Cat scans

Q:My daughter is being pressured by her doctors to have repeated CAT scans. I know there are side effects, but need some ammunition to show the doctors. Can you help? D T, Forest Row.....

A:In the Seventies, computed axial tomography (CAT), now usually known as CT scans, revolutionized x-ray diagnosis, particularly of soft tissue of the body, offering pictures with up to 20 times the detail of ordinary x-rays. CAT scans take a 360 degree series of cross sectional x-ray images from multiple angles up to 30 shots by passing a pencil thin beam through a particular portion of the body, sometimes with the use of a contrast agent. This information is then passed through a computer which reconstructs the image on a video screen, allowing the operator to see this portion of the body from any angle. It is also stored so that the doctor can take photographs of the video screen, or call up the information in the future.

Although doctors have attempted to claim that CT scanning reduces the need for other tests like brain scans and arteriography or exploratory surgery, this may be a false saving. While no doubt CAT scanning represents the height of 20th century technology, it also poses far more risks than most other tests, blasting you with far higher doses of radiation. In 1991, the NRPB concluded that CAT scans accounted for only 2 per cent of the total UK x-ray examinations but 20 per cent of the overall collective dose and so were the largest single source of exposure from x-rays. (The Lancet, 1992; 340: 299). This risk is magnified if you don't stay stock still during the half minute or so of the test and it has to be repeated. In the UK, use may still be conservative (largely because scanners are so expensive); in Japan nearly one eighth of the population was getting CT scanned as far back as 1979.

Furthermore, although all the early studies showed that CAT scans reduced diagnosis time, helped doctors to understand their diagnosis, reassured doctors about their diagnosis or treatment plans, and avoided the need for other tests, very few demonstrated that this knowledge in any way reduced illness, shortened hospital stay or prevented death (The Lancet, 1976; 1: 847-8). There are also questions of accuracy in certain instances. Despite the dangers of high dose radiation in children, particularly of their sexual organs, it often used to diagnose hernia. Nevertheless, one study found that it misdiagnosed as normal one third of children with hernias (British Medical Journal, 10 April 1993). CAT scans also miss subtle intercranial disease (The Lancet, June 25, 1994).

Despite any real demonstration of value, other than as a diagnostic toy, use of CT scanning has moved briskly apace. Patients who have a seizure are scanned, even before a clinical history is taken to rule out alcohol withdrawal (British Medical Journal, 15 October, 1994). So beloved is this gadgetry that is has even been used to research the cause of the common cold, the researchers concluding that their study patients had (wait for it) swelling of the mucus membranes (New England Journal of Medicine, 1994; 330: 25-30).

Besides megadoses of radiation, CT scans (indeed all x-rays) have long been known to cause cataracts and other lense opacities such as nuclear sclerosis (American Journal of Public Health, April 1993) and could affect thyroid function (Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr, February 1992).

Also, the contrast agent used has been known to cause patients' kidneys to pack up (Lakartidningen, March 11, 1992). And radiation exposure varies considerably, depending on the equipment and individual technique (Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr, July 1994).

Because of the inherent dangers, CAT scans should be used only in exceptional circumstances, when there is no safer diagnostic tool available. If one CAT scan needs to be performed on your daughter, hold your ground that one is enough. And during it, make sure that her reproductive organs and also her thyroid gland are shielded, which can reduce scatter radiation by 20 per cent (Rofo Fortschr Geb Rontgenstr Neuen Bildget Verfahr, February 1992)