Metastasis is when cancer cells spread from their original location to another part of the body (i.e., the cancer metastasizes). During this process, cancer cells breakaway from the initial (primary) tumor, travel through the blood or lymphatic system, and create new tumors in different organs. Tumors are defined by their tissue of origin (e.g., lung cancer that spreads to the bones is considered lung cancer, not bone cancer).

It is important to distinguish the difference between local and distant metastasis. Local metastasis refers to when the cancer has spread to nearby tissues or lymph nodes, while distant metastasis is when cancer travels to further parts of the body, such as the lungs, liver, or bones.

Additionally, metastases can be categorized as early or advanced based on the extent of the spread. Early metastasis typically involves fewer, smaller cancerous spots and tends to be more localized to the tissue of origin, whereas advanced metastasis indicates more widespread and numerous cancerous sites, often associated with later stages of the disease. These differences matter in the context of determining how the cancer is staged and what treatment options are best.