The significance of highlighting the oestrogen receptor low category in breast cancer

Abstract The latest ASCO/CAP guideline has recommended to report oestrogen receptor (ER) low cases (ER lo ; 1–10%) as “ER low positive category”, prompting us to compare the clinicopathologic features, biomarkers, survival and treatment of the ER lo cases with other subgroups (ER negative (ER neg ) and ER high (ER hi )). ER lo cases revealed more similar clinicopathologic and biomarker profiles (including younger age, larger tumour, high proliferation, HER2 and basal markers expression) to ER neg than ER hi cancers. The ER lo cases receiving hormonal therapy showed a similarly poor outcome as ER neg cancers. However, majority of ER lo cases were downstaged to stage I in the 8th AJCC pathological prognostic staging, highlighting a risk of potential under treatment. Overall, our data highlighted the differences of ER lo from other ER pos cases and their management should be considered separately.