Bethesda, MD—The risk of developing an additional malignant tumor in the other breast is reduced in women on tamoxifen therapy, according to a new study, which adds that the risk progressively decreases with increased duration of therapy.

About 5% of patients develop contralateral breast cancer (CBC) within 10 years after their breast cancer diagnosis, according to the report in JAMA Oncology. It cites previous research indicating that tamoxifen citrate therapy can reduce primary cancer recurrence risk, improve survival, and lower CBC risk, as well as trials suggesting a lower CBC risk with the use of aromatase inhibitors (AIs).

The study team led by National Institute of Health researchers raised questions, however, about the magnitude and duration of the protective associations in real-world treatment scenarios.

To help find answers, the researchers focused on CBC risk among 7,451 patients diagnosed with a first primary unilateral invasive breast cancer at the Kaiser Permanente Institute for Health Research in Colorado or the Kaiser Permanente Northwest Center for Health Research in Oregon between 1990 and 2008.

Participants included 7,451 mostly white women—median age at initial breast cancer diagnosis of 60.6 years—with 248 women developing CBC (45 in situ and 203 invasive) during 6.3 years of follow-up.

More than half, 52%, were on tamoxifen therapy at a median time period of 3.3 years. About 25% also used AIs—about half of those taking them with tamoxifen and half without, for a median time period of between 2 and 3 years, according to the report.

Results indicate that the risk of CBC decreased the longer tamoxifen was used—a 66% reduction in current users with 4 years of tamoxifen use, compared with nonusers of tamoxifen. Even at least 5 years after discontinuing tamoxifen therapy, reductions in risk were smaller but still significant, study authors note, adding that AI use without tamoxifen therapy was also associated with reduced risk of CBC.

In current users, the relative risk per year of tamoxifen use was 0.76 going up to 0.85 in the past users, the study points out. AI use without tamoxifen therapy, meanwhile, was associated with reduced CBC risk of 0.48.

Risk reductions were most apparent among women whose primary and CBCs were estrogen-receptor positive, the report adds.

“Among those surviving at least five years, tamoxifen use for at least four years was estimated to prevent three CBCs per 100 women by 10 years after an estrogen receptor-positive first breast cancer, an absolute risk reduction that is consistent with findings from clinical trials,” the researchers conclude. “If adjuvant endocrine therapy is indicated for breast cancer treatment, these findings in concert with trial data suggest that women should be encouraged to complete the full course.”

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