Abstract P4-05-11: The clinical importance of the new category, ER low-positive in the ER expression HER2 negative early breast cancer

Abstract <Background>In the latest updated ASCO/CAP guideline, a new category, ER low-positive was introduced, which are determined by the 1% to10 % of positive staining of the tumor nuclei for ER. However clinical meaning of ER-low positive was unclear compared with ER- positive or ER-negative in HER2 negative breast cancer. <Purpose> In order to clarify the clinicopathological features of ER-low positive breast cancer, we compared the clinicopathological factors and prognosis among three groups, ER negative, ER-low positive, and ER-positive groups in the patients with HER2 negative breast cancer. <Patients and Method> A total of 1,882 patients with HER2 negative breast cancer who underwent surgery between 2001 and 2018 were included in this study. We divided them into three groups, ER negative (<1%), ER-low positive (1-10%), and ER positive (10%≦). The relationships between ER status and clinicopathological characteristics and prognosis were evaluated. <Result>According to ER status, 553(29.4%), 183(9.7%), 1146(60.9%) patients were divided into ER negative, low-positive and positive groups. Compared with ER-positive group, patients in the ER negative and low-positive groups were of higher age (p<.0001), and those tumors were significantly associated with the larger tumor size (p<.0001), higher histological grade (p<.0001) and more administration of adjuvant chemotherapy (p<0.0001). Most of the patients with ER low-positive and ER-positive tumors had received the adjuvant endocrine therapy. The prognosis of the patients with ER low-positive tumors were poorer than that of those with ER positive tumors in terms of relapse-free survival (RFS p<.0001), distant metastasis-free survival (DMFS p<.0001) and overall survival (OS p<.0001).In the node negative patients, the prognosis of the ER-negative and ER low-positive groups were equivalent, and poorer than that of the ER-positive group (RFS; p=0.0068, DMFS; p=0.0013, OS: p=0.0032). In addition, in pStage III patients, prognosis of the ER-negative and ER low-positive groups were also equivalent, and poorer than that of the ER-positive group (RFS; p=0.008, DMFS; p=0.0021, OS: p=0.0010). <Conclusion> This study showed that the prognosis of the patients with ER negative and ER low-positive were similar, and poorer than that of those with ER positive tumors. Therefore, the new category of ER-low positive is clinically especially important in order to determine the appropriate adjuvant therapy. Citation Format: Tajiri Wakako. The clinical importance of the new category, ER low-positive in the ER expression HER2 negative early breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-05-11.