Oestrogen receptor low positive breast cancer: associations with prognosis

PURPOSE: In this study of oestrogen receptor (ER) Low Positive breast cancers (BC) in three large cohorts of BC patients, we assess associations between levels of ER expression and tumour characteristics and prognosis. METHODS: Cases were stratified into patients unlikely to have received adjuvant therapy according to treatment guidelines at time of diagnosis (before 1995), and those who could have received adjuvant therapy (diagnosed in 1995 or later). ER status was divided into < 1%; ≥ 1 < 10%; ≥ 10%. Results were correlated with time of diagnosis, histopathological grade, proliferation status, and molecular subtypes, using Pearson’s Chi-square test. For prognosis, hazard ratios and cumulative incidence of death from BC were used. RESULTS: Of the 1955 tumours, 65 (3.3%) were ER Low Positive (ER ≥ 1 < 10%). Overall, the highest proportion of ER Low Positive tumours was observed among Luminal B (HER2 +) subtype (9.4%) and grade 3 tumours (4.3%). The risk of death from BC was lower in ER Low Positive and ER ≥ 10% compared to ER-negative cases. Compared to patients diagnosed before 1995, women diagnosed in 1995 or later showed a higher proportion of ER Low Positive BCs, and their tumours were of smaller size, lower grade, and lower proliferative status. There was no significant difference in prognosis compared to those with ER ≥ 10% tumours. CONCLUSION: Women with ER Low Positive tumours diagnosed in a time period when adjuvant therapy was available had tumours of smaller size, lower grade, and lower proliferative status, and similar prognosis to those with ER ≥ 10% compared to women diagnosed earlier.