  Background. The DURATIONS design has been recently proposed as a practical
alternative to a standard two-arm non-inferiority design when the goal is to
optimise some continuous aspect of treatment administration, e.g. duration or
frequency, preserving efficacy but improving on secondary outcomes such as
safety, costs or convenience. The main features of this design are that (i) it
randomises patients to a moderate number of arms across the continuum and (ii)
it uses a model to share information across arms. While papers published to
date about the design have focused on analysis aspects, here we show how to
design such a trial in practice. We use the REFINE-Lung trial as an example;
this is a trial seeking the optimal frequency of immunotherapy treatment for
non-small cell lung cancer patients. Because the aspect of treatment
administration to optimise is frequency, rather than duration, we propose to
rename the design as Multi-Arm Multi-Stage Response Over Continuous
Intervention (MAMS-ROCI). Methods. We show how simulations can be used to
design such a trial. We propose to use the ADEMP framework to plan such
simulations, clearly specifying aims, data generating mechanisms, estimands,
methods and performance measures before coding and analysing the simulations.
We discuss the possible choices to be made using the REFINE-Lung trial as an
example. Results. We describe all the choices made while designing the
REFINE-Lung trial, and the results of the simulations performed. We justify our
choice of total sample size based on these results. Conclusions. MAMS-ROCI
trials can be designed using simulation studies that have to be carefully
planned and conducted. REFINE-Lung has been designed using such an approach and
we have shown how researchers could similarly design their own MAMS-ROCI trial.
