WEDNESDAY, Feb. 4 (HealthDay News) -- A new analysis shows that the drop in breast cancer cases that began in 2003 is indeed due to women stopping hormone replacement therapy (HRT) after a large, U.S. study showed surprisingly higher rates of heart problems and breast cancer among users of some kinds of HRT.

Some experts have suggested such a theory explains the trend, while others have said it might be related to changes in mammography use.

"If you stop hormones, the risk of breast cancer [associated with hormone use] rapidly declines," said Dr. Rowan Chlebowski, a medical oncologist at Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center and lead author of the analysis in the Feb. 5 issue of the New England Journal of Medicine.

Based on that analysis and another recent review, the researchers also suggested that the "safe" period for combined use of progestin and estrogen to relieve postmenopausal symptoms was probably about two years, not the approximately five years researchers have discussed previously.

To get to the root of the drop in breast cancer cases, Chlebowski and his colleagues looked at data from the Women's Health Initiative (WHI) clinical trial (more than 15,000 women were assigned to either a placebo or HRT) and an observational study in which more than 25,000 women were on HRT, and more than 16,000 were not.

The part of the WHI that looked at combined hormone therapy was halted in 2002, as soon as researchers saw surprisingly higher rates of heart problems and breast cancer in women assigned to take the combined HRT (but not estrogen alone).

When Chlebowski's group looked in more detail and at shorter intervals for the effects of HRT, they found that during the initial two years, the HRT group participating in the clinical trial had fewer breast cancer diagnoses than the placebo group. However, the number of breast cancer cases in that group increased over the 5.6-year study period. The risk decreased rapidly in both groups after they stopped the pills, even though both groups had mammograms with similar frequency.

That fact weakens the argument that the drop in breast cancer cases was due to fewer women getting mammograms, he noted.

"There was a rapid decline in breast cancer incidence after stopping hormones, while mammography use didn't change between the groups," Chlebowski explained.