If you have invasive breast cancer, your surgeon will probably remove some of the lymph nodes under your arm during your lumpectomy or mastectomy. Examining your lymph nodes helps your doctors figure out the extent of cancer involvement. Cancer in the lymph nodes is associated with an increased risk of having cancer cells in other parts of your body.

Your lymph nodes act as filters for your body's lymphatic drainage system. That's why the lymph nodes are likely to "catch" or filter out cancer cells that might be floating in the fluid that drains away from the cancerous area of the breast. Doctors look at the different kinds of nodes that are involved with your breast:

• The nodes around your collarbone and neck (supraclavicular, infraclavicular, and cervical nodes) are examined manually (by hand). Your doctor will feel this area for signs of enlarged nodes.
• The nodes under your arm (axillary lymph nodes) are also examined manually and are relatively easy to get to during surgery. Surgery to remove some or all of the lymph nodes under your arm is called axillary lymph node dissection.

You can learn more about possible side effects of lymph node removal in our section on Arm Lymphedema.