A metastasis is the spread of tumour cells from the place where the tumour started to a different part of the body. Common locations for a metastasis include lymph nodes, lungs, liver, bones, or brain. Before tumour cells spread to another area of the body, they must enter a blood vessel or lymphatic vessel. Pathologists describe this as lymphovascular invasion.
When performing surgery to remove a tumour, lymph nodes in the areas of a tumour are often also removed. A pathologist then examines the lymph nodes under a microscope to look for tumour cells. Tumour cells found in a lymph node are called a lymph node metastasis.
A small sample of tissue can also be removed from an organ such as the lungs or liver to look for tumour cells that may have traveled there from a tumour in a different part of the body.
The ability to metastasize is usually associated with cancers, however, there are some non-cancerous tumours that are also known to metastasize.
For many types of cancers, the patient is harmed more by the metastatic disease than by the primary tumour (which can often be removed surgically). In addition, some types of cancer are much more likely to metastasize to lymph nodes while others metastasize to other organs such as the lungs, liver, bones, or brain.
For most types of cancers, metastatic disease is used to determine the pathologic stage. Tumour cells found in a lymph node are used to determine the nodal stage (pN). Tumour cells found in another organ are used to determine the metastatic stage (pM). Metastatic disease increases the overall pathologic stage and is associated with worse prognosis.