Differences in the incidence of endometrial cancer were small
but real. Among some 30,000 randomized women, there were 78 endometrial
cancer cases in the tamoxifen group vs 21 in the controls.
"Clearly a real difference," Prof. Peto said, "but
only three excess cases per 1,000 women." In terms of deaths
from endometrial cancer, the excess for tamoxifen users was only
one per 1,000 (19 endometrial cancer deaths for tamoxifen vs 2
in the control group).
He also noted that the metaanalysis has completely refuted any
concerns about increases in colorectal cancer with tamoxifen use.
Although the incidence of endometrial cancer rises with longer
tamoxifen use, "so do the benefits," Prof. Peto pointed
out. For those receiving an average of 4 years of tamoxifen, the
number of endometrial cancers rose to 9 per 1,000, but the reduction
in contralateral breast cancer was 50%, compared with 20% and
30% for 1 and 2 years of tamoxifen use, respectively. "But
it is still uncertain what will be seen with really long-term
tamoxifen treatment," he said.