Introduction

We aimed to investigate the prognostic significance of estrogen receptor status (ER)-low status in breast cancer (BC). Additionally, we compared the ER-low subgroup with ER-negative and ER-high BC according to clinical, histopathological and molecular factors.

Material and methods

The analysis included medical records of BC patients treated at the National Institute of Oncology in Gliwice, Poland, between 2002 and 2018.

Results

Of the 657 tumours, 14% were classified as ER-low, 52% as ER-high and 34% as ER-negative. Patients with ER-low tumours were more likely to present with T3–T4 disease compared with those with ER-high (19% vs. 12%, p = 0.089) and less frequently than ER-negative (19% vs. 29%, p = 0.092). Patients with ER-low tumours were less likely to be progesterone receptor status (PgR) negative (19% vs. 92%, p < 0.001) and G3 (22% vs. 49%, p < 0.001) compared with ER-negative. BRCA P/LP (pathogenic/likely pathogenic) variants were less frequent in ER-low than in ER-negative tumours (5% vs. 20%, p = 0.001). Overall survival (OS) was similar in both ER-high and ER-low groups (p = 0.333; 5-year OS: 93% vs. 92%, 7-year OS: 86% vs. 83%). Overall survival was non-significantly better for those with ER-low compared with ER-negative (p = 0.107).

Conclusions

Estrogen receptor-low status appears to be associated with a higher T stage and PgR negativity compared with ER-high tumours, and with a lower grade and fewer BRCA P/LP variants compared with ER-negative tumours. A trend toward improved OS was observed in ER-low compared with ER-negative patients.