Published this month in the journal Cancer Epidemiology Biomarkers and Prevention, the five year study of 1557 women with breast cancer looked at the odds of having a false negative result on mammogram in women who attended first round screening in 1994 and 1995 or second round screening in 1995 and 1996.
Chief investigator, Associate Professor Anne Kavanagh from the University's Key Centre for Women's Health in Society said that 24 per cent (371) of the 1557 women included in the study were taking HRT and had reduced mammographic sensitivity.
"This means it is easier to miss cancers in women taking HRT and we calculate that if these women had the same level of sensitivity as women not on HRT an additional 76 cancers would have been found," she says.
The study confirms the results of previous work by the same researchers who have found that higher breast density is associated with an increased risk of breast cancer.
Breast density - the glandular tissue in the breast that appears white on mammogram - is higher in some women taking HRT, which makes it more difficult to find breast cancer.
Associate Professor Kavanagh says, "Higher breast density is part of the reason why tumours are not detected in these women but we believe there are other factors also coming into play and at the moment we don't know what they are."
"Despite these issues, we would still urge women on HRT to continue to have a mammogram every two years as it remains the most effective screening tool for breast cancer."
"Most importantly, women should receive information about the effects of HRT on mammography before they make decisions about whether or not to start on HRT."
Chief Executive Officer of BreastScreen Victoria, Ms Onella Stagoll, called on all women over 50, and particularly those on HRT, to make regular two-yearly screening a priority.
"A BreastScreen mammogram every two years is the best way to find cancer early," said Ms Stagoll. "Finding breast cancer at an early stage offers women the best chance of successful treatment and recovery."
For more information about BreastScreen services or to make an appointment anywhere in Australia call 13 20 50.