Monitoring rules for stopping accrual in comparative survival studies

It is useful to separate the decision to stop accrual from the decision to perform a final data analysis in comparative survival trials. Rules for monitoring survival studies are presented that allow for early termination of accrual if evidence mounts against one therapy and if the decision to stop accrual will not prolong the time to analysis unduly. It is shown that these rules for stopping accrual do not detectably perturb the final analysis, which includes a logrank test of no treatment effect and estimation of the log relative hazard with confidence intervals. These methods take advantage of the reservoir of information represented by patients still at risk at the time accrual ceases. Especially with moderate or rapid accrual rates the average number of patients accrued is often reduced substantially and the time to analysis prolonged only slightly.