Estrogen and its Effects on Breast Cancer


I recently returned from the Annual Meeting of the American Society for Reproductive Medicine where I had the opportunity to interview Richard Santen, one of the leading authorities on estrogen and breast cancer. Today I’d like to share with you information on estrogen’s impact on breast cancer.

When asked to talk about whether or not estrogen causes breast cancer, Santen’s answer was not black and white: it depends. “We didn’t realize until several years ago that it takes up to 20 years for a breast tumor to grow from one cell to a cluster of cells that are big enough to find on a mammogram. If that’s the case, and we are giving women menopausal hormonal therapy, its possible that therapy would actually cause those occult tumors to grow more rapidly and therefore be diagnosed earlier on a mammogram. So, we think that a lot of the studies that have been published is really an effect of estrogen to cause preexisting tumors, too small to be seen, to grow more rapidly and then to be diagnosed.”

Santen then went on to explain that based on a model they have developed, it is estimated that only 6 percent of the tumors that were seen in the Women’s Health Initiative study were brand new tumors; 94 percent of the tumors were already there at the time the study was started, but they were too small to be seen.

Estrogen also changes the way the breasts look, often making it more challenging to see a breast tumor. Santen remarked, “We know as women get older, the amount of density of the breast decreases and it’s a lot easier to see a cancer in a breast that has little density. Hormone therapy in menopausal women increases breast density to some extent and it masks the ability to make the diagnosis. So this is one of the complexities when you give hormone therapy to menopausal women.”

Another interesting topic that Santen spoke of during the conference was the use of tamoxifen, which is sort of an anti-estrogen that may not be preventing cancer but rather slowing down the growth of preexisting tumors. Santen said, “If you think of an anti-estrogen influencing an occult tumor, and it causes that tumor to grow more slowly, or even makes it smaller there would be fewer women diagnosed with breast cancer after the five years of being on tamoxifen. Really what is happening is that tamoxifen is just altering those preexisting tumors, but not actually causing prevention of breast cancer.”

The study of breast cancer is an ever-evolving field. Richard Santon’s new studies have opened up different thought processes around estrogen and its effects on breast cancer. What are your concerns? Have you experienced taking tamoxifen? Please share below in the comments section.

Until next time,

Dr. Mache Seibel, Professor, University of Massachusetts Medical School

And Founder of My Menopause Magazine

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Professor, University of Massachusetts Medical School

(617) 916-1880


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