Breast cancer screening is less effective in thinner women, researchers have found.

Breast cancer screening is less effective in thinner women, researchers have found.

Cancers were harder to detect in the breasts of lower-weight women who were also more likely to develop cancers between screenings, according to a study.

The reason is thought to be that thin women have denser breasts, making it harder to distinguish cancerous lumps. For this reason, mass breast screening of women under 50 is not recommended in the UK.

However, it has not previously been suggested that thin women had denser breasts, making breast screening more difficult in this group.

The density of the breast is determined by the amount of connective tissue which shows up black on a mammogram. Heavier women have more fat in their breasts which is translucent, making it easier to spot cancerous lumps.

The authors of the study, from the Cancer Research UK Epidemiology Unit at Radcliffe Infirmary, Oxford, stressed yesterday that breast screening was still worthwhile for thin women.

"It is not ineffective in this group, it is just less effective," said Emily Banks, the deputy director of the unit and the leader of the research team.

Thin women were at lower risk of breast cancer than fatter women, Dr Banks added. "Even if screening detects fewer cancers in thinner women, the overall protective effect in the two groups of women may not be that different," she said.

The study also found that screening was less effective in women on hormone replacement therapy (HRT) and in those who had had previous breast surgery to remove benign (non-cancerous) lumps or cysts.

The finding on HRT will add to concern about the risks of the therapy. The new research suggests that HRT makes the cancer harder to detect.

The study, published in the British Medical Journal, questioned 122,000 women aged 50 to 64 about personal factors before routine breast screening. The women, who were part of the Million Women study into the effects of HRT which reported a year ago, were then monitored for 12 months to examine how the factors influenced the accuracy of screening.

Three factors - use of HRT, previous breast surgery and a body mass index below 25 - that is, normal weight - were found to increase the chances of breast cancer being diagnosed between the three-yearly NHS screenings (suggesting that it had been missed at screening).

Women with these factors were also more likely to be recalled for further tests following screening, without having breast cancer.

The seven other factors examined, including age, family history of breast cancer, physical activity, smoking and alcohol consumption, had no significant effect.

Gillian Reeves, an epidemiologist and the co-author of the study, said: "These results highlight how important it is that women remain breast aware between the three-yearly NHS screening intervals."