For many postmenopausal women with early-stage breast cancer, treatment includes an aromatase inhibitor. Current guidelines recommend that women stay on an aromatase inhibitor for five years. But might staying on treatment for 10 years confer additional benefit? A recent study suggests the answer is yes, for some women.

About 70 percent of women with breast cancer have tumors that are fueled by estrogen. Following surgery, these tumors are typically treated with an anti-estrogen drug. Many postmenopausal women take one of three equally effective aromatase inhibitors—Arimidex (anastrozole), Aromasin (exemestane) or Femara (letrozole)—daily for five years. 


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The study, in the July 21, 2016, New England Journal of Medicine, randomized women who had already been on an aromatase inhibitor for five years to get five additional years of treatment with either Femara or a placebo. The women on Femara were significantly less likely to have their cancer recur than those on the placebo.

Aromatase inhibitors have side effects such as vaginal dryness, hot flashes, and muscle and joint pain. A study presented at the 2011 San Antonio Breast Cancer Symposium found that about one-third of women don’t stay on an aromatase inhibitor for the full five years, primarily because of these side effects.

In the new study, reported quality of life was the same in women on Femara or on the placebo. Paul Goss, a medical oncologist at Massachusetts General Hospital Cancer Center and Harvard Medical School in Boston who led the study, attributes this to the type of women who enrolled in the study. “They were a select group of women,” he says. “By definition, all of these patients had tolerated treatment and stayed on it.”

Anti-Estrogen Treatment for Post-Menopausal Women

Current guidelines recommend that postmenopausal women with early-stage, hormone receptor-positive breast cancer take one of the following:

  • tamoxifen for 10 years;
  • an aromatase inhibitor for five years;
  • tamoxifen for five years, followed by an aromatase inhibitor for up to five years, for a total of 10 years of hormonal therapy; or
  • tamoxifen for two to three years, followed by up to five years of an aromatase inhibitor, for a total of seven to eight years of hormonal therapy.

“You would only offer patients the option of five more years of an aromatase inhibitor if they had stayed on the aromatase inhibitor for five years,” he adds.

Some oncologists have already been extending treatment. “What we’ve done for a long time is offer women who are at high risk for recurrence a longer duration of treatment with an aromatase inhibitor if they are tolerating it after five years, and that was in the absence of data,” says William J. Gradishar, a medical oncologist at the Robert H. Lurie Comprehensive Cancer Center of Northwestern University in Chicago.

The new study shows “the dominant effect of taking an aromatase inhibitor longer was preventing a second breast cancer, not preventing metastatic disease,” says Gradishar. “But to reap the benefits, you have to tolerate the treatment and experience few side effects, and there is no shortage of patients who can’t wait for the moment they hit the five-year mark and can stop because they are tired of the musculoskeletal problems.”

Goss says he believes the findings will change the standard of care. “If the aromatase inhibitor is not affecting a woman’s quality of life for the first five years, it makes sense to continue,” he says. “The way I see it, the question is why shouldn’t you continue on?”