6.42).
- Patients exposed to a dose equivalent to 5 to 10 CT scans had over a three-fold increased risk of developing leukaemia (RR 3.18, 95% CI 1.46 to 6.94).
However, it should be noted that the risks of developing either cancer are generally low. Therefore, in absolute terms, the risk of developing a brain tumour or leukaemia following childhood CT scans was still small. The researchers estimated that if 10,000 children under the age of 10 each received one CT scan, this would be associated in a single additional patient developing a brain tumour or leukaemia over the next 10 years: a 0.01% increase in cases.
How did the researchers interpret the results?
The researchers concluded that, while the absolute increase in risk was low, “radiation doses from CT scans ought to be kept as low as possible.”
This well-conducted research suggests that, among children, the risk of developing a brain tumour or leukaemia goes up as doses of radiation from CT scans increase. This absolute increase in risk, however, is small.
CT scans (like many other scans such as X-rays) expose the body to doses of ionising radiation, although it is not entirely certain that this exposure increases leukaemia or brain tumour risk. However, as the researchers rightly pointed out, when considering whether to conduct a CT scan, doctors should weigh up the benefits of the scan against its small increase in brain tumour and leukaemia risk. As with any other test or treatment, doctors must consider whether a CT scan is the best option for the patient and what the overall balance of benefits and harms is.
The researchers added that the radiation doses of CT scans should be kept as low as possible, and that CT should only be used once other diagnostic tests with lower or zero radiation doses (such as ultrasound or MRI scans) have already been used or ruled out.
The three-fold risk increases covered in the media headlines are relative increases, and the absolute risk increase of developing a brain tumour or leukaemia following childhood CT scans is well below 1%.
UK regulations already state that CT scans should only be used when clinically justified, and the UK is reported to have lower levels of CT scanning than other countries. CT scans are an invaluable diagnostic tool in many medical situations. Although this research suggests a potential link with brain cancers and leukaemia, the benefits of this accurate and fast test appear to outweigh the risks associated with radiation exposure.
Analysis by Bazian