295P - Long-term clinical outcomes and treatment strategy considerations by ER expression level in breast cancer

Background

The expression status of estrogen receptor (ER) in breast cancer tissues is one of the predictors of therapeutic response to endocrine therapy and prognosis. The 2020 ASCO/CAP guidelines proposed that 1-10% of ER-positive cells should be classified as ER low positive. The purpose of this study was to investigate the effect of the percentage of ER-positive cells in breast cancer tissues on long-term prognosis.

Methods

We analyzed the perioperative treatment and prognosis of 3079 ER-positive HER2-negative patients treated at our institution from 1981 to 2022, excluding stage 0 patients, according to the level of ER expression. The criteria for determining ER were as follows: a positive ER cell rate of less than 1% was defined as negative, between 1% and 10% as low positive, between 10% and 2/3 as middle positive, and more than 2/3 as high positive.

Results

The prognosis was poor in the ER-negative group and good in the high positive group. Interestingly, there was no significant difference in prognosis between the low and middle positive groups, while there was a significant difference in both disease-free survival (DFS) and overall survival (OS) between the middle and high positive groups (p=0.0009, p

Conclusions

The ER middle positive and high positive groups are often clinically lumped together as ER-positive, but the middle positive group had a prognosis rather similar to that of the low positive group. The present results indicate that treatment strategies should be based on the level of expression among ER-positive patients.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

Yumi Wanifuchi-Endo.

Funding

Has not received any funding.

Disclosure

T. Toyama: Non-Financial Interests, Personal and Institutional, Invited Speaker: Eli Lilly, Pfizer, Daiichi Sankyo, Chugai; Non-Financial Interests, Personal, Invited Speaker: Kyowa Kirin, Taiho, Eisai, AstraZeneca. All other authors have declared no conflicts of interest.