Chae SY, et al. - In this prospective cohort study conducted in South Korea, researchers assessed whether 16α-[¹⁸F]fluoro-17β-oestradiol (¹⁸F-FES) PET-CT would be an accurate and safe diagnostic tool for the determination of oestrogen receptor status in patients with recurrent or metastatic breast cancer. Patients with breast cancer, with first recurrence or metastatic disease at presentation, having an Eastern Cooperative Oncology Group performance status of 0–2, and patients aged 19 years or older were considered eligible for inclusion in the study. They investigated whether qualitative ¹⁸F-FES PET-CT interpretation agreed to the findings of oestrogen receptor expression by immunohistochemical assay. These two modalities demonstrated high negative percent agreement. This implies that oestrogen receptor assays could be replaced with positive ¹⁸F-FES uptake by recurrent or metastatic oestrogen receptor-positive breast cancer lesions in this setting. In cases where non-feasibility of retesting oestrogen receptor status is encountered, ¹⁸F-FES PET-CT should be included in the staging evaluation.