The federally funded phase III study involved more than 3,100 postmenopausal women with a kind of breast cancer known as ductal carcinoma in situ, which was treated by removing the cancerous lump followed by a radiation regimen.
Some women were then randomly assigned to receive tamoxifen and others anastrazole.
"The good news is tamoxifen and anastrazole are both very effective, but it seems that women have better chances of staying well with anastrazole," said lead study author Richard Margolese, a professor of surgical oncology at The Jewish General Hospital, McGill University in Montreal, Canada.
"Women should also consider differences in side effects when discussing treatment options with their doctors."
The drugs were similarly tolerated by patients with few side effects, though anastrazole tends to speed up osteoporosis and put women at higher risk of bone fracture, while tamoxifen has been associated with higher rates of uterine cancer.
The differences in these complications were not statistically significant when the two drugs were compared, researcher said.
Both drugs block estrogen -- which can fuel the growth of hormone receptor-positive breast cancer -- but in different ways.
Tamoxifen blocks the access of estrogen to the cancer cells, while anastrazole is an aromatase inhibitor which suppresses the manufacturing of estrogen.
Women diagnosed with DCIS face an increased risk of getting invasive breast cancer later in life, but deaths from DCIS are uncommon after the tumor is removed followed by radiation therapy.
The patients were followed for an average of 8.6 years, during which time 114 breast cancers were detected in the tamoxifen group compared to 84 in the anastrazole group.
Eight women died of breast cancer in the tamoxifen group and five in the anastrazole group.
The 10-year overall survival rates were comparable in the two groups (92.5% for anastrazole and 92.1% for tamoxifen).
When looking at the subgroup of women over 60, however, researchers also found that anastrazole may not be superior to tamoxifen.
"Women with DCIS already have several great treatment options, and now they have one more," said ASCO expert Don Dizon of Massachusetts General Hospital.
"Aromatase inhibitors offer important advantages, but patients and their doctors should still consider the full range of options, including tamoxifen or even foregoing adjuvant treatment, as every approach carries its own risks and benefits."
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