Women who understand the risk of over-detection and over-diagnosis associated with mammography screening have lower intentions to have a breast screening test, according to a new study. Over-detection and over-diagnosis refers to the diagnosis and treatment of breast cancer that would not have presented clinically during a woman's lifetime.

Such a diagnosis, and the resulting overtreatment, can harm women physically and emotionally.

"Mammography screening can reduce breast cancer deaths but most women are unaware that inconsequential disease can also be detected by screening, leading to over-diagnosis and overtreatment," says study author, Prof Kirsten McCaffery of the University of Sydney, who says their first of its kind study "underlines the ethical imperative for women to have clear decision support materials so that they can make more informed decisions about whether they want to have a breast screening mammogram."

"A health system centred around the individual should assist decision-making that incorporates a woman's values and preferences, irrespective of whether her eventual choice is to screen or not.

"Therefore, momentum is shifting from uninformative and persuasive approaches to screening communication to clear and balanced information, giving people the opportunity to make informed choices based on their assessment of the trade-offs between potential outcomes."

The study is the first research to assess the effect of including over-detection information in decision support materials on choices of women on the threshold of becoming candidates for mammography screening - women aged 48-50 years.

The women who participated in the study had not had mammography in the past two years and did not have a personal or strong family history of breast cancer.

The randomised control trial of 879 women found that:

Compared to controls, in women provided with decision support containing explanatory and quantitative information about over-detection:

• significantly more women had adequate knowledge about breast cancer screening and made an informed choice whether to be screened;

The intervention decision aid contained evidence-based information about important outcomes of breast screening over 20 years, compared with no screening -- that is, breast cancer mortality reduction, over-detection, and false positives. The control version omitted all content about over-detection but was otherwise identical to the intervention decision aid.

Quantitative evidence included in the decision aids was taken from an updated version of a published model of mammography screening outcomes for women in Australia.

The model incorporates estimates of both over-detection and the reduction in breast cancer mortality from screening, which were derived from a meta-analysis of randomised trial data and adjusted to account for the effect of regularly undergoing screening.

These estimates were applied to current Australian data for incidence and mortality to quantify cumulative outcomes of biennial screening from age 50-69 years versus no screening over this period.

This information was presented in a short booklet for women, combining text and visual formats. The expected frequencies of outcomes were illustrated with icon arrays depicting the absolute numbers affected per 1000 women screened over 20 years from age 50 years.