Scientists have found evidence that the reduced effectiveness of breast screening in women in their 40s is mainly due to their tumours being harder to detect, rather than because they grow faster.

Previous research has suggested that breast screening is less effective in women in their 40s than in older women.

Two reasons have been identified for this - firstly, that younger women tend to have denser breast tissue which makes it harder to detect tumours on a mammogram; and secondly, because younger women's tumours tend to grow more quickly.

A research team at the Stanford University School of Medicine set out to discover which of these two factors has the greatest effect on screening outcomes.

They used a computer-simulated model - called the Breast Cancer Screening Simulator - to estimate the relative effect of the two factors on mammograms of women in their 40s, 50s and 60s.

They estimated the average tumour size detectable on a mammogram and the average tumour growth rate in women from different age groups and used this model to quantify the effect of each of these factors on the effectiveness of mammography.

Writing in the Journal of the National Cancer Institute, they concluded that lower tumour detectability accounted for 79 per cent of the reduced sensitivity of breast screening in younger women, while faster tumour growth accounted for just 21 per cent.

Dr Sylvia Plevritis and her colleagues observed: "The age-specific differences in mammographic tumour detection contribute more than age-specific differences in tumour growth rates to the lowered performance of mammography screening in younger women."

They added: "More research is needed to not only establish a better relationship between mammographic breast density and breast cancer risk, but also to understand the differences in tumour characteristics in dense versus non-dense breast tissue."

Hazel Nunn, senior health information manager at Cancer Research UK, said: "This study helps to explain how different factors contribute to breast screening being less effective in women under 50. It suggests that younger women's denser breasts are the main reason that screening isn't beneficial to women under 50 rather than how quickly their tumours grow."