Tamoxifen benefits in breast cancer extend decades

Taking tamoxifen for 5 years to prevent breast cancer results in a prophylactic effect that extends over the next 20 years, according to a new report published in in The Lancet Oncology. The findings, from long-term follow-up of the International Breast Cancer Intervention Study (IBIS-1), show that the benefit is even greater in women who did not take menopausal hormone therapy during the treatment period.

More than 7,000 women ages 35 to 70 year were enrolled in IBIS-1, which spanned 1992 to 2001. Randomization was to either a daily 20-mg dose of oral tamoxifen or placebo for 5 years. Both patients and investigators were blinded to treatment assignment. The primary end point was occurrence of breast cancer, either invasive or ductal carcinoma in situ (DCIS). Breast cancer occurrence and mortality were assessed using Cox proportional hazard models.

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After a median follow up of 16.0 years (interquartile range 14.1-17.6), 601 cases of breast cancer have been reported (251 [7.0%] in 3579 patients in the tamoxifen group vs 350 [9.8%] in 3575 women in the placebo group; hazard ratio [HR] 0.71 [95% CI 0.60-0.83], P<0.0001). The risk of developing cancer was similar in years 0 to 10 (226 [6.3%] in 3575 women in the placebo group vs 163 [4.6%] in 3579 women in the tamoxifen group; hazard ratio [HR] 0.72 [95% CI 0.59-0.88], P=0.001) and after 10 years (124 [3.8%] in 3295 women vs 88 [2.6%] in 3343, respectively; HR 0.69 [0.53-0.91], P=0.009).

The greatest reduction in risk was seen in DCIS (HR 0.65 [95% CI 0.43-1.00], P=0.05) and invasive estrogen receptor (ER)-positive breast cancer (HR 0.66 [95% CI 0.54-0.81], P<0.0001). No effect was seen in invasive ER-negative breast cancer (HR 1.05 [95% CI 0.71-1.57] P=0.8).

An increase in deaths from endometrial cancer was confirmed, but no effect on all-cause or breast cancer mortality. Their findings, researchers noted, “substantially improve the benefit-to-harm ratio for the use of tamoxifen to prevent breast cancer in high-risk women, with an estimated 22 women being needed to be treated for 5 years to prevent one breast cancer in the next 20 years.”

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