Mammography rates were steady in Canada during the study period. (Becky Shink/Lansing State Journal/Associated Press)

A Canadian study confirms international declines in new breast cancer cases among post-menopausal women when the use of hormone replacement therapy fell sharply.

Many Canadian women stopped hormone replacement therapy, or HRT, in 2002 after a large U.S. clinical trial suggested the breast cancer, heart and stroke risks of taking HRT outweighed the benefits.

In Thursday's issue of the Journal of the National Cancer Institute, researchers from the Canadian Cancer Society said that the number of new breast cancers in about 1,200 Canadian women aged 50 to 69 fell by almost 10 per cent between 2002 and 2004.

"During the period 2002-2004, there was a link between the declines in the use of hormone replacement therapy and breast cancer incidence among Canadian women aged 50-69 years, in the absence of any change in mammography rates," the study's authors concluded.

The findings confirm what studies in other countries such as the U.S. have already showed: when women stop using the therapy, the rate of breast cancer falls.

"The fact that incidence has declined so quickly with the drop in HRT use goes back to the fact that we're not entirely sure about how HRT works on breast cancer incidence," said Dr. Prithwish De, the study's lead investigator and an epidemiologist at the Canadian Cancer Society.

"Its not necessarily a one-to-one relationship as we might assume."

The Canadian study also showed that the drop in breast cancer incidence only lasted a few years before it started to go up again, which suggests HRT might fuel existing cancers, said Dr. Steven Narod of Women's College Research Institute in Toronto.

Growth encouraged

"The suggestion is the HRT is not causing the breast cancer but causing it to grow more quickly once it's established," Narod said.

More followup data is needed to test whether HRT acts as a promoter rather than a cause of breast cancer, the researchers said.

Specialists like Narod and De suggest the therapy should only be used for severe causes of menopausal symptoms.

The Society of Obstetricians and Gynecologists of Canada advises that combined HRT continues to be the best option to treat moderate to severe menopausal symptoms, including hot flashes, night sweats and mood changes. The drugs should be prescribed at the lowest effective dosage for "the appropriate duration," the group's guidelines say.

Rosslyn Yeo, 67, of Bowmanville, Ont., took HRT from 1993 to 2002 when she was in her 50s.

"I did suffer many, many months without sleep, but I'm very happy to say that I think I'm healthier now not being on that product," said Yeo, who now uses an alternative therapy.