Women are prescribed adjuvant therapies, such as chemotherapy and endocrine drugs, after breast cancer surgery to reduce the risk of cancer recurrence. The effects of these therapies have been assumed to remain constant over time, but a new study published in the Journal of the National Cancer Institute (2015; doi:10.1093/jnci/djv304) suggests the opposite is true.

The study, from the Cancer Therapy & Research Center (CTRC) at The University of Texas Health Science Center at San Antonio, looked at 19 breast cancer adjuvant therapy clinical trials of the National Surgical Adjuvant Breast and Bowel Project (NSABP). Therapies were evaluated for relative benefit, defined as the percentage by which they reduced recurrence and death among study participants.

Researchers found time-varying treatment effects of adjuvant therapies in nearly half of the trials (9 of 19).

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“In some trials, the benefit diminished at specific points of time after surgery,” said study senior author Ismail Jatoi, MD, PhD, FACS. “In other trials, there was no benefit early on, but then there was a delayed benefit that emerged more than 1 year after surgery.”

In 1 clinical trial, the researchers found that a regimen provided initial benefit but then a subsequent disadvantage to patients. He explained that the findings may change the way oncologists talk to their patients about the effects of treatments they receive.

“We are seeing more and more long-term survivors of breast cancer who had these treatments many years ago,” Jatoi said. “The question is, if these treatment effects have waned, should we consider extended adjuvant treatment regimens for the long term in some patients.”

Adjuvant therapy trials should be designed and interpreted with this in mind, Jatoi said.