A pre-surgical window-of-opportunity study with irosustat was undertaken in estrogen receptor-positive (ER+) breast cancer to assess the effect of irosustat on tumour cell proliferation as measured by 3′-deoxy-3′-[18F] fluorothymidine uptake measured by PET scanning (FLT-PET) and Ki67. Here, MKI67 is linked to neoplasm.