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CT scans: The cancer time bomb

MagazineApril 2009 (Vol. 20 Issue 1)CT scans: The cancer time bomb

The CT (computed tomog-raphy) scan-once known as the CAT scan-is arguably the most impress-ive x-ray diagnostic tool widely available to the doctor nowadays

The CT (computed tomog-raphy) scan-once known as the CAT scan-is arguably the most impress-ive x-ray diagnostic tool widely available to the doctor nowadays. Its whole-body, three-dimensional imagery can provide a highly detailed picture of human anatomy. It is also one of the most sensitive early-warning detectors of cancer, internal bleeding, heart problems, stroke and other neurological disorders.

As a result, 62 million CT scans are carried out in the US alone every year, including at least four million on children-an exponential increase on the three million scans of both adults and children that were carried out in 1980. What's more, in the UK, its use has doubled in just the last 10 years.

However, these figures have been boosted by the growing numbers of 'worried well', who are paying as much as lb2000 ($4000) for a just-in-case scan through private clinics.

Japan has the highest per capita CT usage in the world, having 64 scanners per one million population compared with only 26 machines per million in the US.

Not to be left too far behind, the UK government recently announced an annual budget of lb20 million to buy around 15 more scanners for National Health Service hospitals, which currently has a relatively paltry six scanners per million people.

Cause for concern

However, patients are not being told about the extraordinarily high levels of ionizing radiation they are exposing themselves to every time they submit themselves for a CT scan. Leading radiologist David Brenner, at Columbia University in New York, has estimated that a standard course of two to three CT scans is equivalent to the radiation levels at Hiroshima or Nagasaki (N Engl J Med, 2007; 357: 2277-84), while the Royal College of Physicians of Edinburgh reckons that having just one scan is equivalent to around 500 standard chest x-rays.

This rush for a CT scan may be producing a cancer time-bomb for future generations, warns Professor Brenner. He estimates that CT scans could be responsible for as much as 2 per cent of all cancer cases in the US (N Engl J Med, 2007; 357: 2277-84). As around 1.44 million Americans develop cancer every year, this suggests that the scans alone could be responsible for 29,000 new cases annually-far above the accepted figure of 6000 cancer cases that are currently attributed to the CT scan in the US.

Not surprisingly, children are especially vulnerable, Professor Brenner believes. Not only do they have many more years during which to develop some sort of cancer,but they are also far more sensitive to the radiation from the scan, which is routinely used to detect appendicitis.

His concerns and estimates have been echoed in the UK by the government's advisory group COMARE (Committee on Medical Aspects of Radiation in the Environ-ment). The group's current chair-man, Professor Alex Elliott, also believes that CT scans could be responsible for up to 2 per cent of all new cancers and, even conser-vatively speaking, are almost certainly to blame for 1 per cent, which means that they may already be more than twice as dangerous as the current estimates suggest.

Professor Elliott and COMARE are especially concerned over the growing numbers of CT scans that the 'worried well' are having. The most popular just-in-case scans are colonography for cancer of the colon, lung screening for current and former smokers, cardiac screening for heart health and whole-body screening for an overall health review. As radiation levels from the scans are so high, the risk, according to COMARE, far out-weighs any benefits (COMARE Twelfth Report. The impact of personally initiated X-ray computed tomography scanning for the health assessment of asymptomatic individuals. Health Protection Agency, 2007; online at www.comare.org.uk/documents/ COMARE12thReport. pdf).

The risk of cancer

One average CT scan produces a radiation dose of 15 mSv (milli-sieverts) in an adult, and twice that amount in a newborn. A typical CT course involves two to three scans, which means that an adult will be exposed to a cumulative 45-mSv dose of radiation. What's more, most patients undergoing CT of the abdomen or pelvis undergo more than one scan on the same day, while 30 per cent of patients having a general scan are subjected to at least three scans, 7 per centto five scans and 4 per cent to nine or more scans (J Radiol Prot, 2000; 20: 353-9).

To put the resulting radiation dosages into a real-life perspective, the citizens of the Japanese cities of Nagasaki and Hiroshima were exposed to slightly less than 50 mSv when the atomic bomb was dropped (Radiat Res, 2004; 162: 377-89). And, as subsequent researches have shown, those atomic-bomb survivors who were exposed to mean radiation levels of 40 mSv were far more likely to develop cancer (Radiat Res, 2007; 168: 1-64).

