  Platform trials gained popularity during the last few years as they increase
flexibility compared to multi-arm trials by allowing new experimental arms
entering when the trial already started. Using a shared control group in
platform trials increases the trial efficiency compared to separate trials.
Because of the later entry of some of the experimental treatment arms, the
shared control group includes concurrent and non-concurrent control data. For a
given experimental arm, non-concurrent controls refer to patients allocated to
the control arm before the arm enters the trial, while concurrent controls
refer to control patients that are randomised concurrently to the experimental
arm. Using non-concurrent controls can result in bias in the estimate in case
of time trends if the appropriate methodology is not used and the assumptions
are not met. In this paper, we faced two main objectives. In the first, we
aimed to identify the methods currently available for incorporating
non-concurrent controls, clarify the key concepts and assumptions, and name the
main characteristics of each method. For this purpose, we systematically
searched research articles on methods to include non-concurrent controls. The
second objective is to summarise the current regulatory view on non-concurrent
controls to clarify the key concepts and current guidance. Therefore, we
conducted a systematic search in regulatory guidelines regarding using
non-concurrent controls and summarised the most relevant arguments and
recommended methods. Finally, we discuss the advantages and potential caveats
of using non-concurrent controls.
