Behind The Medical Headlines

'Full-body' CT scans: Are they worth the cost in money and radiation exposure?

• Dr PL Allan, Consultant Radiologist, Royal Infirmary of Edinburgh, Edinburgh, Scotland
• Mr JR Williams, Consultant Medical Physicist and Radiation Protection Adviser, Lothian University Hospitals NHS Trust, Edinburgh, Scotland

Summary

There has been an increasing trend in some countries for the use of ‘full-body’ CT scans as check-ups to screen for possible disease. However, such scans can be very expensive and can also result in unnecessary exposure to radiation. Dr Paul Allan and Jerry Williams review the evidence on the efficacy and safety of ‘full-body’ CT scans.

Declaration of interests: No conflict of interests declared

There has been an increasing trend in some countries, such as the US and Australia, for imaging centres to offer the general public ‘full-body’ computerised tomography (CT) scans to screen for possible disease. The individual undergoes scanning of the head and neck, thorax, abdomen and pelvis; the images are then reviewed and a report is produced within a matter of days. These scans are sold on the premise that a negative/normal result will reassure a concerned person; whereas a positive result might allow potentially serious disease to be diagnosed at an early stage allowing a better chance of effecting a cure. However, this procedure is not based on any meaningful evidence that it provides any worthwhile benefit for the individual. Indeed, apart from having to pay anything up to $1000 for the scan (in Australia), there are other disadvantages and risks to be considered. These include the radiation dose received by the patient, the costs of any further investigations arising from the result of the scan, especially if these are to exclude disease suggested but not confirmed on the original scan (false positive outcome) and a possible false sense of security instilled in those with negative/normal reports (false negative outcome). This practice has served to highlight concerns rising from the increasing use of CT scanning, the complexity of some of the new scanning techniques and the subsequent increase in the radiation dose to both the individual and the general population.

Computerised tomography technology has advanced dramatically in the last ten years. In particular the latest generation of multi-slice scanners can image the whole body in less than 30 seconds. With the production of impressive 3-D reconstructions and other fancy techniques it is easy to forget that relatively high doses of radiation are used in the production of these images. A chest X-ray typically delivers an effective dose of 0.02 mSv (millisieverts), equivalent to about three days’ exposure in the UK to natural sources of radiation in the environment. A CT scan of the abdomen or pelvis however delivers an effective dose of about 10 mSv to the patient, which is equivalent to 500 chest X-rays, or 4.5 years’ exposure to background radiation.¹ The estimated risk of inducing fatal cancer is one in 2,000 at this level of dose.

In the last ten to 15 years, developments in general X-ray technology have driven examination doses down by as much as 50% for some procedures. Paradoxically developments in CT technology have pushed doses up. Improvements in scanning speed and the ability to perform extensive scan sequences (up to 1,200 slices for a single examination, compared with 40-50 on older scanners) increases the diagnostic potential but also increases the dose. Even if like-for-like scan sequences are performed on a modern multi-slice scanner the dose may increase by 20-40% compared to a conventional single slice machine.² In 1989 CT scans accounted for 2% of all X-ray examinations but contributed 20% of the collective dose to the general population from diagnostic imaging.³ By 1998 these figures had risen to 4% of examinations and 40% of the collective dose.⁴ It is likely that the contribution from CT will continue to rise for a variety of reasons:

1. Increased numbers of scans are being requested as more scanners are now available, the applications of CT scanning increase and because scans are used as a substitute for careful history taking, clinical examination and the application of clinical acumen and experience.

2. New scanners can perform various reconstructions from the data introducing new, exciting examination techniques such as virtual colonoscopy or bronchoscopy and CT angiography. But these require large numbers of thin section slices for the reconstructions.

3. In addition the ability to make multiple short scans through particular areas allows multiple vascular phase examinations (arterial, capillary, venous, parenchymal, etc.), or ‘real time’ CT fluoroscopy for interventional procedures.

Coming back to ‘full body’ CT scans for asymptomatic people, the Food and Drug Administration (FDA) in the US is of the view that ‘the harms [arising from these scans] currently appear to be far more likely and in some cases may not be insignificant’.⁵ So their advice would seem to be to keep your money in your pocket if you are in general good health.

Walk-in CT screening units will not be introduced in the UK (or elsewhere in Europe) since our legislation requires individual medical exposures to be justified in terms of potential benefit against radiation risk.⁶ However, it is important that all of us in the clinical community, both clinicians and radiologists, are aware of the doses involved in these ‘exciting’ new techniques. We should consider alternative imaging techniques such as ultrasound and magnetic resonance imaging (MRI) which, while not providing colour-coded 3D constructions of the mediastinum or abdominal structures, can still provide the diagnostic information necessary for patient management, without the significant exposure to ionising radiation. If a CT scan is required then every care should be taken to minimise the dose, particularly in paediatric cases, so that the necessary diagnostic information is obtained with the minimum patient exposure. Anyone requiring further information on this subject should read a recent review by Golding and Shrimpton, this provides an excellent summary of the issues involved.⁷

References

1. Royal College of Radiologists (RCR) Guidelines. (MBUR4) 1998.
2. Berland LL, Smith JK. Multidetector-array CT: once again, technology creates new opportunities. Radiology 1998; 209:327-9.
3. Shrimpton PC, Jones DG, Hillier MC et al. Survey of CT practice in the UK Part 2: dosimetric aspects (R249). Chilton: NRPB; 1991.
4. Shrimpton PC, Edyvean S. CT scanner dosimetry. Br J Radiol 1998; *71*:1-3.
5. Whole-Body CT Screening: Should I or shouldn’t I get one?
6. The Ionising Radiation (Medical Exposure) Regulations 2000. (Statutory Instruments 2000 No. 1059). London: The Stationery Office; 2000.
7. Golding SJ, Shrimpton PC. Radiation dose in CT: are we meeting the challenge? Br J Radiol 2002; 75:1-4.
8. Dixon AK, Whole-body CT health screening. Br J Radiol 2004; 77:370-1.
9. Hillman, BJ. CT Screening: who benefits and who pays? Radiology 2003; 228:26-8.