Breast cancer is still the most common cancer worldwide. But the way breast cancer
is viewed has changed drastically since its molecular hallmarks were extensively characterised,
now including immunohistochemical markers (eg, ER, PR, HER2 [ERBB2], and proliferation
marker protein Ki-67 [MKI67]), genomic markers (eg, BRCA1, BRCA2, and PIK3CA), and
immunomarkers (eg, tumour-infiltrating lymphocytes and PD-L1). New biomarker combinations
are the basis for increasingly complex diagnostic algorithms. Neoadjuvant combination
therapy, often including targeted agents, is a standard of care (especially in HER2-positive
and triple-negative breast cancer), and the basis for de-escalation of surgery in
the breast and axilla and for risk-adapted post-neoadjuvant strategies. Radiotherapy
remains an important cornerstone of breast cancer therapy, but de-escalation schemes
have become the standard of care. ER-positive tumours are treated with 5-10 years
of endocrine therapy and chemotherapy, based on an individual risk assessment. For
metastatic breast cancer, standard therapy options include targeted approaches such
as CDK4 and CDK6 inhibitors, PI3K inhibitors, PARP inhibitors, and anti-PD-L1 immunotherapy,
depending on tumour type and molecular profile. This range of treatment options reflects
the complexity of breast cancer therapy today.