299P - Characterization and clinical outcomes of low hormonal receptor positive / HER2 negative early breast cancer

Background

Estrogen receptor (ER) is a prognostic and predictive biomarker in breast cancer. In early breast cancer (EBC), adjuvant endocrine therapy (ET) has become a standard of care in hormone receptor (HR) positive cancers (ER/ progesterone (PR) expression by IHC stain of >1%). However, a subgroup of tumor with ER expression of 1-10% (ER low) was adopted by the ASCO in 2020 as a new entity with different features and prognosis from ER high. Conflicting data arose regarding the benefit of adjuvant ET in ER low. The study aimed to evaluate the characteristics and outcomes including benefit of adjuvant ET of HR low patients in comparison to HR high and HR negative EBC.

Methods

The study is a retrospective study of patients with stage I-III HER2 negative EBC between 2011-2015 who had at least 5 years of follow-up. They were categorized into 3 groups: HR high (ER>10% and/or PR>20%), HR low (ER 1-10% and/or PR 0-20%), and HR negative (ER and PR<1%). Demographic data, survival outcomes (iDFS and OS) of the cohort and subgroups who received ET were described using appropriate statistical methods.

Results

A total of 1,402 patients were analyzed with a HR low prevalence of 5.1%. Patients with HR low carried a distinct characteristic from HR high and more similar to HR negative. They presented with advance pathologic features (larger tumor size, higher stage, and higher Ki-67). Both iDFS and OS of HR low were identical to HR negative and significantly worse than HR high (5-year iDFS of HR low, HR negative and HR high: 74.7%, 75% and 90.2%, p < 0.001). Age at diagnosis, high pT stage, high pN stage, and high Ki-67 were associated with worse outcomes in multivariate analysis models of iDFS and OS. Adjuvant ET was prescribed to 82% of HR low patients. Patients with HR low who received ET had a significant improvement in iDFS compared to those without ET (5-year iDFS 82.8% vs 38.5%, p 0.001) and it was comparable to that of patients with HR high who received adjuvant ET (HR 1.23, p-value 0.447).

Conclusions

HR low is a rare entity among HER2 negative EBC. Although the clinicopathological features were paralleled to HR negative EBC, the responsiveness to endocrine therapy was non-negligible. Larger real-world data are needed to define the role of adjuvant hormonal treatment in this setting.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.