  Computation of sample size is important when designing clinical trials. The
presence of competing risks makes the design of clinical trials with
time-to-event endpoints cumbersome. A model based on the subdistribution hazard
ratio (SHR) is commonly used for trials under competing risks. However, this
approach has some limitations related to model assumptions and clinical
interpretation. Considering such limitations, the difference in restricted mean
time lost (RMTLd) is recommended as an alternative indicator. In this paper, we
propose a sample size calculation method based on the RMTLd for the Weibull
distribution (RMTLdWeibull) for clinical trials, which considers experimental
conditions such as equal allocation, uniform accrual, uniform loss to
follow-up, and administrative censoring. Simulation results show that sample
size calculation based on the RMTLdWeibull can generally achieve a predefined
power level and maintain relative robustness. Moreover, the performance of the
sample size calculation based on the RMTLdWeibull is similar or superior to
that based on the SHR. Even if the event time does not follow the Weibull
distribution, the sample size calculation based on the RMTLdWeibull still
performs well. The results also verify the performance of the sample size
calculation method based on the RMTLdWeibull. From the perspective of the
results of this study, clinical interpretation, application conditions and
statistical performance, we recommend that when designing clinical trials in
the presence of competing risks, the RMTLd indicator be applied for sample size
calculation and subsequent effect size measurement.
