This study is investigating the biological characteristics of early oestrogen receptor (ER) positive, human epidermal growth factor receptor 2 (HER2) negative breast cancer (BC) in older woman (aged > 70 years) who were treated with adjuvant endocrine treatment (ET). It will use surgical tissue previously collected as part of routine care from patients aged 70 years or older at diagnosis with ER positive, HER2 negative stage I-III BC treated The Royal Marsden NHS Foundation Trust (RMH).

The overarching aim of this study is to define the biological characteristics of early BC in older women in terms of tumour microenvironment (TME), molecular and genomic features. This analysis will include assessment of tumour infiltrating lymphocytes (TILs), and gene expression profiling with NanoString using the Breast Cancer 360 (BC360TM) assay measuring ribonucleic acid (RNA) expression signatures of genes involved in proliferation, endothelial, angiogenesis, cytotoxicity, stroma, inflammatory chemokines, and apoptosis. This analysis will examine various biological pathways, aiming to inform suitability for certain treatments including cyclin-dependent kinase inhibitors (CDKi), chemotherapy, immunotherapy, or targeted treatments such as phosphoinositide 3-kinases (PI3K) inhibitors, or deoxyribonucleic acid (DNA) damage response.

Inclusion Criteria:

• Post-menopausal women aged >70 years at the time of diagnosis of invasive breast cancer.
• ER positive (Allred score >5/8), HER2 negative early invasive BC.
• Stage I-III BC.
• Surgical tissue available.
• No pre-surgical systemic therapy (i.e., no pre-operative ET or chemotherapy).
• Planned adjuvant ET with an aromatase-inhibitor.
• No adjuvant chemotherapy.
• Surgery was performed between 1st January 2014 and 31st December 2016.

Exclusion Criteria:

• Pre- or perimenopausal.
• ER negative or HER2-positive BC.
• Multifocal cancer where not all of foci ER positive HER2 negative
• Bilateral breast cancer.
• Stage IV breast cancer.
• Benign histology or DCIS.
• Pre-surgical treatment.
• Prior chemotherapy for BC.

This is a non-interventional, translational study, using surgical tissue (collected as part of routine care) from older women (aged > 70 years at the time of diagnosis) with early (stage I-III), ER positive, HER2 negative BC who were treated curatively with primary surgery. Patients would not have had any prior treatment with (neo-) adjuvant chemotherapy but received an aromatase-inhibitor in the adjuvant setting.