The cancer itself, treatment, or infection can block lymph nodes or vessels in the lymphatic system, which can lead to swelling. When treatment removes or damages lymph nodes or vessels, fluid builds up in surrounding tissue causing swelling. Because breast cancer surgery usually involves the removal of one or more lymph nodes, lymphedema is a common complication of treatment. Radiation therapy can also cause damage to lymphatic vessels. The more lymph nodes and vessels removed, the higher the risk of developing lymphedema.