If a lymph vessel or node becomes blocked, the lymph fluid is unable to pass along it. Because the excess tissue fluid cannot drain away normally, it builds up and causes swelling. The medical name for swelling is oedema (pronounced e-dee-ma). When the swelling is due to a problem in the lymph system, it is called lymphoedema.

Lymphoedema is not always related to cancer.

Non-cancer related lymphoedema

Also called primary lymphoedema, this occurs because the lymphatic system has not formed properly, but this is rare. It can also be caused by other medical conditions that affect the lymphatic system.

Cancer-related lymphoedema

Also called secondary lymphoedema, this is caused by cancer or its treatment. It can occur if the lymph nodes are blocked with cancer or if they have been removed by surgery. Radiotherapy can also cause lymphoedema by causing a build-up of scar tissue within the lymph nodes.

Not everyone who has radiotherapy to the lymph nodes will get lymphoedema. Lymphoedema is more likely to occur if surgery and radiotherapy are both given together to the same area - for example, when lymph nodes are removed from the armpit during surgery for breast cancer and the area is then treated with radiotherapy.

Surgery or radiotherapy given on their own to a part of the body are less likely to result in lymphoedema.