Table 1.

Main classification systems of breast cancer

Classifier	Variables
Presentation	•Detection (Screen‐detected versus symptomatic). •Stage (Early stage, locally advanced, or metastatic). •*Signs and symptoms (inflammatory BC, lump size, consistency, shape, and fixation, skin and nipple changes, axilla, laterality, and focality) •Menopausal status (premenopausal versus postmenopausal). •Others: gender, age, ethnic origin, family history.
Imaging	Mass shape, margin, depth, and site, breast composition, calcification, axillary findings, laterality
Pathological
Morphological classification (mainly diagnostic and prognostic)
Tumour differentiation
Tumour type:	Several tumour types (currently at least 18 tumour types) are described and some types include multiple variants 20 based on the combination of cytological, architecture features, and secretory activity and stromal features
Tumour grade	Three grades 19 based on the degree of differentiation and similarity to TDLUs
Disease extent
Tumour stage	Invasive tumour size, infiltration of other tissues, lymph node status, and assessment of lesions at distant sites
Other factors
	Lymphovascular invasion (present or absent), presence and extent of the in situ lesions (DCIS), stromal features such as TILs, Paget's disease, focality, bilaterality, and excision status
Molecular classification (mainly predictive but can provide diagnostic and prognostic value)
Single gene classifier	Oestrogen receptor and HER2 are the most important classifiers to guide treatment decision with the addition of PDL1 Other markers include progesterone receptor (PR), KI67 as prognostic markers Familial predisposition genes such as BRCA1, BRCA2, and PALB2.
Multiple gene classifier	Multigene prognostic signatures are composed of multiple genes assessed together to assess risk in certain BC groups mainly the luminal class.
Global gene expression and genomic classification	•Intrinsic molecular subtypes including luminal, basal and HER2 enriched. •Mutation signatures •Integrated class classification based on a combination of transcriptomic and genomic (e.g. gene copy number) classification
Therapy classification	•Systemic therapy naïve versus treated patients. •Neoadjuvant therapy treated versus adjuvant treated patients. •Type of therapy (hormone, cytotoxic, targeted, or immunotherapy). •Line of therapy (first‐line therapy versus second‐ or third‐line)