Mammograms are not as effective at detecting potentially life-threatening breast cancers in thin women, those taking hormone replacement therapy (HRT) and those who have had previous breast surgery as they are in other women, scientists reveal today.

Women over 50 and under 64 - this is soon to be under 70 - are invited for breast cancer screening using an X-ray of the breast called a mammogram every three years. The £52m-a-year programme detected nearly seven cancers for every 1,000 women screened in 2001-2, but some are missed and some suspect lumps turn out not to be cancers.

The Million Women Study, based at the Cancer Research UK epidemiology unit in Oxford, has produced a number of definitive pieces of work on the causes of breast cancer. Today, the deputy director, Emily Banks, and colleagues publish a new analysis in the British Medical Journal, which shows that screening benefits some women more than others.

The researchers took a sample of 122,355 women aged between 50 and 64 who were being screened for breast cancer and looked at the outcome over the following 12 months. In all, 726 were subsequently diagnosed with breast cancer, but 97 of those had been cleared by the mammogram.

Many more - 3,885 - were given a positive result after screening, and recalled for further tests - which, however, found there was no cancer.

Three groups of women were more likely to get wrong results from breast screening: those on hormone replacement therapy; those who had previously had breast surgery for reasons other than cancer; and those who are thin, with a body mass index of under 25. The researchers say mammograms are known to be less reliable in women with dense breast tissue, which is why they are not generally given to women under 50. These three groups of women may all have denser breast tissue.

"Reduced mammographic sensitivity may lessen the benefit conferred by screening. Thus, our results suggest mammography may be less efficient, and possibly less effective at reducing mortality in users of HRT, in women with previous breast surgery and in thin women compared with other women," they write.

But Julietta Patnick, coordinator of the NHS breast screening programme, urged women in the three groups not to cancel mammograms.