One main lymph node area (the armpit, or "axilla") and two secondary lymph node areas (the internal mammary and supraclavicular regions) filter the lymph fluid draining away from the breast area.
Since the job of the lymph nodes is to filter out "bad guys" like cancer cells, this is a logical place to look for breast cancer cells that have escaped the original tumor and are trying to go elsewhere in the body. Cancer cells may also leave the breast through the bloodstream and bypass the lymph nodes. However, the presence ("node-positive") or absence ("node-negative") of cancer in the lymph nodes is one of the most important signposts your doctor will use to determine the best treatment for you.
Another purpose of lymph node dissection is to remove cancer that might be in the nodes. This is done, so that the cancer can't grow further in the lymph node area or shed cells that could go elsewhere.
Doctors once believed that removing as many lymph nodes as possible would reduce the risk of cancer ever spreading to the rest of the body. The hope was that if you caught every last possible cell that could be in those lymph nodes and "cured" the armpit and breast of any cancer, you could "cure" the rest of the body. But lymph node removal does nothing to fight cancer cells that may have already spread elsewhere. That's where systemic, or whole-body, treatment comes in—to kill any cells that escaped the original breast cancer or the adjacent lymph nodes.
Until recently, doctors wanted to take out as many nodes as possible to determine as accurately as possible how many lymph nodes are involved with cancer. The number of involved lymph nodes strongly predicts the nature of the cancer and the kind of treatment needed to fight it.