  Duration of response (DOR) and time to response (TTR) are typically evaluated
as secondary endpoints in early-stage clinical studies in oncology when
efficacy is assessed by the best overall response (BOR) and presented as the
overall response rate (ORR). Despite common use of DOR and TTR in particular in
single-arm studies, the definition of these endpoints and the questions they
are intended to answer remain unclear. Motivated by the estimand framework, we
present relevant scientific questions of interest for DOR and TTR and propose
corresponding estimand definitions. We elaborate on how to deal with relevant
intercurrent events which should follow the same considerations as implemented
for the primary response estimand. A case study in mantle cell lymphoma
illustrates the implementation of relevant estimands of DOR and TTR. We close
the paper with practical recommendations to implement DOR and TTR in clinical
study protocols.
