Lee Rodwell writes that the price of saving lives through scanning has to be weighed against the effectiveness of treatments
Screening for cancer sounds like an obvious thing to do. After all, it must make sense to pick up cancers as early as possible, before they spread, when they are easier to treat. The bottom line is, screening can save lives.

But it is not so simple. Existing tests may be unreliable. Screening large numbers of people is expensive. The costs have to be weighed against whether treatments are available or effective, if detection and treatment at an early stage is more beneficial than later, and if, overall, screening is likely to do more good than harm.

Dr Anne Szarewski of the Margaret Pyke Family Planning Centre in London says cervical screening comes closest to the ideal model. "With smear tests we can detect changes which are not even cancerous and treat women before they are in any danger," she says

In Britain, screening for cervical cancer has been available since 1967, but it is only since 1988 that district health authorities set up computerised call/recall systems. Prior to this, at least two-thirds of women who had invasive cervical cancer had never had a smear test. Now, though, there are strong indications that screening is beginning to reach those most at risk.

Even so, quality control has been a problem. Concerns reached a peak last June when 91,000 smear tests in Kent and Canterbury Hospital had to be rechecked. Five women died and many more developed cancer.

A government report which followed revealed that one-third of smear test centres were not meeting standards. To tackle the problems they said that in future all laboratories were to be accredited and that staff would be obliged to take regular refresher training on a regular basis. An action team, with representatives from leading cancer charities as well as members of the medical profession, was set up to monitor progress.

The whole business forcibly illustrated the undeniable truth that screening is only as good as the people doing it. It is not a perfect science. And smear testing also provides a good example of the way it is necessary to weigh the cost of screening against the number of lives you can save.

If a woman is screened every five years between the ages of 20 and 64, her risk of developing invasive cervical cancer is reduced by 84 per cent. If she is screened annually, the risk is reduced to 93 per cent. However, instead of having nine smears in her lifetime, she will have had 45.

"For the individual woman, clearly having an annual smear test is preferable," says Dr Szarewski. "But in a national screening programme which has to balance costs and benefits, the costs would clearly outweigh the relatively few extra lives saved."

Far more women die from breast cancer than from cervical cancer, and breast screening is available on the NHS for women over 50. Screening can improve the prognosis for many women who are found to have cancer. In addition, treatments for early-stage cancers are less radical: a woman is less likely to lose a breast. But there are those who say screening also leads to over-diagnosis of questionable abnormalities, and point out that there is, in any case, much debate about the best way to treat breast cancer.

Many multi-centred trials are under way to test the best combination of treatments for breast cancer.

Kate Law, head of clinical programmes at the Cancer Research Campaign says: "Mammography, as with all screening tools, is not perfect. However, it does pick up 95 per cent of cancer in women over the age of 50 and the Campaign strongly advises all women in this age-group to attend for screening.

"For the future, we would anticipate far more sensitive screening tools, a simple blood test for instance, which would indicate a potential problem long before a lump was detectable."

But while screening may help to reduce the number of deaths from cancer, it has no impact on the numbers of people getting cancer in the first place.

Dr Szarewski says: "If we ever find a vaccine against cervical cancer we will simply vaccinate all teenage girls. Prevention is better than cure - and prevention is better than screening. Screening is what you do when you can't prevent disease."