  We propose a Bayesian nonparametric utility-based group sequential design for
a randomized clinical trial to compare a gel sealant to standard care for
resolving air leaks after pulmonary resection. Clinically, resolving air leaks
in the days soon after surgery is highly important, since longer resolution
time produces undesirable complications that require extended hospitalization.
The problem of comparing treatments is complicated by the fact that the
resolution time distributions are skewed and multi-modal, so using means is
misleading. We address these challenges by assuming Bayesian nonparametric
probability models for the resolution time distributions and basing the
comparative test on weighted means. The weights are elicited as clinical
utilities of the resolution times. The proposed design uses posterior expected
utilities as group sequential test criteria. The procedure's frequentist
properties are studied by extensive simulations.