The well-established link between radiation levels and cancer has been further amplified by a study of 400,000 workers in the nuclear industry. These workers, who are exposed to an average radiation level of 20 mSv, were found to have significantly higher rates of cancer than their matching fellow workers in other, non-nuclear, industries (BMJ, 2005; 331: 77).

In addition, researchers have analyzed the clinical characteristics from the atomic-bomb statistics, and estimated that children are at a far higher risk than adults of developing cancer if they have been exposed to radiation at a young age. This is because young people are more radiosensitive as a result of having a far higher proportion of cells that are dividing and repro-ducing (Biological Effects of Ionizing Radiation (BEIR) VII. Washington, DC: National Academies Press, 2005).

In a separate study carried out by Columbia University, the scientists estimated that CTCA (computed tomography coronary angiography), a specialist application of the CT technology that is used to look for coronary artery disease, can increase cancer risk by 1 per cent in young women. Indeed, women in general, and people who have had multiple scans, were also at greater cancer risk than were men and the elderly (JAMA, 2007; 298: 317-23).

Astonishingly, given that this is a technology that emits such high levels of radiation, no one has ever carried out a large-scale study into CT safety. David Brenner and his colleague at Columbia, Eric Hall, estimate that the cancer risk from CT scans has increased simply because the numbers of scans being carried out have also increased-and dramatically so.

The current estimate of new cancer cases attributable to CT scans is 0.4 per cent of the total cases, which translates to around 6000 new cancer cases in the US every year. But this estimate was based on the numbers of CT scans performed between 1991 and 1996, when between only 15 million and 20 million scans were being per-formed annually in the US (Lancet, 2004; 363: 345-51).

Today, that figure has risen to 62 million scans a year just in the US alone, which prompted Brenner and Hall to adjust the cancer risk accordingly. Their adjusted esti-mate is that the CT scan is now responsible for 1.5 per cent to 2 per cent of all new cancers, or up to 29,000 new cases in the US alone.

Their estimate is supported by America's Food and Drug Admin-istration (FDA), which reckons that one standard 10-mSv scan will cause one fatal cancer for every 2000 scans carried out. By today's usage, this means that CT scanning is responsible for 31,000 deaths due to cancer each year, which is an even more depressing picture than that painted by Brenner and Hall, whose figures refer to disease incidences only (Circulation, 2006; 114: 1761-91).

And as the US is not the only country that avidly uses the CT scan, it would not be unreasonable to estimate that the worldwide cancer rates that could be laid at the doorstep of the technology is around 100,000 new cases a year.

For a procedure that's known to be responsible for 31,000 cancer deaths a year in the US alone, you might assume that the patient who is about to receive a radiation dose equivalent to the Hiroshima blast would have given his informed consent.

Sadly, you'd be wrong. As leading clinical cardiologist Eugenio Pica-no, at the Institute of Clinical Physiology of the National Research Council in Pisa, Italy, has said: "Even for procedures with high radiation doses, there is no explicit or implicit mention of long-term risks. The risk remains unheard by the patient and unspoken by the doctor" (Cardiovasc Ultrasound, 2007; 5: 37).

Patients remain ignorant

Concerned by the growing cancer risks and the lack of informed consent from the patient, the American College of Radiology prepared a white paper on best practices in 2007. In essence, the College concluded that ignorance lies at the heart of the issue. In many instances, the doctor himself is not fully aware of the true risks that come with the procedures, and the patient is equally as ignorant simply because there's no one to tell him (J Am Coll Radiol, 2007; 4: 272-84). Nevertheless, there's no doubt that the informed consent should include an outline of the potential cancer risk of CT along with the possibility of other, safer, scanning technologies such as ultrasound and MRI.

The CT scan also falls outside the boundary of 'acceptable risk', as defined by the UK's Royal Society Study Group (Br J Radiol, 2003; 76: 763- 5), which also underscores the idea that patients' informed consent-including true risk calculations-is imperative.

Radiologists have long suspected that scans may lead to cancer, but have reasoned that the risk must be negligible compared with the 'spontaneous' cancer risk, which is put at one in four chances, or a 25- per-cent risk (Office for National Statis-tics. Health Statistics Quarterly, 2000). Such a position is justified only if you accept that people only develop cancer spontaneously-and from no other causes.

Is CT worth it?

CT advocates argue that the benefits of this technology far outweigh any risks, especially as, they claim, early-stage cancers and heart problems can be most easily detected by the outstanding images the scans generate. As Dr John Giles, clinical director of Lifescan, the UK's largest private clinic offering CT screening, has said: "We confidently estimate that we have saved the lives of over 2000 people who were identified with a potential life-threatening disease and able to receive early treatment" (The Guardian, 20 Decem-ber 2007).

