TORONTO — The first Canadian study of its kind is adding to a growing body of international evidence suggesting that the use of hormone replacement therapy may raise the risk of breast cancer.
However, some doctors counter that such studies do not prove a cause-and-effect association between taking hormones and the onset of breast cancer, and stress there could be many other factors playing a role in the development of the disease.
The study by the Canadian Cancer Society found there was a significant decrease in the rate of new breast cancers among post-menopausal women between 2002 and 2004 — coinciding with a huge drop in the use of hormone replacement therapy, or HRT.
Many Canadian women stopped taking hormones in 2002 after a massive U.S. clinical trial — the Women’s Health Initiative — suggested the risks of taking HRT outweighed the benefits. That study suggested taking hormones appeared to increase the risk of breast cancer, heart attack, stroke and blood clots in the lungs.
Following release of that data, the proportion of Canadian women taking HRT began falling dramatically. In 2004, just five per cent of women aged 50 to 69 were on the drugs, compared to 13 per cent in 2002.
“The drop in breast cancer incidence was fairly significant,” said lead author Prithwish De, an epidemiologist with the Canadian Cancer Society.
“We saw a 10 per cent drop in the incidence rate of breast cancer from 296 per 100,000 women to about 278 per 100,000.”
At the same time, the rate of mammography did not change and so was not a factor, say the authors, whose report was published online Thursday in the Journal of the National Cancer Institute.
De said the study is the first national analysis of HRT use and breast cancer incidence rates in Canada. It follows a similar U.S. study published in 2007, which found disease rates plunged in 2003, the year after millions of American women stopped taking hormone pills.
“It certainly gives a Canadian perspective to the growing international evidence around the association between breast cancer incidence and HRT,” he said.
“It also supports the Canadian Cancer Society position (that) women should avoid using HRT for any reason other than managing severe menopausal symptoms.”
Yet, the study also turned up an interesting finding that has led some to question the validity of the HRT-breast cancer link.
The decline in breast cancer incidence continued until 2005, dropping to 266 per 100,000 women, after which the annual rate began to rebound — rising to about 279 per 100,000 in 2006 — even though hormone therapy use was virtually unchanged from 2002.
Dr. Jennifer Blake, chief of obstetrics and gynecology at Sunnybrook Health Sciences Centre in Toronto, said after examining the study data that “it’s very hard for me to be convinced.”
“There’s just so many unknowns,” said Blake, who was not involved in the study. “I think we have to be very clear when we look at these studies that we’re not able to make any kind of a causative relationship. All you’re saying is that there’s an observation of an association and you don’t have any way of knowing whether they’re related.”
She said there are many risk factors for breast cancer — including obesity and the age at which a woman has her first period, first child and goes into menopause. As well, the risk of developing the disease increases with age.
The short window between the drops in hormone use and breast cancer incidence also raises a red flag, said Blake, since many studies have shown that it takes about five years from discontinuing hormone therapy for breast cancer risk to return to “baseline.”
Dr. Christine Derzko, an obstetrician and gynecologist at St. Michael’s Hospital in Toronto, pointed out that it takes about 10 years for a breast tumour to develop to detectable levels from the rise of the first cancerous cell.