FRIDAY, Dec. 11, 2015 (HealthDay News) -- Postmenopausal women who have an early, noninvasive form of breast cancer had similar recurrence rates of disease whether they took the drug tamoxifen or the aromatase inhibitor anastrozole after surgery, new research shows.
However, the side effects of the two medications differed greatly, said study author Jack Cuzick, director of the Wolfson Institute of Preventive Medicine at Queen Mary University of London, England.
His team looked at nearly 3,000 women, all past menopause, who had hormone-receptor positive ductal carcinoma in situ (DCIS) breast cancer and underwent surgery to excise it. With DCIS, the cells that line the milk ducts have changed but not spread into the surrounding breast tissue.
Half the women were randomly assigned to take 1 milligram (mg) a day of anastrozole (Arimidex), while the other half took 20 mg a day of tamoxifen (Nolvadex). Each group also took a placebo pill to look like the pill they were not assigned to take, so they wouldn't be biased about side effects.
The aim of the study was to look at breast cancer recurrence, and to see whether it was more likely with one drug than the other.
The researchers said that aromatase inhibitors such as anastrozole have been shown in other studies to be better than tamoxifen in postmenopausal women who have invasive cancers. The less-researched area, Cuzick explained, is DCIS.
After a follow-up of about seven years, those who took anastrozole had an 11 percent lower breast cancer recurrence rate than those who took tamoxifen, but that difference was not substantial from a statistical point of view, Cuzick said.
"Our results show anastrozole to be slightly better, but it was not significant," he said. However, the side effect profiles were slightly better with anastrozole, he added.
"Tamoxifen has [potential] blood clot problems, and those don't occur with anastrozole," Cuzick said. And while those on anastrozole reported more aches and pains, those on tamoxifen were more likely to have hot flashes.
Tamoxifen blocks estrogen receptors in the breast cells to hamper cancer growth. Anastrozole stops estrogen production in fat tissue, which makes small amounts of hormone.
Cuzick plans to present the findings Friday at the San Antonio Breast Cancer Symposium, in Texas. The study is also being published simultaneously in The Lancet.
In a related study to be reported at the same meeting on Friday, UCLA researchers led by Dr. Patricia Ganz found no difference in quality-of-life measures among women who took the two drugs after treatment for DCIS.
But they did find the expected differences in side effects. Those patients on tamoxifen had more hot flashes, while those on anastrozole had more vaginal problems and muscle aches. Research presented at meetings is considered preliminary until published in a peer-reviewed journal.
The results of the new DCIS research "are exactly what we see in invasive breast cancer," said Dr. Joanne Mortimer, director of the Women's Cancer Programs and co-director of the Breast Cancer Program at the City of Hope Comprehensive Cancer Center, in Duarte, Calif.
DCIS has been in the news recently, she said, "because it is not an invasive cancer, yet is treated as aggressively."
Some have challenged the need to even treat DCIS, Mortimer said, suggesting doctors wait until invasive cancers develop. The new findings suggest that approach is not warranted, she added.
"The fact that DCIS and invasive cancer both respond to endocrine therapy [tamoxifen and anastrozole] suggests that we should treat DCIS, so that invasive cancers don't develop," she said.
For women trying to decide with their doctor which drug to take, Mortimer said the message is this: "Both of these hormonal therapies are effective and if one causes side effects, switching to the other is reasonable."
To learn more about hormone therapy for breast cancer, visit the American Cancer Society.