The breast cancer rate in the U.S. fell a whopping 12 per cent among post-menopausal women in 2003 – immediately after warnings that they should stop taking hormone replacement therapy.

Authors of the new data, to be published in Thursday's edition of the New England Journal of Medicine, say it is the most graphic proof to date of the damage wrought by HRT.

But other professionals have warned against running ahead of the data, calling for close, critical analysis of the purported HRT link.

Peter Ravdin, one of the study authors, said the change in breast cancer rates in the U.S. between 2002 and 2003 was the “the largest single drop ... within a single year I am aware of.”

The lower incidence rate translates into 14,000 fewer cases of breast cancer in the U.S. in that single year.

“Something went right in 2003, and it seems that it was the decrease in the use of hormone therapy,” said Dr. Ravdin, who is based at the department of biostatistics at M.D. Anderson Cancer Center in Houston.

Robert Reid, professor of obstetrics and gynecology at Queens University in Kingston, Ont. was dubious when he read an advance copy of the paper.

Such a dramatic drop was unlikely to be in evidence almost immediately after the HRT backlash in mid-2002 because cancer takes so long to develop, Dr. Reid said.

It is also not yet known if HRT causes cancer, or merely makes existing cancers bigger and therefore detectable, he said.

“It takes five years from a cell mutating before it's detected in mammography, and up to 10 years before you can feel a lump. Is it realistic to think [with] the news coming out in 2002, in less than six months there were less cancers?” he asked.

Combine that with the tendency for women to stop having regular mammograms once they stop taking HRT, and the low rate of HRT use by women aged over 70 -- one of the main ages cited in the study that experience a drop in cancer rate -- and Dr. Reid said there are enough question marks to consider the research “a little premature.”

While Dr. Ravdin and the researchers say they are only inferring that the widespread abandonment of HRT is behind the drop, they say it is the most likely explanation.

It was July 2002 when the blockbuster Women's Health Initiative study revealed that HRT, a drug treatment used by millions of women to treat symptoms of menopause, actually increased the risk of breast cancer and cardiovascular disease.

Until that time, HRT had been widely touted as a way to stave off heart disease in older women, in addition to lessening the symptoms of menopause.

As a result of the research and the widespread media coverage it garnered, women abandoned HRT in droves.

About one-third of women over the age of 50 were actively taking HRT, and more than half had taken the drugs at some point, making them among the biggest sellers in North America.

Prescriptions for HRT plummeted 38 per cent in the year after publication of the WHI and they continue to fall.

The new research notes that incidence of breast cancer fell most precipitously, 11.8 per cent, in women aged 50-69, those most likely to take HRT. In the 70 and over age group, the rate went down 11.1 per cent.

But in women under the age of 50, where HRT use was probably not a factor, the breast cancer rate rose slightly, by 1.3 per cent.

The new research shows too that almost all the decline was accounted for by a drop in estrogen-receptor-positive tumours – those that would be most directly affected by use of HRT.

The immediacy of the drop was of interest, according to the American Society for Reproductive Medicine, but they too were left with unanswered questions.

Although HRT use kept decreasing by an extra 20 per cent through 2004, the registry data used in the new research paper shows no accompanying decline in breast cancer incidence for 2004.

“This raises questions about causality and, as the authors themselves note, other factors including nutrition and a decline in mammography screening could contribute to the observed decreases,” the Society stated.

HRT entails taking substitutes for estrogen and progesterone after natural levels of these key female hormones diminish after menopause.

In estrogen-receptor-positive tumours, HRT would fuel the growth of cancer. Stopping the treatment would effectively starve a tumour, leaving it so small as to undetectable in a mammogram, according to the paper.

Heather Logan, director, cancer control policy at the Canadian Cancer Society, said the research is “very suggestive” of a link between a drop in HRT use and a decline in breast cancer incidence.

She said it is a “reasonable assumption” that the same trend occurred in Canada, but the data has not been analyzed in this manner. “It's a question we should be thinking about answering in Canada,” Ms. Logan said.

She noted that the CCS was one of the first groups to warn women about the risks of long-term use of HRT. “This study adds further evidence that we need to be really careful about our use of non-essential hormones,” Ms. Logan said.

Dr. Ravdin said the long-term impact on breast cancer rates remains unclear. The change in “hormonal milieu” from the discontinuance of HRT may slow the growth slightly or temporarily, and that means breast cancer rates should stabilize, then rise again. The paper notes that after the 2003 drop, rates have stabilized again.

When results of the Women's Health Initiative, a long-term study of 16,600 women aged 50 to 79, were published in mid-2002, researchers concluded that long-term use of HRT presented more potential dangers than benefits to overall health.

The study showed that HRT increased the risk of breast cancer by 26 per cent, stroke by 41 per cent, heart attacks by 29 per cent, the risk of cardiovascular disease by 22 per cent and that the rate of blood clots doubled. On the plus side, HRT diminished the risk of osteoporosis and hip fractures, and was effective in treating menopausal symptoms like hot flashes and vaginal dryness.

Yet, discussion and debate about the merits of HRT continues to this day, as large numbers of women continue to take the drugs, though largely on a short-term basis now.

Just last week, new research suggested that HRT does not increase the risk of heart disease in women in their 50s, that the real dangers come only with prolonged use.