*2.1. Patient Population*

In this retrospective study, we included women ≥18 years with histologically proven ER+, grade 1–2, clinical stage IIB/III or locoregional recurrent BC, treated at the Amsterdam UMC (VUmc) and The Netherlands Cancer Institute-Antoni van Leeuwenhoek (NKI-AvL) in the Netherlands between 2008–2016 and 2014–2015, respectively. All patients underwent FDG PET/CT for staging and had their follow-up visits for at least 18 months. Patients with other malignancies in the last five years prior to diagnosis of (recurrent) BC were excluded.

Prior to inclusion, patients provided written informed consent, except when it was not possible to approach them for consent, due to various reasons (e.g., death or no contact details available). The study was approved by the local Medical Ethics Committee of the VUmc (No. 2017.382).
