[What Is an Estimand?-The Most Important Clinical Question in Oncology Trials]

Patients can experience different disease journeys and clinical trials that investigate the benefit of oncology treatments need to account for this diversity. When defining the treatment effect of interest in a trial, researchers thus have to account for events occurring after treatment initiation, such as the start of a new therapy, before observing the variable of interest. We review the estimand framework recently introduced by the International Council for Harmonisation(ICH, 2019)to structure discussions on the relationship between patient journeys and the treatment effect of interest in oncology trials. This framework is expected to improve coherence between trial objectives, design, analysis, reporting and interpretation, as illustrated in this article by examples in oncology disease settings.