Most breast cancers develop from the cells which make up the lining of the glands in the breasts and the fine tubes (ducts) which lead from those glands to the nipple.  

Cancers which develop from these lining cells (also known as epithelial cells) are called carcinomas.

Breast cancers can be grouped into two main types: ‘in situ’ carcinomas and ‘invasive’ carcinomas.

In situ carcinomas are where there are cancerous changes in the epithelial cells but these malignant cells are still limited to the lining of the glands or ducts, and have not begun to invade, or penetrate, into the surrounding tissue.   Invasive carcinomas are where the cancerous cells have broken through the lining layer and begun to eat into, and damage, the tissue surrounding the breast ducts.

If a lump is removed from the breast and is found to contain a cancer then more than 95 out of every 100 will be invasive carcinomas.

There are several different types of invasive carcinoma of the breast.  These can only be told apart by looking at them under the microscope, where the cells of the different types have different appearances.  Although some 16 different forms of invasive breast carcinoma have been described the majority fall into one of three types.  These are:

• ductal carcinomas of no specific type (IDCNST): this is by far the commonest of the invasive breast cancers.   It makes up between 6 to 8 out of every 10 of invasive breast cancers.
• infiltrating lobular carcinomas: these make up slightly more than 1 in 10 of all invasive breast cancers.  Breast cancers can be ‘graded’ by their appearance under the microscope to give an idea of how aggressively they will behave.   Infiltrating lobular carcinomas are usually of intermediate grade with a reasonable outlook but some are high grade and more aggressive: these are called pleomorphic lobular carcinomas. 
• tubular carcinomas:  these are uncommon, making up between 1 in 25 to 1 in 50 invasive breast cancers (but they are more common among those cancers picked up by breast screening).  These are low grade tumours which means they usually have a very good outlook with more  than 9 out 10 of these cancers being cured.

Sometimes a breast cancer may be made up of a mixture of several different types of invasive cancer. 

Other types of invasive breast cancer are rare.  These include mucinous, medullary, cribriform, spindle cell/metaplastic, adenoid cystic and squamous cancers.