Why would a woman who has undergone a left radical mastectomy have a swollen left arm?

Lymph nodes removed: A radical mastectomy involves removal of all the breast tissue, the chest wall muscle, and the axillary nodes. This is rarely done today unless the tumor directly involves the muscle. The axilla contains nodes that not only drain the breast but also fluid from the arm. If too many of the letter are removed or the pathways damaged fluid backs up and causes swelling.

Lymphedema: Swelling after surgery is secondary lymphedema due to lymph node excision with radical mastectomy.