A woman's breast consists of lobules. Lobules are milk-producing glands. There is also fatty and connective tissue surrounding the ducts and lobules - this is called stroma. Breast cancer by the name itself is a cancer that develops from breast tissue.The most common breast cancers start in the cells around the ducts. Others can start in the cells that line the lobules. A smaller number of breast cancers can start in other parts of the breast.Breast cancer is a tumor that has become malignant - it has developed from the breast cells. A 'malignant' tumor can spread to other parts of the body - it may also invade surrounding tissue. When it spreads around the body, we call it 'metastasis'.

The human body has two ways of moving fluid about. One is through the blood stream and the other through lymphatic vessels. It is believed if cancer cells are in lymphatic system, they are most likely to be present in the bloodstream and can spread to other organs in the body. It is difficult to test for cancer cells in the bloodstream.If breast cancer cells have got to the nodes under the arm (axillary), it will most likely swell. Whether or not it has swollen there, will decide what type of treatment a patient should have. If cancer cells are found in more lymph nodes, then the likelihood of it turning up in different parts of the body is greater. However, there is no hard and fast rule here. Women have had swellings in many nodes and did not develop metastasis, while some women with no swellings in their nodes did.