Population and patient risk from CT scans

David Sutton
Ninewells Hospital & Medical School, Dundee, UK

Address for correspondence:
David Sutton, PhD
Head of Radiation Physics,
Department of Medical Physics, Ninewells Hospital,
Dundee, DD1 9SY, UK
Tel: +44-(0)-1382-632604 Fax: +44-(0)-1382-640177
Email: d.g.sutton@dundee.ac.uk


Abstract
The recent rapid increase in the use of computed tomography (CT) scanning has been paralleled by increased concern about the doses of radiation involved and, in particular, about the public health consequences in terms of increased cancer induction. Despite the fact that CT only accounts for ~10-15% of all radiological procedures, it contributes around 50% of the collective dose to the population arising from diagnostic radiology. Because of the relatively high organ doses associated with CT, it is possible to produce 'broadbrush' estimates of the number of cancers induced in the population as a result of its use. It is also possible to estimate the Lifetime Attributable Risk of cancer induction resulting from a single CT scan on an individual patient. This article reviews the available evidence and methodology and considers risks to both the general population and to individual patients.

Background
Over the past few years, there has been a significant increase in the number of computed tomography (CT) scans performed in most of the developed world. The increase has been due, in no small part, to the introduction of first spiral single detector (SSD) and then multi detector CT (MDCT) scanners which have allowed much faster scanning and wider scan coverage than was previously available. As a result, there has been an increase in the number of applications for CT, which has been paralleled by increasing concern about the doses of radiation involved, with particular unease about the public health consequences in terms of increased cancer induction. For example, in 2000, the International Commission on Radiological Protection (ICRP) published a report on managing patient dose in CT, motivated by the relatively high radiation dose and the increasing frequency and variety of examinations following the development of SSD technology[1].

A further ICRP publication appeared seven years later, addressing the patient dose in MDCT[2]. This was a response to the by then ubiquitous use of MDCT and the introduction of more novel applications, with the potential for even greater population doses. The potential for CT-induced cancers in children has justifiably received particular attention in the scientific literature[3-8] and there have also been increasing numbers of articles concerned with the general issue of CT dose[9-12].

Figure 1. Figure 1. Number of CT procedures performed per year in England (a) and the USA (b) between 1998 and 2007.
Figure 1. Number of CT procedures performed per year in England (a) and the USA (b) between 1998 and 2007.

Figure 1 shows the increase in CT scans in England and the USA over the last ten years[13-15]. The average annual increase in the number of scans is about 10% in England and 11.5% in the USA; this growth is faster than for other imaging modalities and shows no sign of slowing down. In 2002, CT scans accounted for 5.5% of all imaging involving ionising radiation in the UK, and by 2007, this had risen to 11%. Figure 2 shows 2007 data for percentage of CT scans performed compared to other radiological procedures[13-15].
Figure 2. Figure 2. Relative numbers of radiological procedures in England (a) and the USA (b) in 2007.
Figure 2. Relative numbers of radiological procedures in England (a) and the USA (b) in 2007.

Evidence for cancer risk associated with diagnostic radiology
There is no doubt that the development of cancer is a late consequence of exposure to ionising radiation. The relationship between radiation dose and risk is the subject of much research, most of which is epidemiological and based on populations exposed to high doses and relatively high dose rates. However, there are some low-dose studies which can be used to check the statistical validity of extrapolating from high-dose studies to lower doses[16]. The accepted model of dose response is a linear quadratic model in which the initial linear element is followed by a quadratic response as the dose increases. The major debate is about the effects at low doses - is the response linear or does it follow some other pattern? For example, is there a threshold below which there are no effects or is there a zone in which low doses of radiation are actually beneficial? The debate over the theory that radiation effects at low doses conform to the linear no threshold (LNT) hypothesis is concerned with organ doses below 100 mGy. The controversy is greatest at organ doses <10 mGy, where the risks cannot be quantified epidemiologically because of the enormous sample sizes that would be required[17].

There are arguments on both sides,[18,19] but new analysis of the data provides plausible evidence to support quantification of the risk associated with organ doses of 10-20 mGy, using the LNT approach. In 2003, Brenner et al. concluded that there was good evidence to show an excess risk of cancer for organ doses in excess of 34-50 mGy and reasonable evidence for some increased risk above 5 mGy[17]. Subsequently, the results of a 15-nation study of over 400,000 nuclear workers who received an average occupational dose of 20 mSv were published[20]. Although the interpretation of some of these results is controversial, Hall and Brenner have taken them in their entirety and included them in a meta-analysis with other sources, including the Japanese Long Term Survival Study (LSS) data.[21] Their results indicate a statistically significant linear relationship between excess cancer risk and radiation dose at levels lower than previously seen.

This implies that there can be no argument about an excess cancer risk associated with CT, where organ doses are above 10 mGy for many procedures. For example, the dose to the stomach, colon and bladder (some of the most radiosensitive organs) from a single MDCT scan of the abdomen and pelvis is typically [16-20] mGy, depending on the equipment parameters selected. (In comparison, the dose to the stomach is <1mGy from a plain X-ray of the abdomen in an average person). Many CT procedures involve more than one scan, and many patients receive more than one procedure, hence organ doses >45-60 mGy are not uncommon.

