Estrogen and Cancer Risk

Were you surprised by the front page headline of USA Today this week (12/8/00) announcing that "Estrogen may join carcinogen list"?

Does this reflect any new information about the safety of estrogen for women who take birth control pills or who take hormone replacement therapy?  In a word, "NO".  We have long known about estrogen's ability to increase a woman's risk of cancer of the lining of the womb unless she takes progesterone as well; information about estrogen's impact on breast cancer is more controversial.

What is this list?

The "list" is actually the federal "Report on Carcinogens" which will come out in 2002.  Published every other year since 1978 by the National Toxicology Program, it is simply a list of substances which may cause any type of cancer in humans.  It does not include any risk/benefit assessment or suggest that these substances are of danger to all people.  The list is mainly used as a reference tool.

What do we know about estrogen and cancer?

First, estrogen has long been known to cause endometrial hyperplasia, or an overgrowth of the lining of the womb, which may develop into cancer.  Women who have had their uterus removed--who have had a hysterectomy--are at no increased risk of this problem.  For women who do have their uterus in place, and want to take estrogen, we know that giving progesterone simultaneously, can prevent the risks of endometrial hyperplasia.  This issue is not controversial.

What is controversial is the association--if any--between estrogen and breast cancer.  Numerous studies have been done to evaluate this issue and the results are conflicting, in both women taking estrogen in the form of menopausal hormone replacement therapy and in oral contraceptives. 

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.

What about birth control pills and cancer risk? 

Studies done many years ago on birth control pills (BCP's) and breast cancer showed an increased risk of breast cancer--with pills that contained much higher doses of estrogen.  Studies of BCP's using today's average doses show no increased risk of breast cancer.  In addition, several studies have shown that BCP usage for more than 5 years can decrease a woman's risk of ovarian cancer, endometrial cancer and colon cancer.

Created: 12/11/2000  -  Donnica Moore, M.D.