PATHOLOGICAL COMPLETE RESPONSE OF ESTROGEN RECEPTOR-LOW VERSUS ESTROGEN RECEPTOR-HIGH AND NEGATIVE BREAST TUMORS

Background: Estrogen receptor (ER) is a remarkable predictive/prognostic biomarker in breast cancer (BC). Tumors with 1-10% cells positive for ER by immunohistochemistry (IHC) are ER-low and, probably, have a more aggressive phenotype. Methods: We retrospectively analyzed consecutive stage I-III BC patients who received neoadjuvant chemotherapy (NAC) at our institution. Luminal HER2 negative + triple negative were included and divided into ER-low (ER ≤ 10%), ER-high (ER > 10%), ER- (ER negative), according to IHC of the primary tumor biopsy. The primary endpoint was pathological complete response (pCR): compared between ER-low versus ER-high and ER-. χ2/Fisher’s tests were used to compare categorical variables and logistic regression for multivariable analysis. Results: From 2007-2018, 850 patients were included (44 ER-low; 466 ER-high; 340 ER-). There were imbalances between ER-low and ER-high: ERlow had more grade 3 (74.4% vs 24.1%; p<0.001), cT1/2 (47.7% vs 31.3%;p=0.03), cN0 (45.5% vs 29%;p=0.03), progesterone receptor-low: PR-low (81.8% vs 25.1%; p<0.001); received more NAC carboplatin (29.5% vs 0.6%; p<0.001), NAC dose dense AC (47.7% vs 26.6%; p=0.005) and adjuvant capecitabine (14.3% vs 1.8%;p<0.001). Characteristics were well balanced between ER-low and ER-, except for higher rates of PR-low (95.6% vs 81.8%; p=0.002) and NAC carboplatin (48.5% vs 29.5%; p=0.02) for ER-. pCR was superior for ER-low vs ER-high (27.3% vs 8.2%; p<0.001) and inferior for ER-low vs ER- (27.3% vs 46.2%; p=0.02). In multivariable analysis, ER-low was associated with inferior pCR in the group ER-low + ER- (p=0.03); this association was not seen for ER-low + ER-high. In both groups, NAC carboplatin, premenopause and cT1/T2 improved pCR. Disease progression rates (DP) tended to be higher for ER-low vs ER-high (38.6% vs 26%; p=0.07) and death rates (DR) were higher (27.3% vs 11.4%; p=0.007). ER-low increased DR in multivariable analysis. DP and DR were similar between ER-low and ER- (38.6% vs 27.9%, p=0.16; and 25% vs 19.1%, p=0.42). Median progression-free and overall survivals were not reached. Conclusions: ER-low had a more aggressive behavior than ER-high in this cohort, and, although it led to an inferior pCR than ER-, it had similar DP and DR to this group.