Why does breast cancer spread to other parts of the body?

Cancer is by definition abnormal growth of tissue which spreads and causes damage. The actual mechanism why and how it happens is the subject of a great deal of study but appears to involve small groups of cancer cells invading and breaking off in blood and lymph vessels and being carried along to distant tissues.

It is a bit like a tree falling into a river during a storm and being carried downstream. The cancer cells somehow stick in the new tissue and develop a growing clump of cancer cells called a metastasis. The reason for this may be as simple as the blood vessel is too small and the clump sticks and starts to grow. The metastasis disrupts the tissue and causes disease.

In the bone it weakens causing pain and fractures. It often goes to the liver lungs and brain. If it has spread and become detectable in one tissue it is likely to be present elsewhere. There is no way to cut them all out.

Chemotherapy can treat the whole body and may be effective. Radiotherapy can be used on local areas to control pain.