Metastasis is the spread of a disease from one organ or part to another non-adjacent organ or body part. The new site is called a metastatic lesion. Metastasis typically occurs via the bloodstream or lymphatics, but can also occur through direct extension. Cancer cells are most likely to metastasize to organs that are in close proximity to the original tumor, as well as organs that have a good blood supply. Common sites of metastasis include the lungs, liver, brain, and bones.
The process by which cancer cells metastasize is complex and not fully understood. It is thought that cancer cells first detach from the primary tumor and enter the bloodstream or lymphatics. They then travel to other parts of the body and begin to grow and form new tumors (metastases). Cancer cells that metastasize often undergo changes that allow them to survive in their new environment better than normal cells would be able to survive. For example, they may become more resistant to cell death or more adept at evading the immune system.
There are several factors that contribute to whether or not a cancer will metastasize. These include:
• The type of cancer: Some types of cancer are more likely than others to metastasize. For example, breast cancer and melanoma are relatively common cancers that often metastasize; whereas, thyroid cancer and testicular cancer tend not to spread beyond their original site very often.
• The stage of the cancer: Metastasis is more likely when a tumor is larger and has spread beyond its original location (i). Early-stage cancers are less likely than late-stage cancers to have already begun spreading elsewhere in the body at diagnosis. However, even early-stage cancers can sometimes spread rapidly and become difficult to treat effectively (ii). In contrast, some late-stage cancers may never develop any distant metastases despite being large primary tumors (iii).
• The grade of the tumor: Tumors with higher grades (i.e., those that look more abnormal under a microscope) are generally more likely than low-grade tumors