241MO - Patient characteristics and real-world outcomes in HER2 negative/ ER zero and ER low patients treated as triple-negative breast cancer in Sweden 2008-2020

Background

Estrogen receptor-low (ER-low) Her2-negative breast cancer has similar pathological and molecular characteristics as triple-negative breast cancer (TNBC), and it is questionable whether it should be considered a separate entity. When the international guidelines lowered the cutoffs for ER to ≥1% in 2010, the ≥10% threshold was kept in Sweden. ER-low breast cancer (ER 1-9%) has thus in Sweden been treated as TNBC, which is interesting now that the international community recognize limited data for benefit of endocrine therapy in the ER-low group The benefit of endocrine therapy in these tumors is under debate and discussion. We aimed to describe real-world patient and tumor characteristics, treatment patterns and overall survival in a Swedish population-based cohort of patients with HER2 negative/ ER zero and ER low breast cancer treated as TNBC.

Methods

TNBC cases diagnosed in Sweden 2008-2020 were included in a population-based cohort study. Patient, tumour and treatment characteristics were analysed by ER status (ER-negative 0% vs ER-low 1-9%), and associations between subgroups compared using χ² test. Endpoints were overall survival (OS) and distant disease-free survival (DDFS). Kaplan-Meier curves were used to describe time-to-event endpoints and Cox proportional hazards models to estimate adjusted hazard ratios.

Results

Of the 5657 tumors, 90.1% were ER-negative and 9.9% ER-low. In the unadjusted analysis of OS, ER-low disease was associated with a borderline significantly better OS than ER-negative disease (n=3893, HR 0.83 (0.70-1.001), p=0.051), but this was restricted to patients not given chemotherapy (n=1764, HR 0.65 (0.50-0.88), p=0.002). ER-status 0% vs 1-9% did not affect OS in the multivariable analysis (HR 1.09, 95% CI 0.89-1.34). DDFS did not differ by ER-status 0% vs 1-9% (n=1299, HR 0.97 for ER-negative vs ER-low (0.62-1.53)). After preoperative treatment, the importance of pCR for OS did not significantly differ between ER-negative or ER-low disease.

Conclusions

ER-low breast cancer has characteristics and prognosis similar to ER-negative breast cancer when treated as TNBC. The use of ≥10% as threshold for ER positivity is supported by this study.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

Karolinska Institutet.

Funding

MSD.

Disclosure

I. Fredriksson: Other, Institutional, Coordinating PI: MSD. J. Hartman: Financial Interests, Personal, Full or part-time Employment: Stratipath. D. Sönmez: Financial Interests, Personal, Full or part-time Employment: MSD. H. Lindman: Other, Institutional, Invited Speaker: MSD. All other authors have declared no conflicts of interest.