In some women, cancer cells break away from the primary breast cancer and spread to other parts of the body in the bloodstream or lymphatic system (see diagram below). The cells may lie dormant (inactive) for years before they begin to grow again.

The lymphatic system is part of the immune system − the body's natural defence against infection and disease. It is a complex system made up of organs, such as bone marrow, the thymus, the spleen, and lymph nodes (lymph glands). The lymph nodes throughout the body are connected by a network of tiny lymphatic ducts.

Some time after breast cancer treatment, if cancer cells have broken away and spread to other organs of the body they may begin to grow and cause symptoms. This is known as secondary breast cancer.

Secondary cancers are also called metastases, so another name for secondary breast cancer is metastatic breast cancer. The most common places that breast cancer cells spread to are the bone, liver, lung or brain. Secondary breast cancer can also affect the lymph glands. It is most likely to affect the lymph nodes in the armpits or the lower part of the neck.

Breast cancer cells do not usually spread to many places in the body at once. Although it is possible for secondary breast cancer to affect more than one place at a time, it more commonly affects just one or two parts of the body.

Each woman's situation is individually assessed and the appropriate treatment given depending on where the breast cancer has spread to. A woman with secondary breast cancer affecting the bones will have different symptoms and may need different treatment from a woman with secondary breast cancer affecting the liver.