Breast tumors are tested to see if they are estrogen receptor (ER) and/or progesterone receptor (PR) positive or negative. Hormone receptor tests are both prognostic and predictive. In general, tumors that are ER+ and/or PR+ are slightly slower growing and have a slightly better prognosis than tumors that aren’t. Hormone receptors also provide information about treatment options. If your tumor is ER+ and/or PR+, then your cancer can be treated with a hormone therapy. For this reason, these tumors are also sometimes referred to as “hormone sensitive.”
Hormone therapies slow or stop cancer's growth by changing the hormonal milieu. For early stage cancer, these treatments include tamoxifen and a class of drugs called aromatase inhibitors or AIs. Currently three aromatase inhibitors are approved for use by the U.S. Food and Drug Administration (FDA): anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin). Studies suggest that all three are equally effective. Women with metastatic breast cancer also have other hormone therapy options, including fulvesrant (Faslodex), megestrol acetate (Megace), and tormifene (Fareston).
Tamoxifen is an oral treatment. It is a selective estrogen receptor modulator (SERM), meaning it blocks estrogen from getting into breast cells. But although tamoxifen is anti-estrogenic in the breast, it is estrogenic in others parts of the body, such as the uterus and the bones. This is good for the bones, but not good for the uterus, and this is the reason why tamoxifen slightly increases a woman’s risk of developing uterine cancer.
The aromatase inhibitors reduce estrogen by blocking an enzyme called aromatase and keeping it from converting androgens into estrogen. Both premenopausal and postmenopausal women can use tamoxifen as hormonal therapy. But for a woman to take an aromatase inhibitor, she must be postmenopausal. That's because postmenopausal women get most of their estrogen from the conversion of androgens into estrogen by the aromatase enzyme, while premenopausal women get most of their estrogen directly from their ovaries. There are drugs that a premenopausal woman can take to put her into menopause that are used as breast cancer treatments. These include goserelin (Zoladex) and leuprolide (Lupron).
All three aromatase inhibitors have known side effects. The most common is bone and joint pain. Other side effects women report include fatigue, dizziness, hot flashes, and weight gain. All of these side effects can affect your quality of life, and you may be able to tolerate some more than others. If you find the side effects are keeping you from taking the hormone therapy that you were prescribed, you can talk to your doctor about switching to one of the other aromatase inhibitors. You can also discuss switching to tamoxifen.
In June 2014, ASCO updated its hormone treatment guidelines. The new guidelines incorporate new research findings, including a large study that found that 10 years of tamoxifen was more effective than five years of tamoxifen followed by a placebo. (There were 617 recurrences and 331 deaths that occurred among the 3,428 women who stayed on tamoxifen compared to 711 recurrences and 397 deaths among the 3,418 women who were on the placebo.)
The new treatment guidelines for women with hormone-sensitive breast cancer are:
- Tamoxifen for five years.
- After five years assess, menopausal status. If not yet menopausal, consider continuing on tamoxifen for five more years. If menopausal, consider staying on tamoxifen or switching to an aromatase inhibitor.
- Tamoxifen for 10 years or
- An aromatase inhibitor for five years or
- Tamoxifen for five years followed by an aromatase inhibitor for up to five years or
- Tamoxifen for two to three years followed by an AI for up to five years (Note: There is insufficient evidence to recommend taking an AI for greater than five years.)
Learn more about treatment with hormone therapy and what these guidelines means for you here.