For the new study, the authors looked at 16,608 women who participated in the WHI from 1993 to 1998. The women were randomly assigned to receive combined hormone replacement therapy (estrogen plus progesterone) or a placebo.
Mammograms and breast exams were conducted annually and biopsies performed, if indicated.
More than one in 10 women had otherwise avoidable mammogram abnormalities (an increase of 11 percent), while one out of 25 women had otherwise avoidable breast biopsies (an increase of 4 percent), after taking the hormone therapy for five years.
Ten percent of women in the HRT group had to have a biopsy, compared to 6.1 percent in the placebo group. Yet the biopsies only detected 14.8 percent of cancers in the HRT group, compared with 19.6 percent in the placebo group.
"Your breasts become denser [with HRT], and we all know that mammography isn't as sensitive for the detection of breast cancer in women with dense breasts," Byrne explained.
The increase in abnormal mammograms persisted for at least 12 months even after discontinuing hormone therapy, the study found.
For the medical community, Chlebowski said, this finding "focuses attention that diagnosis is hindered. We have additional imaging modalities, and maybe we should evaluate them to see if we can get rid of this hindrance or delay in diagnosis. It hasn't been a factor for attention before, but it probably should be."
Chlebowski has consulted for several pharmaceutical companies.
A prepared statement from Wyeth Pharmaceuticals, which makes the hormonal product Prempro, said: "While the [study] authors report a link between an increase in abnormal mammograms and breast density among women taking combined estrogen plus progestin, this does not mean they are at an increased risk for breast cancer.
"The data used in this sub-analysis were taken from the combined estrogen plus progestin
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