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

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.