What we do know with relative certainty is that estrogen does not cause breast cancer de novo, but it may cause an existing breast cancer to grow.
We also know that breast cancer risk increases with age: 8 out of 10 breast cancers are in women over 50, whether or not they take estrogen. A woman's breast cancer risk is also based upon her lifelong exposure to her own hormones; the longer she produces estrogen, the more likely she is to develop breast cancer. Women are at lower risk for breast cancer if they got their periods late, have not become overweight, or go through menopause early. We know that women who are on hormone replacement therapy (HRT) are diagnosed with breast cancer more often than women not on HRT, but we also know that women on HRT are much more likely than women not on HRT to have mammograms and annual clinical breast exams.
Over the last 25 years, more than 50 studies have evaluated hormone therapy and breast cancer. According to the National Cancer Institute, these studies varied widely in design and have inconsistent results. While there is one major study of 80,000 nurses that shows that patients who took estrogen had a slightly increased risk of breast cancer, other studies contradict that finding. The most encouraging finding may be one from a study of 42,000 women in Iowa concluding that there was no difference in the survival rate from breast cancer in women on estrogen compared to those on placebo. This study also concluded that a woman who took estrogen and got breast cancer had an increased life expectancy of 2-3 years over a matched control who did not take estrogen and did not get breast cancer. What this suggests is that women on HRT with breast cancer are probably being diagnosed very early. . .and that their risk of dying of other causes is probably reduced. This also suggests that menopausal women on HRT are probably healthier to begin with as well as less likely to smoke.
The cancer biology team from UQ's Diamantina Institute for Cancer, Immunology and Metabolic Medicine, believe their finding will help explain the link between breast cancer and high levels of estrogen.
"What we've shown is that the ability of estrogen to switch this gene on is important for the growth of breast cancer cells," Diamantina cancer biology research leader Professor Tom Gonda said.
The gene they studied, known as MYB, is found in about 70 percent of all breast cancers and is one of several dozen genes called oncogenes that promote cancer growth.
"What's important in breast cancer is the ability of estrogen to turn on MYB rather than there being a mutation in the gene itself," Professor Gonda said.
He said the next step was to take the results, which come from isolated cancer cells grown in the laboratory, and test them in laboratory mice that are a better model for human patients.
"We're trying to show directly that MYB can induce cancerous changes in normal breast cells."
Professor Gonda and his colleagues at UQ worked with researchers in Melbourne, Adelaide and the United States and published their findings in August in the journal Proceedings of the National Academy of Sciences.
He said a drug that blocks the action of MYB might be used to treat breast cancer in the future but he warned that would take many years of hard work.
Note: This story has been adapted from a news release issued by University Of Queensland
Posted on: September 12, 2007, 03:51:31 PM
Like the WHI study, the results of the NHS found no statistically significant increased breast cancer risk when estrogen therapy was used for less than 10 years, Wendy Chen, M.D., and researchers at Harvard here wrote.
However, they said, the longer a woman used estrogen, the greater her risk of breast cancer. For those who had taken estrogen for more than 20 years, the increased risk was highly significant.
The NHS, a prospective cohort study, with a 1980 baseline of postmenopausal women who had had a hysterectomy, eventually included 28,835 women in the 2000-2002 follow-up period.
A total of 934 invasive breast cancers were included in the analysis, with breast cancer risk increasing with duration of unopposed estrogen among longer-term users and especially in tumors that were estrogen-receptor positive and progesterone-receptor positive, the researchers reported.
Analysis of relative risks over 20 years found the risk began to increase after 10 years, so that by 20 years, it was more than 40% greater.
The multivariate relative risks (RRs) and 95% confidence intervals (CIs) for breast cancer with current use of unopposed estrogen were as follows (P for trend .001):
Less than five years, RR 0.96 (95% CI, 0.75-1.22);
Five to 9.9 years, RR 0.90 (95% CI, 0.73-1.12);
10 to 14.9 years, RR 1.06 (95% CI, 0.87-1.30);
15 to 19.9 years RR 1.18 (95% CI, 0.95-1.48);
20 years or longer RR 1.42 (95% CI, 1.13-1.77).