The truth is, although CT is highly sensitive, it still produces false-negative findings-failing to detect a problem where one is present-in one out of every 20 scans. This ratio worsens to three out of every 20 scans for early-stage diagnosis, where CT is likely to pick up a false-positive-seeing a problem where there isn't one (Radiology, 2005; 234: 415-22).

Even in patients whose disease is more advanced-and thus more likely to be correctly visualized-CT can still produce false-negative or false-positive results in 12 per cent of all patients scanned.

The accuracy of the scan also varies depending upon the dis-ease. In one study of 23 patients with rectal cancer, CT successfully recognized the problem in 18(78 per cent) of the patients (Cancer, 2007; 54: 512-6). In contrast, in those who had lung cancer, it was inaccurate-it failed to detect the tumours that were present-in up to 47 per cent of all cases (Ann Thorac Surg, 2007; 84: 1830-6).

One study carried out by the medical examiner's office in San Antonio, Texas, concluded that the results of CT scanning are too unreliable to be used as testimony in court. In that instance, the researchers pointed out that its accuracy could range from 0 per cent for cerebral lacerations to up to 75 per cent in cases involving liver injury. Interestingly, those researchers were unable to find any scans that were accurate above that level. As they concluded: "CT scans are an inadequate detection tool for forensic pathologists, where a definitive diagnosis is required, because they have a low level of accuracy in detecting traumatic injuries. If the evidence of traumais based solely on CT scan reports, there is a high possibility of erroneous accusations, indictments and convictions" (J Trauma, 2007; 63: 625-9).

The worried well

If the CT scan can be inaccurate in detecting a disease that is already suspected, how helpful is it likely to be at picking up potential problems among people who are supposedly healthy, but who are seeking the reassurance of a just-in-case checkup?

A scan is likely to detect a genuine health problem in only 2.8 per cent of patients who are undergoing a general checkup; yet, it can also produce a 14.8-per-cent rate of false-positives, where it detects a problem that isn't there. Put another way, this means that it will give a clean bill of health to 82 per cent of the worried well when, in fact, 97 per cent of them are healthy (COMARE Twelfth Report. The impact of personally initiated X-ray comput-ed tomography scanning for the health assessment of asymptomatic individuals. Health Protection Agency, 2007).

Furthermore, not only will nearly 15 per cent of the worried well be subjected to more costly tests, and further worry, but all of these healthy individuals will have exposed themselves to levels of radiation that are carcinogenic-and all for no good reason.

However, despite these concerns, the market for just-in-case scans continues to mushroom, especially in countries such as the US and Australia. Usually, private clinics will offer full-body scans for fees of around lb2000 ($4000), and they have used catch-phrases such as 'the ultimate screening miracle' in marketing their services.

The UK's COMARE is so concern-ed by the growth in the private-scanning market that it is calling for regulations to outlaw its use for conditions such as spinal prob-lems, osteoporosis and body-fat determinations.

What do we do now?

There's no doubt that CT scanning is a technology that has run away with itself. Doctors are eager for the patient to try it, and everyone is astonished by the extraordinary 3-D images that it produces.

However, it's a technology that is now grossly overused. From having started in the early 1970s as a way to capture more detailed images of the brain, today it is used to assess and even monitor trauma, seizure risk and chronic headaches. In children, it's used to determine the risk of acute appendicitis.

It's also become a defensive mechanism that is now included as part of the general checkup. In one poll among paediatric radiologists, it was found that up to one-thirdof all CT scans could either be replaced by a safer process or simply not performed at all (Pediatr Radiol, 2002; 32: 242-4). Indeed, if such a conservative approach were to be adopted, it would mean that 10,000 people every year would not develop cancer in the US alone, according to the FDA's and Professor Brenner's estimates.

Another problem, and one that was highlighted last year by the American College of Radiology in its white paper, is that doctors themselves do not fully appreciate the radiation risks associated with CT scanning and, instead, place it in the same category of risk as the standard x-ray. In one survey of emergency doctors and radiolog-ists, 75 per cent underestimated the radiation levels from a CT scan, and 91 per cent of emergency-ward physicians simply did not believe that the scans could increase a person's lifetime risk of cancer (Radiology, 2004; 231: 393-8).

