The lymphatic system is often referred to as the body’s "secondary circulatory system". The lymphatic system collects excess fluid in the body’s tissues and returns it to the bloodstream.

Lymph formation occurs at the microscopic level. During the exchange of fluid and molecules between the blood circulation and body tissues, blood capillaries may not reabsorb all of the fluid. Surrounding lymphatic capillaries absorb the excess fluid. The fluid is then filtered and transported back by the lymphatic system into large veins near the heart.

The lymphatic system can play a very worrisome role in the spread of breast cancer. Components of the lymphatic system called lymph nodes are distributed at specific locations throughout the body. There is also an extensive network of lymphatic vessels in every woman’s breast tissue, which is important in regulating the local fluid balance as well as in filtering out harmful substances.

The lymph vessels in the breast may inadvertently supply cancerous cells with access to a highway along which the cancerous cells can move to other parts of the body. This process is called metastasis and may result in the formation of a secondary cancer mass in a different location of the body. Regular breast self examinations can help to detect tumors earlier in their growth, hopefully before they spread quickly or metastasize.