How Cancer Spreads
As a malignant tumor grows, it may spread locally, invading and sometimes destroying other tissues, or cells may break away from the tumor and get into the lymphatic vessels, or into the blood vessels, and travel to distant parts of the body. Some of the breakaway cells will be trapped in the lymph nodes of the armpit, or axilla. Examination of these nodes by a procedure called axillary lymph node dissection, can help determine the stage (the degree of spread) of the cancer.
If cancer cells escape beyond the lymph nodes, or enter the circulatory system directly, they can spread to the liver, brain, lungs, and bones, forming new tumors called metastases. These distant metastases are the most worrisome, because they can damage vital organs. This advanced stage of breast cancer, called metastatic cancer, is less common and its management is more difficult.
To make sure that no cancer cells remain anywhere in the body, it is often necessary to use systemic therapy—therapy that reaches all the organs, in all parts of the body, by means of the blood stream. This is explained in the SYSTEMIC THERAPY section.