Dose and risk
As already discussed, the radiation doses from CT scans are among the highest of all diagnostic exposures. The concept of 'effective dose' is one way of characterising the associated risk. This represents the uniform whole body dose that would result in the same radiation risk as the actual non-uniform dose received.[22] The unit of effective dose is the Sievert (Sv) and the risk of inducing a fatal or non-fatal cancer (as well as inheritable defects) can be expressed in terms of percentage risk per Sv.

Assessment of effective dose
Effective dose is difficult to assess since it cannot be measured directly; the absorbed doses to a variety of organs are estimated and then weighted and summed according to a scheme devised and recently revised by ICRP.[23,24] Nevertheless, effective dose is directly related to stochastic radiation risk and it provides an understandable link between the radiation dose and the probability of harm.[22]

Usefulness of effective dose
Effective dose can be used to estimate the risk of induction of a fatal cancer by multiplying it by a probability coefficient for fatal cancer induction which, for example, the ICRP gives as 5.5 x 10-2 per Sv for a general population.[22,24]

Effective dose is an adequate tool for estimating the approximate risk to individuals from diagnostic exposures, since only order of magnitude answers are required, but it cannot be used to make estimates about anything else.[25,26] There are three major reasons for this:

  • The age distribution for patients undergoing medical examinations is different from that of the general or working population groups for which it was derived. For example, the cancer risk for males associated with the same radiation dose is four times higher in the first year of life than between the ages of 20 and 50 years, while for females it is almost twice as high[27]. The age profile of patients also varies from one procedure to another.
  • Estimation of effective dose is particularly difficult when organs and tissues receive only partial or heterogeneous exposures, as is the case for diagnostic exposures.[24]
  • There are considerable uncertainties in the risk factors because of the way in which they were obtained.

Despite these caveats, effective dose is an appropriate measure to compare the relative risks from diagnostic procedures and to compare the use of different technologies and procedures between hospitals or even countries, provided that patient populations are similar in age and sex.[22,24]

For these reasons, the risk associated with diagnostic procedures using ionising radiation is best evaluated using appropriate risk values for the individual tissues at risk and for the age and sex distributions of the individuals undergoing the procedures.[24,27] Calculations of risk, based on the product of organ dose and the organ-specific fatal cancer probability, can differ by as much as 50% from those derived from effective dose.[22]

Collective effective dose
Effective doses can however be summed over a population to produce the 'collective effective dose', measured in man Sv. This can be used to provide information about the total radiation burden to a population as a result of CT scanning.[23,24] As with effective dose, the use of this metric is open to abuse and must be interpreted carefully, taking into account factors such as the age and sex of the exposed population, the dose distribution in time and the total number of exposed persons.[24]

It is generally accepted that ICRP risk factors for the induction of cancer by low-dose radiation are associated with a large degree of uncertainty, because they are mainly extrapolated from data on medium- and highdose ranges, and rely on the LNT hypothesis.[24,25] The uncertainty is greatest at lower doses, where errors in summation may be greater. It is tempting to look at the collective effective dose arising from a particular diagnostic examination and draw conclusions about the excess cancer risk by applying the ICRP risk factors. However, all such estimates must be treated with caution unless they are rigorously scrutinised.

Figure 3. Figure 3. Estimated contribution to the overall collective dose from radiological procedures in England (a) and the USA (b).
Figure 3. Estimated contribution to the overall collective dose from radiological procedures in England (a) and the USA (b).
Doses from CT scans
What about the overall contribution of CT to population dose? Because CT delivers higher doses than conventional procedures, one would expect that the percentage dose distribution would differ from the numerical distribution of procedures and indeed the difference is dramatic. Figure 3 illustrates the relative contribution of CT to the collective effective dose from radiological examinations for both the US and English populations; the proportion of the radiation dose to the population from radiological procedures resulting from CT is 54% in England, and 49% in the USA. (These figures have been estimated for the purposes of this article from existing data sources.[13-15,28,29] So despite its relatively low frequency, CT delivers by far the biggest radiation dose to the population.

Although the overall contribution of CT to the population dose is similar in England and the USA, the collective effective dose is much greater in the USA, even after adjusting for the population size. As also shown in Figure 3, the collective effective dose from CT in the USA is estimated at 440,000 man Sv compared with 18,000 man Sv in England, a ratio of ~25, whereas the ratio of population size is about 5. In 2007, the dose of radiation attributed to CT was about 0.35 mSv per head of population in England and 1.5 mSv in the USA. Much of this discrepancy can be explained by the higher number of CT scans per head undertaken in the USA as shown in Figure 4.
Figure 4. Figure 4. Number of CT scans performed per head of population by year in England and the USA between 1998 and 2007. The average ratio over the period of the number of scans in the USA to the number in England is 4.5.
Figure 4. Number of CT scans performed per head of population by year in England and the USA between 1998 and 2007. The average ratio over the period of the number of scans in the USA to the number in England is 4.5.