It may well be that only when doctors themselves fully appre-ciate the cancer risks that are associated with this 'gee-whiz' technology will the numbers of CT scans start to fall and, with it, the ever-increasing rates of cancer across the world.

Bryan Hubbard

Ticket to scan

It's been said that the CT (computed tomography) scanner was the Beatles' greatest legacy. Profits from the group's record sales certainly helped EMI, their recording company, to fund the development of the prototype scanners.

The technology was originally called 'EMI scans', but it soon became known as 'computed axial tomography', or CAT scans, a term that many still use today.

The idea behind CT is attributed to Godfrey Newbold Hounsfield, who worked at Thorn EMI's Central Research Laboratories in Hayes, Middlesex, in the UK. Hounsfield himself had doubtlessly been inspired by the Italian radiologist Alessandro Vallebona, who in the 1930s developed the idea of a 'super-x-ray' that would create an image of a body by recording it from various angles at the same time, a technique he called 'tomography'.

Hounsfield started work on his scanner in 1967, at the same time that Allan Cormack, at Tufts University in Massa-chusetts, was working on his own version. They both shared the Nobel Prize in Physiology and Medicine in 1979 for their efforts.

In addition, Hounsfield has given his name to the units (HU) in the standardized scale that is used to quantify the radiodensities of air and water that allow CT scans of the human body to be created and interpreted.

The first CT scanner was installed in the Atkinson Morley Hospital in Wimbledon, in South West London, in 1972. At that time, its use was limited to brain scans. It took between four and seven minutes to perform one 180-degree scan, and another seven minutes to process each image.

Although the technology continued to advance over the years, the greatest acceleration has taken place only in the last six years. The recent development of multidetector or multislice CT has increased scanning speed, and has made it possible to do the high-resolution reconstruction of images that produces an almost 3-D effect.

The time taken to scan and to obtain an image has also been dramatically decreased. A chest examination that once required 10 separate breath-holds of 10 seconds each can now be performed with a single 10-second breath-hold.

CT software has also advanced, so that a study of 1000 images can now be constructed in a mere 30 seconds.

Is CT for you?

If you're claustrophobic, the CT scanner is not for you. Many patients are subject to panic attacks as they are being placed in the tunnel-like opening of the scanner-to the point where the test has to be stopped. Other patients are frightened by the whirring noise that the scanner makes while it takes the x-rays.

Some individuals also react badly to the x-ray dye that is injected into their veins. The dye makes it easier for the radiologist to visualize organs, blood vessels or tumours. Those who are about to undergo an abdominal scan are asked to drink about a litre of a sweet, viscous white liquid containing gastrografin, an aniseed-flavoured dye, which can also be administered rectally.

As the radiologist usually forgets to mention that you're about to be exposed to a radiation level similar to the bombing of Hiroshima, it's fairly unlikely that he's going to tell you that you should never take gastrografin if you have thyroid problems. You should also never be given the dye if you are taking a beta-blocker for high blood pressure. As gastrografin is based on iodine, you also shouldn't take it if you're allergic to the element.

Finally, don't take gastrografin if you have weak kidneys-as it's likely to make them even weaker-or if you suffer from asthma.

Side-effects are "rare", says gastrografin's manufacturer Bayer, but theydo happen: they include nausea and vomiting, shock and diarrhoea-and ifit gets into your lungs, it can cause a buildup of fluid in the lungs, leading to breathlessness.

People with common sense should also think twice before subjecting themselves to a CT scan. Remember: this is a 'super-x-ray' that is equivalent to up to 500 standard x-rays-and all given at the same time.

Medicine is the foremost major source of man-made irradiation-far in excess of the nuclear industry, for instance. Furthermore, the CT scan repre-sents 47 per cent of the total radiation dose from all medical procedures.This is because, with a CT scan, multiple x-ray beams are being trans-mitted simultaneously, often with just 1 degree of difference between each one.

The strength of each beam is measured according to the area of the body it has passed through. Beams that pass through the lungs, for instance, will appear darker than those that pass through the more solid tissues such as bone. On this basis, a computer is then able to build up a picture of the organ or tissue as a pattern of darker or lighter points.

While the initial image is two-dimensional, some of the more modern scanners are able to recreate the image in 3-D.

Although the CT scan was developed as a way of 'seeing' the brain, itis today used for every area of the body in an attempt to provide an early-warning system of any developing deformity, deterioration and disease. It is especially useful where lung cancer, bleeding in the brain or a brain tumour is suspected.


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