One of the scary things about tumors is their ability to spread in the body. Today, let’s talk about tumor metastasis. What is metastasis? Tumor cells can spread to localized normal tissues, regional spread through metastasis to nearby lymph nodes, tissues or organs, and also spread to distant parts of the body. Once a tumor forms at a distant location, it is known as a metastatic tumor and may also be called a stage IV tumor, and this process of spreading is called metastasis. When viewed under a microscope, metastatic tumor cells have the characteristics of the primary cancer, and it is this that doctors use to determine that the cells have metastasized from other sites. Metastatic tumors have the same name as the primary cancer, e.g., breast cancer that metastasizes to the lungs is called metastatic breast cancer and is considered stage IV breast cancer, not lung cancer. Sometimes the origin of a metastatic tumor is difficult to determine, and this is referred to as a cancer of unknown origin. If a patient with a previous tumor is diagnosed with a new primary cancer, it is called a second primary cancer. Second primary cancers are rare, and most patients who develop another tumor have a recurrence of the primary tumor. How do tumors metastasize? Tumor cells generally metastasize in the body through the following steps: 1) grow into or invade nearby normal tissues; 2) move along lymph nodes or blood vessel walls; 3) flow through the lymphatic system or blood to other parts of the body; 4) stop in small distant blood vessels, invade the vessel walls and enter into the surrounding tissues; 5) grow in that tissue until small tumors form; 6) cause new blood vessels to be created to provide blood supply for the tumor and continue growth. In most cases, tumor cells will die during the metastasis process. However, as long as each step favors the tumor cells, it is possible for a new tumor to form. Metastatic tumor cells can also remain active for many years at a distant site before they begin to grow. Where can tumors metastasize? Tumors can metastasize to many parts of the body. The most common sites of metastasis are bone, liver and lung. The following table shows the common metastatic sites of some tumors: How should metastatic tumors be treated? Once a tumor has spread, it can be difficult to control. While certain types of metastatic tumors can be cured with current methods, more often than not, they cannot. Even so, there are treatments available for these patients, when the goal of treatment is to stop or slow the growth of the tumor and reduce the symptoms it causes. The specific treatment depends on the type of primary tumor, the site of metastasis, previous treatment received, and overall health status.