One of the most important factors influencing treatment plans and overall prognosis for breast cancer is the tumor stage. Tumor stage is related to how large the tumor is, as well as whether or not it has spread to other parts of the body, such as the regional lymph nodes. 

The primary lymph nodes draining the breast are found in the armpit (also known as the axilla). In order to see if the cancer has spread from the breast to the axilla, a surgical biopsy of a small number of axillary lymph nodes can be done at the time of lumpectomy or mastectomy. In certain cases where the lymph nodes are known to be involved with cancer before surgery, removal of a larger number of lymph nodes at the time of lumpectomy or mastectomy will likely be recommended to help with tumor staging and prevent the cancer from coming back in the area.