When breast cancer becomes invasive, the first place it spreads to is the axillary lymph nodes, which are the lymph nodes under your arm. In order to better understand your cancer and determine its stage and how far it has spread, it is important for lymph nodes to be evaluated. If your cancer is not metastatic, the lymph nodes that have cancer will often be removed via surgery (usually at the same time as a lumpectomy or mastectomy). 

Ways to Evaluate Lymph Nodes

Lymph Node Biopsy
If one or more of your lymph nodes are swollen, you will most likely have a biopsy to determine if the lymph node has cancer. The most common types of biopsies are the Fine Needle Aspiration Biopsy and the Core Needle Aspiration Biopsy. A Core Needle Aspiration biopsy is slightly more informative that a Fine Needle Aspiration Biopsy, but it does involve a slightly larger needle (still relatively small). 
If the lymph node biopsy is positive for cancer cells, you will most likely need further surgery to remove the cancerous lymph nodes in order to remove the cancer and reduce the recurrence risk. 

There are two common types of lymph node surgeries: a sentinel lymph node biopsy (SLNB) and an axillary lymph node dissection (ALND) 

Sentinel Lymph Node Biopsy 
A sentinel lymph node biopsy is a procedure that identifies the sentinel lymph nodes (the first axillary lymph nodes that cancer would spread to). A surgeon will inject a radioactive blue dye into the breast. This dye will then travel to the 1st draining lymph nodes (or the sentinel lymph nodes), allowing the surgeon to identify them. Then, the surgeon will remove these lymph nodes, and they will be checked for cancer cells. 
If these lymph nodes are positive for cancer cells (>.2mm), the surgeon will most likely perform an axillary lymph node dissection (ALND). There are a few exceptions to this (such as women who have micrometastasis (0.2-2mm of cancer) in their lymph nodes and are getting a mastectomy already). However, if the SLNB is negative (<.2mm of cancer), an ALND is unnecessary. 
*Those with inflammatory breast cancer usually do not get SLNB's, as inflammatory breast cancer plugs up the lymphatic system in the skin, and therefore the blue dye method used in SLNB's will not be accurate. 

Axillary Lymph Node Dissection (ALND)
An axillary lymph node dissection is a procedure in which a large number of axillary lymph nodes are removed (usually after an SLNB or during a mastectomy/lumpectomy). An axillary lymph node dissection will most likely be necessary if: 
-an SLNB has shown macrometastasis (>2mm) of cancer cells in the sentinel lymph nodes
-a lymph node biopsy (FNA or CNB) shows cancer in the lymph nodes

Side Effects of SLNB/ALND 
Although both SLNB's and ALND's have similar side effects, these side effects are more common after an ALND, as it is a larger and more invasive procedure. 
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Possible side effects include: 

-pain, blood clots, and infections are possible 

-numbness

-lymphedema is a possible side effect of lymph node surgery. Lymphedema is tissue swelling (edema) caused by blockage in the lymphatic system (due to lymph node surgery) that prevents the drainage of protein-rich fluid. This condition can lead to a decreased range of motion in the affected arm, along with an increased risk of sepsis and skin infections. However, this condition is treatable with certain medications and other methods such as compression sleeves. Furthermore, exercise is a very effective way to reduce the risk of lymphedema, with a risk reduction of 15%; 22% of patients experience some form of lymphedema, but only 7% of patients who exercise experience lymphedema.