Canadian breast cancer cases dropped six per cent in 2003, CBC News has learned.

Similarly, researchers in the U.S. announced last week that rates in the U.S. declined over the same period, which they tied to reduced use of hormone replacement therapy, or HRT.

More recent statistics may confirm the downward trend in breast cancer rates, Dr. Steven Narod says.
(CBC)

The decline in Canada is not as large as the seven per cent decline registered in the U.S. between 2002 and 2003. That decline was for estrogen receptor positive breast cancer, in which tumours depend on hormones to grow.

Doctors suspect that women stopping HRT is the only explanation. Use of HRT in Canada dropped significantly after the treatment was linked in 2002 to an increased risk of breast cancer, strokes and heart attacks.

"The fact that we saw it in Canada and the fact that we saw it in the United States makes it a very intriguing question," said Dr. Steven Narod, a breast cancer specialist at Women's College Hospital in Toronto. "But that's not yet conclusive."

Dr. Elaine Jolly says it's important not to rule out other causes for the decline in breast cancer rates.
(CBC)

In 2002, when doctors wrote more than 11 million HRT prescriptions for Canadian women, there were 102 cases of breast cancer per 100,000 women. 

The following year, hormone prescriptions plummeted to 8.5 million, and the breast cancer rate fell to 96 cases — a drop of six per cent.

Hormone replacement therapy is generally prescribed to relieve the symptoms of menopause, such as hot flashes.

Researchers suspect that many times women have small, undetectable tumours and HRT may be fuelling growth of the tumours. When the therapy stops, it is thought that the tumours stop growing large enough to be seen on mammograms.

Seeking confirmation

So far, the cases have declined over one year, and researchers caution they will be looking for national numbers for 2004 and onward to confirm the trend.

More recent provincial data from Alberta, for example, show that the number of cases has stayed the same since the drop in 2003.

Many gynecologists are unconvinced that HRT use could have such a rapid impact on breast cancer rates. Unlike in the U.S., researchers don't yet know if the drop seen in Canada was for hormone-fuelled cancers.

"We have to look at other causations," said Dr. Elaine Jolly of the Women's Health Centre in Ottawa. "Were mammograms stable, did they decrease, did women lose a lot of weight?"

Such population statistics cannot be used to reach definitive conclusions about cause and effect relationships.