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Structure of the Breast

Breast cancer begins to develop in the cells of the breast’s mammary glands. The breast is made up mainly of mammary glands (also called lobules); of milk ducts and fatty tissue (adipose).

When a cancerous tumour settles in the mammary glands and the cancerous cells do not spread to other parts of the body, the breast cancer is considered non-invasive or “in situ” (“stays in place”). This is the most common type of breast cancer in women. At this stage this cancer can be cured in the vast majority of cases.

Sometimes the cancerous tumour can break through the original tissue membrane, at which point the breast cancer becomes invasive or infiltrating. In this case, the cancerous cells have moved away from their original tissue and been carried around the lactation ducts, notably the lymph nodes, through the blood vessels or lymph ducts. Most invasive cancers can be treated and disappear altogether.


The infiltrating cancer cells can also settle in other organs, generally the bones, the lungs or the liver. They spread outside the breast and form new masses called metastasis. These new tumours are not new forms of cancer, but rather the same breast cancer that develops in other parts of the body. This form of cancer is called “metastatic”.