- PFS of 9 months in the control arm (letrozole in monotherapy). Using a two-sided log-rank test, for a 5% α level, 344 patients (172 in each treatment arm) will be required for 270 events to occur, which will provide an 80% power for detecting a hazard ratio of 0.69 (corresponding to a PFS median of 13 months in the bevacizumab arm). This sample size has been adjusted for an intermediate analysis when 2/3 of the total of required events have occurred. This intermediate analysis can be avoided if, at the time in which it must be carried out, it is estimated that the final analysis will be carried out in 4 months.