The causes of upper limb edema in breast cancer are as follows: 1. If breast cancer is not treated and there is upper limb edema, especially on the affected side, it is likely to be caused by enlarged lymph nodes in the axilla, which compress the lymphatic vessels and lead to poor lymphatic return, resulting in upper limb edema, which is generally mild; 2. If the lymphatic vessels or collateral circulation are damaged, poor lymph node reflux may also lead to upper limb edema. Postoperative fluid accumulation or infection in the axillary area can lead to poor local circulation, resulting in upper extremity edema. Postoperative fibrous scar formation and poor microcirculation can also lead to upper limb edema. 3. Radiation therapy for postoperative patients, especially for the supraclavicular region and axillary region, can further aggravate upper limb edema on the affected side. If the patient has mild postoperative upper limb edema, the symptoms can be reduced by elevating the affected limb and physical therapy. However, if severe edema develops, the patient is advised to see the doctor again and undergo further treatment.