Staley, who is today cancer-free and with peace of mind she never thought she would get, said she wonders what would have happened had she made it to the "magical five-year" treatment endpoint. Studies have found that even two years on tamoxifen can bring benefits, and ongoing studies are investigating whether a 10-year treatment course is even better than a five-year course.
The study also confirms that women whose tumors are even mildly hormone-sensitive -- as low as 1 percent positive for estrogen receptors in standard lab tests -- can benefit from tamoxifen.
"Recently, we started calling these low-level-expressing tumors positive and have been offering these women tamoxifen," said Dr. Banu Arun, co-director of Clinical Cancer Genetics at the University of Texas MD Anderson Cancer Center in Houston. "This further supports that what we've been doing was correct."
The new study also found that tamoxifen extends survival, even in women who've had chemotherapy, debunking a common misconception that one or the other would do, Arun said.
Tamoxifen is also used to prevent breast cancer in women at high risk for the disease because of their genetic makeup. But only 30 percent of these women opt to take the drug, said Arun, with 70 percent opting for regular screenings instead.
"It's easier for me to tell my patients with invasive breast cancer that they need to take the drug because the benefits far outweigh the risks," said Arun. "But in high-risk women who don't have cancer, it's harder to make that decision. At the end of the day, it's our patients' choice. But it's our job to help them decide."