During breast cancer surgery (mastectomy or lumpectomy), some of the lymph nodes in your underarm (called axillary lymph nodes) may be removed. They are checked to see if they contain cancer cells. When axillary lymph nodes are removed during breast surgery (with sentinel node biopsy or axillary dissection) or treated with radiation therapy, some of the lymph vessels can be blocked. This may prevent lymph fluid from leaving the area. Lymphedema occurs when lymph fluid collects in the arm (or other area such as the hand, fingers, chest/breast or back), causing it to swell (edema). The swelling may be so slight it’s barely seen or felt. Or, it may be so great the arm grows very large. In severe cases, lymphedema can cause pain and limit movement. Also, it can be quite upsetting to have one arm larger than the other, even if the change is small.
Source: Susan G. Komen 2019