Risks from CT scans
What does this mean in terms of excess risk of cancer induction in the population as a whole? We have examined the limitations of collective effective dose, in particular the uncertainty in risk factors for low doses as a result of their reliance on the LNT hypothesis. However, as discussed above, CT is not a low-dose procedure and is associated with organ doses of tens of mGy. In addition, recent evidence has indicated that there are convincing data to support the use of the LNT hypothesis for organ doses approaching 10 mGy. In this context, it has been estimated that over 85% of the collective effective dose from CT use in the USA arises from chest, abdomen/ pelvis and angiographic examinations.[14]

Therefore, it is possible, in the case of CT (as opposed to plain film radiography), to use the concept of collective dose to produce generic estimates of cancer risks which take a 'broad-brush' approach and ignore the variation of risk with age and gender and provide an indication of the public health consequences of the radiation used in CT.21 If we do this and apply the ICRP risk factor for the whole population of 5.5% per Sv to the estimates for collective effective dose, then the predicted number of fatal cancers based on current usage patterns is >950 per year in England and >24,000 per year in the USA.

It must be stressed that these are generic estimates and reflect neither the proportion of CT procedures performed on children (~10% in the USA), nor that many CT scans are performed on older people in whom the risk factors are lower. However, they do serve to make their point and are supported by what appear to be realistic assumptions. These numbers equate to 0.78% - about 1 in 125 - of annual cancer deaths in the UK and 4.3% - about 1 in 22 - of annual cancer deaths in the USA, based on the most recent statistics.[30,31]

Risks can be loosely attributed to individual CT scans using the concept of effective dose; in very approximate terms, a CT examination with an effective dose of 10 mSv may be associated with an increase in the possibility of fatal cancer of approximately 1 in 2000.[10,32] However, as previously discussed, it is better to evaluate the risk associated with CT scanning using age- and sex-adjusted risk values for individual tissues. This can be done using the BEIR VII committee[27] methodology to evaluate the Lifetime Attributable Risk (LAR) from an individual CT scan.

One study which took this approach investigated the risk associated with 64-slice CTCA (CT coronary angiography) examinations[33]. A major conclusion was that LARs from one standard, non-gated examination ranged from 1 in 143 (0.7%) for a 20-year-old woman to 1 in 3261 (0.03%) for an 81-year-old man. As would be expected, the use of ECG-controlled tube modulation reduced the risks by about 40%. The most likely cancers were lung and breast, in younger women.
Figure 5. Figure 5. Estimated age-dependent gender-averaged percentage lifetime radiation attributable risks from typical CT scans of (a) the head and (b) the abdomen. Reproduced from Br J Radiol with permission.10
Figure 5. Estimated age-dependent gender-averaged percentage lifetime radiation attributable risks from typical CT scans of (a) the head and (b) the abdomen. Reproduced from Br J Radiol with permission[10].

Another study calculated LARs for abdominal and head scans for a range of cancers in patients of different ages.[34] Figure 5 shows that the risks are higher for abdominal scans because of the greater radiosensitivity of the digestive organs. The data show the expected significant age-related variation in risk, which can potentially be reduced in younger patients if scan parameters are adjusted to account for patient size. Without making these changes, the risk per abdominal CT scan ranges from about 0.1% in young children to about 0.02 % in persons aged over [30].

The risks are additive, so that the more CT scans a patient undergoes, the greater the risk of inducing a cancer in that particular patient. Considering the overall risk from a population perspective, the higher the collective effective dose from CT scans, the greater the overall number of attributable fatal cancers. As the number of CT scans is increasing - with no evidence of a slow-down - then it is clear that the associated public health risk is rising.

Addressing the issue of dose in CT
Given the evidence, it is clear that the radiation protection principles of justification and optimisation need to be properly applied to the practice of CT scanning, and this may not be happening at present. CT dose per examination can be reduced using the principle of optimisation and CT usage can potentially be reduced if the justification principle is applied.

The issue of justification (or appropriateness) is very complex. However, in this context, it is interesting to ask why a US patient is almost five times more likely to have a CT scan than a patient in England. Is there an evidence base to suggest that diagnostic outcomes are much better or that there are other factors involved in both countries? The potential increase in population dose from the use of CT as a screening tool in asymptomatic individuals must also be addressed and much has been written on justification in this context.[34,35] Dose reduction through appropriate use of equipment parameters is often addressed,[36] but other factors such as tailoring the scan to the patient are not.[37]

In 2002, Golding and Shrimpton wrote an article entitled 'Radiation dose in CT: are we meeting the challenge?'10 They concluded that the answer was no. Many of the issues that they addressed in 2002 are as pertinent today as they were then and the answer is still no.

Key Learning
  • The number of CT scans being performed is increasing at about 10% per annum
  • CT accounts for 15% of all procedures in radiology but contributes 50% of the population dose resulting from the diagnostic use of ionising radiation
  • The doses from CT are high enough to allow reasonable estimates to be made of the number of cancers induced in the population as a result of its use
  • There is convincing evidence that a CT scan can be associated with the risk of cancer in an individual patient
  • The challenge presented by the issue of radiation dose in CT is not being met

References
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