Women with estrogen receptor-positive who took taxomifen for 10 years had a 25 percent lower recurrence rate in the second decade after diagnosis and a 29 percent lower breast cancer mortality rate than women who stopped taking the drug after five years.
These important findings may also apply to women taking other forms of adjuvant (after primary treatment ) hormone therapy.
"Ask your oncologist how long you should stay on adjuvant therapy."
Scientists in the United Kingdom conducted the ATLAS (Adjuvant Tamoxifen — Longer Against Shorter) study. Nearly 7,000 women with ER-positive (ER+) breast cancer were enrolled in the study between 1996 and 2005.
Tamoxifen blocks the hormone estrogen which drives ER+ breast cancer. It’s most often prescribed to women who have not yet undergone menopause. Other forms of estrogen-blocking therapy are available for postmenopausal women.
“Five years of adjuvant tamoxifen is already an excellent treatment that substantially reduces the 15-year risk for recurrence and death from estrogen receptor (ER)-positive breast cancer, but ATLAS now shows that 10 years of tamoxifen is even more effective,” said Christina Davies, MD, a coordinator in the Clinical Trial Service Unit at Oxford University in the United Kingdom.
She continued, “We already knew that five years of tamoxifen reduces breast cancer mortality in this late period by almost a third in comparison with no tamoxifen. We now know that 10 years of tamoxifen is even better, approximately halving breast cancer mortality during the second decade after diagnosis,” Dr. Davies said in a press release.
Half of the 6,846 women enrolled in the study had breast cancer that had spread to the lymph nodes. Study participants were followed for about eight years.
All of the women had been taking tamoxifen for five years. Investigators assigned some of the women to stop taking the drug after five years, while others were told to continue the therapy for another five years.
The treatment options had no effect on the number of women who saw their cancer return or who died from the disease between five and nine years after diagnosis.
However, women who continued the therapy for 10 years saw significant improvements. Women in the 10-year group had a 25 percent lower recurrence rate and a 29 percent lower breast cancer mortality rate compared with women who stopped after five years.
The risk of dying from breast cancer in the period of 5-14 years after diagnosis was 12.2 percent among women who continued taking tamoxifen compared with 15 percent for those who stopped after five years. Researchers saw the greatest benefits in years 10-14 following diagnosis.
Continuing tamoxifen was not without side effects, with endometrial cancer, which is highly curable, being the most serious. Endometrial cancer was not a concern among pre-menopausal women.
Co-investigator, Richard Gray, MSc, professor of medical statistics at Oxford University presented the results of the ATLAS study at the 2012 CTRC-AACR San Antonio Breast Cancer Symposium. The international conference is being hosted by the Cancer Therapy & Research Center at UT Health Science Center San Antonio, the American Association for Cancer Research and Baylor College of Medicine.
The research is being published simultaneously in The Lancet. No conflicts of interest were reported.