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Now, new research suggests that taking such a drug for an even longer period might not confer any added benefit -- at least in terms of survival.
However, other benefits were noted, so the decision to extend use of drugs like these is one best made on a case-by-case basis, cancer specialists said.
One oncologist who reviewed the new findings said the study has been "eagerly awaited," since many breast cancer survivors are counseled to take an aromatase inhibitor.
"For now, this trial reinforces the need to listen to our patients and weigh out the risk of side effects before extending therapy to 10 years," said Dr. Nina D'Abreo, medical director of the Breast Health Program at Winthrop-University Hospital in Mineola, N.Y.
Many forms of breast cancer may grow in the presence of estrogen, so medicines such as estrogen-suppressing aromatase inhibitors are used to help prevent the disease.
But how long should breast cancer survivors be placed on such drugs?
To help find out, researchers tracked outcomes for almost 4,000 postmenopausal women with early stage hormone receptor-positive breast cancer (meaning the tumors were sensitive to estrogen). The women took either letrozole or a placebo for an average of about two years after an initial five years of aromatase inhibitor therapy.
The result: Women who took letrozole for those extra two years did not experience statistically significant higher rates of either "disease-free" survival or overall survival than those in the placebo group, the study found.
There were some other benefits, however. For example, women who extended their use of letrozole showed a 29 percent reduction in breast cancer recurrence, and a 28 percent reduction in a tumors arising at locations outside the breasts.
The study was led by Dr. Terry Mamounas, medical director of the Comprehensive Breast Program at University of Florida Health Cancer Center in Orlando. He presented the results Wednesday at the annual San Antonio Breast Cancer Symposium in Texas.
Women who took letrozole for the additional years also experienced a "small increase in the risk of [clotting] events after 2.5 years," Mamounas noted in a meeting news release. This slight uptick in cardiovascular risk means that older women considering extra time on the drug "will require careful assessment of potential risks and benefits," he said.
This case-by-case assessment would involve a discussion of various patient factors, such as the patient's age at diagnosis, whether cancer had spread to lymph nodes, other illnesses the patient might have, her bone mineral density, and the side effects she had experienced from the aromatase inhibitor drug, Mamounas said.
For her part, D'Abreo agreed that any decision on whether to take an aromatase inhibitor beyond five years is best made on a case-by-case basis.
"For some subgroups of women with 'high risk' disease, this may be worth the side effects," she said. "Hormone-positive cancers may still recur much later and for our patients who are living longer, healthier lives, there may be benefits of extended therapy beyond the seven-year period of observation on this trial."
Dr. Ruby Sharma is a medical oncologist at Northwell Health Cancer Institute in Lake Success, N.Y. She agreed that based on the findings, "the decision to prescribe additional five years of endocrine therapy needs to be individualized for each particular patient depending on individual risk of recurrence and side effects of treatment."
The study was funded by the U.S. National Cancer Institute and the drug company Novartis, which makes Femara.
Experts note that findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.
-- Robert Preidt
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SOURCES: Nina D'Abreo, M.D., medical director, Breast Health Program, Winthrop-University Hospital, Mineola, N.Y.; Ruby Sharma, M.D., medical oncologist, Northwell Health Cancer Institute, Lake Success, N.Y.; American Association for Cancer Research, news release, Dec. 7, 2016