Breast cancers can have different degrees of malignancy

Breast cancer is the uncontrolled growth of abnormal cells in one or both of the breasts, causing lump (tumour) growth. Certain lumps grow in place without affecting surrounding tissues (benign tumours) while others have the capacity to metastasize and compromise the functioning of other organs. These tumours are life-threatening and are called malignant (cancer). Pathologists distinguish benign and malignant tumours by looking at their microscopic appearance under the microscope.

Also see Causes and Risk factors.

In high-risk populations like Europe, the majority of breast cancers are epithelial tumours localised to the milk ducts (85%) or lobules (15%). Less than 1% are sarcomas and lymphomas arising in other tissues within the breast. Most ductal and lobular cancers of the breast have already spread beyond the epithelium of the milk ducts to the surrounding tissue when diagnosed, and are called invasive. Mammography can detect the cancer in an early stage when it is still non-invasive, see Interventions.

The ICD provides separate codes for malignant invasive- (ICD-9 code 174 in females, 175 in males; ICD-10 code C50) and non-invasive or in situ cancers (ICD-9 code 233.0; ICD-10 code D05).

Stage at first detection is crucial for prognosis

The stage of cancer at first diagnosis is the most important determinant of the outcome of the disease and allows doctors to decide on treatment regimens and make a prognosis. It describes the extent of growth and invasion of other tissues by the tumour. The most widely used staging systems are the TNM developed by the UICC and its re-groupings by the AJCC. Each group summarizes the behaviour (invasive or not), size of the tumour (in centimetres), whether and how many regional lymph nodes are involved and whether it involves other organs. Accurate staging requires that the patient undergoes a set of ad hoc examinations, such as for example chest x-rays to detect metastases to the lungs. For example, tumours up to 1cm in size that have not invaded regional lymph node or other organs are said 'early tumours' and belong to stage group I (AJCC/TNM); tumours that have extended to the skin of the breast or the chest wall (group IIIb) or other non-adjacent organs (group IV) are said advanced.

In addition to stage of disease, the expression of oestrogen receptors in the tumour tissue is an established prognostic marker in breast cancer. Breast cancer cases with positive receptor status tend to have a greater benefit from systemic hormonal therapy.

Also see Interventions.