Why Breast Cancer Treatments Might Work Best Just After Your Period

Breast cancer can be influenced and act differently depending on where you are in your menstrual cycle. Importantly, new data released by Nature and completed by Bornes et al. has shown that breast cancer cells become more susceptible to chemotherapy during the oestrous phase, which is when levels of estrogen in your body are at their lowest.

Right now, a small study has shown that the same effect could be seen in humans, but more research still needs to be done. Scientists hope to prove that breast cancer tumors can be treated more effectively during the oestrous phase and when progesterone levels are lowered, aiming to improve the effects of chemotherapy drugs.

These results are not completely unexpected. A study from 2020 identified that synthetic progestogens could be linked to an increase in breast cancer risk. And research out of The University of Kansas Cancer Center showed that seven out of 10 breast cancer cases are hormone receptor positive breast cancer, showing that hormones and breast cancer have close ties.

While progesterone and estrogen do play an important role in a woman’s menstrual cycle and overall health over time, it is known that many breast cancers are estrogen receptor-positive and will respond to excess estrogen in the body. Researchers believe that excess estrogen exposure can increase the risk of breast cancer caused by estrogen (or, at least, estrogen-dependent cancers that grow more rapidly with its presence).

According to Jake Lee, medical oncology fellow at Memorial Sloan Kettering Cancer Center,

“Our work demonstrates that estrogen can directly induce genomic rearrangements that lead to cancer, so its role in breast cancer development is both that of a catalyst and a cause.”

Understanding the impact estrogen can have on breast cancer, this information just makes sense when considering when to deliver treatments for the best results. Future studies will clarify if women and their healthcare providers can expect better chemotherapy results during the oestrous phase, which is when estrogen levels are at their lowest.

Breast Cancer and Your Hormones: FAQs

How Does the Menstrual Cycle Affect Breast Cancer?

It’s known that estrogen and progesterone can have an impact on cancer cells. Fluctuations in these hormones, as well as the number of menstrual cycles you have, can influence your risk of breast cancer. If cancer does develop, it’s known that around 70% are estrogen receptor-positive and may grow when exposed to estrogen.

What Hormones Are Responsible for Breast Cancer?

The hormones estrogen and progesterone are believed to be responsible, in part, for the development of some breast cancers. These hormones, and particularly the fluctuation of these hormones over time, may play a role in cellular changes that lead to developing cancer cells.

Estrogen, as a hormone, is crucial in the development of breast tissue, while progesterone regulates breast development. With hormone receptive positive breast cancer, hormonal imbalances between progesterone and estrogen may lead to greater tumor growth.

What Is the Role of Estrogen in Breast Cancer?

According to Christy Hagan, PhD and associate professor of biochemistry and molecular biology at The University of Kansas Medical Center, “I am often asked why I study breast cancer and its relationship to hormones, when most people who develop breast cancer are postmenopausal and have low hormone levels. In fact, the large majority of breast cancers in postmenopausal women are powered by hormones.” A study involving Hagan shows that estrogen is the cause of the cellular changes that may lead to estrogen-dependent cancer.

Does Not Having a Period Increase Cancer Risk?

Not having a period, developing polycystic ovary syndrome, or having endometriosis, for example, are all signs of hormonal irregularities. Those hormonal irregularities can lead to problems with the menstrual cycle.

Irregular menstrual cycles are considered any cycles that are shorter than 21 days apart or over 35 days apart. It’s believed that around 1 in 4 women have irregular cycles. While amenorrhea (not having a period) isn’t directly linked to breast cancer, it is important to have irregular menstrual cycles diagnosed and treated. Research has found that the number of menstrual cycles you have in your lifetime can impact your risk of developing breast cancer. And, if your cycles are irregular, those hormonal irregularities could lead to other kinds of cancers, such as endometrial cancer.

Will Reducing Estrogen Stop Breast Cancer?

It is important to note that some breast cancers thrive on the presence of estrogen. Some women may reduce their risk of estrogen receptor positive breast cancers by taking drugs such as tamoxifen, which is a selective estrogen receptor modulator, or SERM. But, what does tamoxifen do to your body? Simply, this medication binds to estrogen receptors to prevent it from encouraging cancer cell growth. Understanding your current risk of breast cancer or the type of breast cancer you have can let you know if managing your hormones will have an impact on its growth or remission.

Is There a Link Between IUDs and Breast Cancer?

There is, but not with all types of IUDs. Hormonal IUDs may increase the risk of developing breast cancer, but only when the IUD contains levonorgestrel. Research has determined that women who use these IUDs have a 40% increased risk of breast cancer, taking the risk from 1 in 204 to around 1.4 in 204 over a seven-year research period. There are alternatives to this kind of IUD, which is why it’s important to discuss your risk of breast cancer before selecting a form of birth control.

This article has been reviewed by Dr. Charles Weaver, Medical Advisor for Learn Look Locate and Medical Oncologist.

Learn Look Locate is proud to present expert insights from Dr. Charles Weaver, founder of Cancer Connect and a key contributor to our mission at Learn Look Locate. Dr. Weaver’s expertise is an integral part of our joint newsletter with Cancer Connect, which has been empowering breast cancer patients for over a decade. The newsletter is distributed monthly to over 30,000 women who have opted in and will support them from diagnosis through treatment and survivorship. You can sign up for the newsletter by scrolling to the top of the page and submitting your email.

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