What is an axillary lymph node dissection?
UCLA Health
If breast cancer spreads, it goes to the axillary, or underarm, lymph nodes first. Axillary lymph node dissection used to be standard for anyone with breast cancer. An axillary lymph node dissection cleans out fatty and lymph node–bearing tissue, which is then sent for pathologic analysis. A variable number of lymph nodes are looked at, usually between about 10 and 30. The downside is that it’s a fairly extensive surgery that can lead to complications, the most common of which is lymphedema, a swelling of the arm on the side of the surgery. Lymphedema can occur in as many as 20% to 25% of people who have had an axillary lymph node dissection.
Axillary lymph node dissection involves removing the level I and level II lymph nodes under the arm for those patients who have cancer present in their lymph nodes. 
In axillary lymph node dissection (ALND), anywhere from about 10 to 40 (though usually less than 20) lymph nodes are removed from the axilla (the area under the arm). This is done so they can be checked for cancer spread. ALND is usually done at the same time as mastectomy or lumpectomy, but it can be done in a second operation. This was once the most common way to check for breast cancer spread to nearby lymph nodes, and it is still done in some patients. For example, an ALND may be done if a previous biopsy showed that one or more of the underarm lymph nodes contained cancer cells.

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