Key considerations for window of opportunity trials and preoperative endocrine therapy in breast cancer

Window of opportunity (WOO) trials are short, pre-surgical trials which aim to evaluate novel therapeutic strategies and answer biological and mechanistic questions. Despite their relative simplicity, their conduct can be challenging. This Perspectives Article considers key factors relevant for the optimal design and interpretation of WOO trials in the context of oestrogen receptor positive breast cancer, and provides insights for clinicians using preoperative endocrine therapy in a non-trial setting.