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INTRODUCTION: Estrogen receptor (ER) loss at metastatic relapse occurs in up to 20% of luminal-like primary breast tumors. Data about clinicopathological features associated with ER loss and its prognostic significance are limited.METHODS: In a nested-case-control study, we compared clinicopathological characteristics and clinical outcomes between a cohort of 51 patients with primary ER+ /HER2- and paired triple-negative metastasis (LUM-TN) and two control cohorts of paired early-metastatic ER+ /HER2- (LUM-LUM, n = 50) and triple-negative (TN-TN, n = 49) breast cancers. Stromal tumor-infiltrating lymphocytes (TILs) were assessed according to the TILs Working Group recommendations as continuous and discrete variables with cutoffs (20%, 40%).RESULTS: LUM-TN tumors had lower ER expression than LUM-LUM tumors, but lower grade and Ki67 than TN-TN cases. Median distant-metastasis free survival was similar for LUM-TN and LUM-LUM cohorts, but significantly longer than in TN-TN cases (log-rank P < 0.001). LUM-TN and TN-TN cohorts had a comparable survival from the time of metastatic recurrence, which was significantly shorter than in patients with LUM-LUM tumors (log-rank P < 0.001). High TILs were associated with worse outcomes in patients with ER loss (P < 0.001).CONCLUSIONS: Breast tumors with ER loss at metastatic relapse have intermediate features and outcomes compared with metastatic luminal-like and ab initio triple-negative tumors. Further investigation on the biological mechanisms underpinning the loss of ER expression is ongoing.