What is breast screening?

The first step in breast screening is a mammogram - a low dose X-ray of each breast. This can detect small changes in breast tissue that may indicate cancers too small to be felt either by the woman herself or by a doctor.

Each breast is placed in turn on the X-ray machine and gently but firmly compressed with a clear plate. Compression is needed to keep the breast still and to get the clearest picture with the lowest amount of radiation possible.

Some women find this slightly uncomfortable and some feel short-lived pain. Research has shown that for most women it is less painful than having a blood test and compares with having blood pressure measured.

At a woman's first screening appointment, two views of the breast are taken - one from above (Craniocaudal) and one into the armpit diagonally across the breast (mediolateral). At subsequent screenings, only a mediolateral view is taken.

What happens next?

About 95% of women get the all clear after the first mammogram and will be routinely invited for screening three years later.

Of those recalled for further investigation, only about one in 10 will be found to have cancer.

Around 30,000 women are diagnosed with the disease in England and Wales each year. With the first round of screening completed, registrations of breast cancer have fallen by 4% between 1991 and 1992.

The latest survival figures for England and Wales show an average 68% of women diagnosed with breast cancer between 1986 and 1990 were alive five years later.

The national screening programme is now on course to save 1,250 lives each year.

Should women expect accuracy all the time?

The Advisory Committee on Breast Cancer Screening is currently preparing information for women to allow informed consent when participating in the breast screening programme.

This will include clear information for women about the possibility of false negative results - when someone is given the all clear but a cancer later emerges.

Research has been commissioned into women's expectations and understanding of the NHS Breast Screening Programme, and the results are still to follow.

Preliminary investigations indicate that the majority of women are unaware that screening can miss cancers.

The NHS Breast Screening Programme acknowledges the rates of sensitivity quoted in the Journal of Epidiemiology and Community Health study - 80% to 94% - are an accurate reflection of the reliability of screening.

What about between screenings?

According to the Institute of Cancer Research, there is an expected rate of interval cancers - cancers occurring between screening sessions - of 12 per 10,000 over two years which then can rise to 13 per 10,000 in the third year.

Interval cancers present in three types:

• True interval cancer - one that occurs between screening
• "Occult" cancer - a "hidden" cancer that cannot be detected by mammogram
• False negative - a cancer that was not detected by mammogram in the first screen

How can false results be avoided?

The NHS Screening Programme constantly works to minimise the false negative results.

We are currently assessing whether Computer Aided Diagnosis (CAD) will help radiologists, and have put together a working party of experts to assess the evidence.

The process may be used as a "sifter" to weed out the normal films automatically, allowing radiologists to concentrate on the abnormal.