  Non-proportional hazards (NPH) have been observed recently in many
immuno-oncology clinical trials. Weighted log-rank tests (WLRT) with suitably
chosen weights can be used to improve the power of detecting the difference of
the two survival curves in the presence of NPH. However, it is not easy to
choose a proper WLRT in practice when both robustness and efficiency are
considered. A versatile maxcombo test was proposed to achieve the balance of
robustness and efficiency and has received increasing attentions in both
methodology development and application. However, survival trials often warrant
interim analyses due to its high cost and long duration. The integration and
application of maxcombo tests in interim analyses often require extensive
simulation studies. In this paper, we propose a simulation-free approach for
group sequential design with maxcombo test in survival trials. The simulation
results support that the proposed approaches successfully control both the type
I error rate and offer great accuracy and flexibility in estimating sample
sizes, at the expense of light computation burden. Notably, our methods display
a strong robustness towards various model misspecifications, and have been
implemented in an R package for free access online.
