Treatment with tamoxifen was associated with a reduced risk for contralateral breast cancer (CBC) during therapy as well as after its cessation, with a progressively decreased risk with increase in the duration of treatment with the drug, according to a study published in JAMA Oncology.1

Researchers led by Gretchen Gierach, PhD, MPH, of the National Cancer Institute in Bethesda, MD, conducted a retrospective cohort study of 7541 women with invasive breast cancer at risk for CBC through Kaiser Permanente Institutes in Oregon and Colorado to determine any association with adjuvant tamoxifen and aromatase inhibitor (AI) therapy.

The researchers analyzed patient data 1 year after diagnosis up to the earliest incidence of a CBC diagnosis, other second cancer diagnosis, death, last tumor registry follow-up, exit from Kaiser Permanente health care plan, or end of study follow-up.

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With a median follow-up of 6.3 years, it was found that 248 women had developed CBC, with 45 in situ and 203 invasive. CBC risk had decreased significantly with an increase in the duration of tamoxifen therapy.

Compared to non-users, current users of tamoxifen had a relative risk per year of 0.76, with about a 66% relative risk reduction for 4 years of use. These risk reductions were smaller for past users but still notable at least 5 years after cessation of tamoxifen therapy.

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AI use without treatment with tamoxifen was associated with a reduced CBC risk, with risk reductions most apparent in women who were estrogen receptor positive.


  1. Gierach GL, Curtis RE, Pfeiffer RM, et al. Association of adjuvant tamoxifen and aromatase inhibitor therapy with contralateral breast cancer risk among US women with breast cancer in a general community setting. JAMA Oncol. 2016 Oct 6. doi: 10.1001/jamaoncol.2016.3340 [Epub ahead of print]