“That was a significant difference. Women who did not use hormone replacement therapy have a much bigger benefit from tamoxifen,” said Cuzick.
Not surprisingly given tamoxifen’s adverse event profile, the incidence of endometrial cancer was higher in the tamoxifen arm versus controls (29 vs 20). However, the large increase in the incidence of non–melanoma skin cancer in the tamoxifen arm (116 vs 84) was unexpected, said Cuzick. “This raises the question as to whether there is any mechanism that tamoxifen might be causing these cancers, something important but which we don’t fully understand.”
In this study, tamoxifen continues to have no impact on breast cancer mortality, with no significant difference seen between the two groups, said Cuzick. There were 31 deaths in the tamoxifen arm versus 26 with placebo. Among women who took tamoxifen, there was a nonsignificant increase in all-cause mortality, but this was smaller than in the 8-year follow-up report, he added. Five deaths from endometrial cancer were reported, versus none in the control groups. Death from all cancers was higher in the tamoxifen arm (83 vs 78).
“After 20 years of follow-up, these results show a clear, long-standing benefit of 5 years of tamoxifen for preventing breast cancer,” Cuzick concluded.
And, since the average age of the participants who have survived thus far is 66 years, “there is a substantial amount of follow-up still to be done,” Cuzick said.
Cuzick J, Sestak I, Cawthorn S, et al. Extended long-term follow up of the IBIS-I breast cancer prevention trial. Paper presented at: San Antonio Breast Cancer Symposium; December 9-13, 2014; San Antonio, TX. Abstract S3-07.
Cuzick J, Sestak I, Cawthorn S, et al. Tamoxifen for prevention of breast cancer: extended long-term follow-up of the IBIS-I breast cancer prevention trial [published online December 11, 2014]. Lancet Oncol. http://dx.doi.org/10.1016/S1470-2045(14)71171-4.
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