| Summary: | Purpose This study aimed to examine the clinical characteristics and outcomes of patients with estrogen receptor-negative (ER−)/progesterone receptor-positive (PR+) early breast cancer. We also aimed to investigate the benefits of adjuvant endocrine therapy (ET) in this patient population. Methods Patients with early breast cancer diagnosed at West China Hospital were divided into the ER−/PR+, ER+, and ER−/PR− groups. The chi-square test was used to analyze differences in clinical and pathological features among the groups. Multivariable Cox and Fine–Gray regression models were used to compare mortality and locoregional recurrence (LRR)/distant recurrence (DR), respectively. We performed a subgroup analysis to determine which ER−/PR+ patients can benefit more from ET. Results From 2008 to 2020, we enrolled 443, 7104, and 2892 patients into the ER−/PR+, ER+, and ER−/PR− groups, respectively. The ER−/PR+ group showed more unfavorable clinical features and aggressive pathological characteristics than the ER+ group. The mortality, LRR, and DR rates were higher in the ER−/PR+ than in the ER+ group. Most clinical features and pathological characteristics were similar between the ER−/PR+ and ER−/PR− group and their outcomes were comparable. In the ER−/PR+ group, patients who received ET showed significantly lower LRR and mortality rates than those who did not; however, no difference was observed in DR. Subgroup analysis suggested that ER−/PR+ patients age ≥ 55 years, and postmenopausal status can benefit from ET. Conclusion ER−/PR+ tumors have more aggressive pathological characteristics and more unfavorable clinical features than ER+ tumors. ET can reduce the LRR and mortality rates in ER−/PR+ patients. Postmenopausal and age ≥ 55 years ER−/PR+ patients can benefit from ET.
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