Canada has become the latest country to report a decline in the incidence of breast cancer in the years right after 2002 — and to link that decline to a concurrent plunge in the popularity of postmenopausal hormone therapy (HT).
For 2002 was the year that women all over the world began fleeing HT in droves. The turning point came when the Women’s Health Initiative (WHI), a clinical trial involving more than 16,000 women, was halted early (a rare occurrence) because it found — to the great surprise of its investigators — that women in the HT arms of the study were at a greater risk of breast cancer, stroke, heart attacks, blood clots and urinary incontinence than those taking a placebo. (Later in-depth looks at the WHI data also linked HT to an increased risk of dementia.)
In the year before WHI was halted (2001), American doctors wrote some 61 million prescriptions for HT. But by 2004, that number had plummeted to 21 million. Over that same period, the annual incidence of breast cancer in the U.S. fell by 8.6 percent. Researchers have subsequently found similar drops in other countries, including Sweden, the Netherlands and Switzerland.
And now, Canada. Last week, researchers with the Canadian Cancer Society reported that between the beginning of 2002 and the end of 2004, their country experienced a 9.6 percent decline in the incidence of breast cancer among women aged 50 to 69 years (the age group most likely to be taking the hormones).
That drop coincided with the two years that saw the largest fall in HT use among Canadian women. Before the WHI clinical trial was halted, 12.7 percent of Canadian women aged 50 to 69 used HT. By 2004, only 4.9 percent did.
Interestingly, the study also found that breast cancer rates began rising again in 2005. The study’s authors say this “rebound” is further evidence of the link between HT and breast cancer. It’s what you would expect to see, they explain, if HT doesn’t cause hormone-sensitive breast tumors to form, but instead speeds their growth.
About 70 percent of breast cancers are estrogen receptor-positive, which means that estrogen helps them grow.
The study has its limitations. The women self-reported their use of HT, for example, and not all the breast cancer tumors were identified by their receptor status.
Still, these findings offer yet more evidence of an association between menopausal HT and breast cancer.
The study was published online last week in the Journal of the National Cancer Institute.