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FRIDAY, Aug. 27 (HealthDayNews) -- A mammogram may be less effective if a woman is on hormone replacement therapy (HRT), a new British study finds.

Mammography's ability to detect and exclude breast cancer may also be less effective for thinner women and those who have had previous breast surgery, according to the report in the Aug. 28 issue of the British Medical Journal.

It was not entirely clear why the three characteristics picked up by the study make the screening less efficient, the researchers said. But what they all have in common is that they make body tissue appear more dense to the X-rays used to examine the breast, "which is a plausible explanation for our findings," the researchers added.

"I wouldn't say these results have any immediate implications for the way we screen women," said Gillian Reeves, a statistical epidemiologist at the Cancer Research UK Epidemiology Unit and lead author of the paper. "The essential advice to women remains the same."

Current American guidelines recommend mammography every year for women over 40.

The effect of HRT on mammography results have been reported in other studies, said Dr. Emily Banks, who was deputy director of the epidemiology unit when the study was done and now is a fellow at the Australian National University.

"To our knowledge, the findings of adverse effect of lower body weight and previous surgery on both sensitivity and specificity together has not been reported before," Banks said.

The findings come from the Million Women Study, a research review of women's health involving approximately 1 million British women aged 50 and over. Last year, the study found a link between HRT and increased risk of breast cancer, a finding that confirmed earlier conclusions reached in the U.S.-based Women's Health Initiative.

The new study included more than 122,000 women ages 50 to 64 who were monitored for a year after mammography to see whether 10 personal characteristics affected the test results.

No effect was found for seven factors: age, family history of breast cancer, physical activity, smoking and/or alcohol consumption, past use of oral contraceptives, whether a woman had given birth, or whether she had tubal ligation to prevent pregnancy.

But there was reduced sensitivity -- the ability to detect cancer -- and specificity -- the ability to rule it out -- for women who had used or were using HRT, those who had had noncancerous breast surgery and those with a BMI under 25.

"We didn't have any specific expectations about the effects of the different factors," Banks said. The study was done, she added, "to gather comprehensive information prior to screening on factors likely to affect breast cancer risk and breast cancer screening, particularly factors related to menopause, reproductive history, and exposure to hormones."

The results do not necessarily rule out age as an influence, Banks added. "This is a relatively narrow age range, and we cannot exclude the possibility of an effect of age outside this age group," she said.

The study results reinforce existing advice, Reeves said, that "it is important to have screening, and that it is important for women to be breast-aware."

But, Banks added, the effect on mammography "should be considered alongside the other risks and benefits of HRT when women are considering whether or not to use HRT."

However, she said, "at present there is no evidence that these women should attend screening more than other women."

More information

To learn more about mammography, visit the National Library of Medicine.

(SOURCES: Gillian Reeves, Ph.D, statistical epidemiologist, Cancer Research UK Epidemiology Unit, Oxford, England; Emily Banks, M.D., fellow, Australian National University, Melbourne; Aug. 28, 2004, British Medical Journal)

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