HEALTH / Common procedures: Mammography

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Screening tests for cancer are based on the concept that the smaller the tumour when it is found, the better the chances of a cure. Breast cancers are notorious for their tendency to spread, but detecting them while they are still small and confined to the breast has proved more difficult than was hoped 30 years ago.

One of the first approaches was to encourage women to learn a technique for systematic self-examination once a month. The results have been disappointing. Large-scale studies have shown that women who examine themselves conscientiously do not gain clear-cut advantages in terms of fewer deaths from breast cancer, and there is some evidence that they become more anxious about their chances of developing it.

Direct screening of the breasts has been done using not only X-rays (mammography) but also heat-sensing equipment (thermography) and ultrasound. The only one of these that has proved effective in detecting small tumours is mammography. The dose of X-rays used is now smaller than in the early research studies, and the risks from the radiation are very small. The test takes only a few minutes, but around 5-7 per cent of women tested are asked to return for further examination. This may involve another mammogram or taking a sample of breast tissue using a needle. On average, one in 10 of those women who are recalled for further tests will be found to have a breast cancer - around six per 1,000 women screened. The cost and inconvenience of the tests which prove to be negative are substantial drawbacks; nine women go through some weeks of anxiety for every one who is found to have an early cancer.

On the other hand, women whose tests are immediately reported as normal are reassured, and the cancers that are detected are mostly small - around one-fifth are less than 1cm in diameter. These very small cancers can be treated by simple surgery, and chemotherapy is rarely recommended.

The gains in terms of lives saved are substantial. Women who have mammography every two years reduce their risk of dying from breast cancer by around 30 per cent.

Mammography still has its problems and its critics. The most contentious aspect is the age at which screening should start. Most of the research studies have involved women aged between 50 and 65. When younger women have been examined, the results have not shown any real evidence of benefit, but often it is younger women who are most keen to be screened.

In the United States the recommended programme is for screening to begin at the age of 40; the test is then done every two years until the age of 50 and every year after that. In Europe the consensus of expert opinion is that mammography is of unproved value for women under 50 (younger women have a lower risk of cancer and their breasts are denser, making small tumours more difficult to detect). On frequency the evidence is more helpful; most of the research studies have been on women screened every two years, and this is the usual interval in screening programmes in northern Europe. The risk of breast cancer rises with age, and

probably there should be no upper age limit for screening.