It is also too early, he said, to judge the ultimate effect on breast cancer deaths among the prevention trials in healthy women. In the IBIS trial only four breast cancer deaths have been reported so far two in the tamoxifen arm and two in the placebo arm. However, the likely potential mortality benefit could be calculated if certain factors were assumed. "For high risk women, we calculate that deaths from breast cancer within 10 years of diagnosis would be reduced by 18%."
Other key findings from IBIS were also generally in line with the other tamoxifen trials, confirming that there was a 2 to 3-fold increase in the risk of endometrial cancer and a 2 to 3-fold increase in the risk of thromboembolism for women taking tamoxifen.
For all of the prevention trials combined there was no effect on all-cause mortality with 112 deaths in the tamoxifen arms and 122 in the placebo arms. However, there were variations, with statistically non-significant reductions in death in two trials, no difference in one trial and a statistically significant excess of deaths in the IBIS trial.
Professor Cuzick said: "In IBIS there were more deaths in the tamoxifen arm with 25 against 10 in the placebo arm. Deaths from cancers other than the breast were higher, but the numbers of these cancers were not increased, suggesting this was likely to be a chance finding. Also, with the exception of venous thromboembolic events, other vascular and cardiac events were not increased. This suggests that, apart from two possible deaths from pulmonary embolism (PE), the increases in other vascular and cardiac deaths were also chance findings.