Metastasis is the process by which cancer cells spread from their original location to other areas of the body, forming new tumors. When cancer metastasizes, it is still identified by its primary site. For instance, breast cancer that has spread to the lungs is termed metastatic breast cancer, not lung cancer. The new growths in distant locations are known as metastatic tumors or metastases.
The Process of Metastasis
The journey of a cancer cell from a primary tumor to a new site is a multi-step process. It begins with local invasion, where cancer cells break away from the original tumor and push into adjacent healthy tissue. The cells secrete digestive enzymes to break down the barriers that normally keep tissues separate, allowing them to migrate.
Following invasion, cancer cells enter a transport system in a step called intravasation. They penetrate the walls of nearby lymph or blood vessels and are carried through the lymphatic system or bloodstream to other parts of the body. This journey is perilous, and most cancer cells do not survive it.
If a cancer cell survives the circulatory journey, it must exit the vessel in a process known as extravasation. The cell adheres to the vessel wall and moves through it into the surrounding tissue at a distant location.
The final stage is colonization, where the cancer cell multiplies to form a new tumor. To grow beyond a microscopic size, this new tumor must develop its own blood supply through a process called angiogenesis. This provides the nutrients needed for the metastatic tumor to grow to a detectable size.
Common Locations for Metastatic Tumors
Cancer can metastasize to almost any part of the body, but the lungs, liver, bones, and brain are the most frequent sites. The specific location depends on the type of primary cancer. For example, colon cancer often spreads to the liver because blood from the colon circulates directly there.
The pattern of metastasis is explained by the “seed and soil” theory. This concept suggests that cancer cells (“seeds”) can only thrive in organs that provide a suitable environment (“soil”). The characteristics of both the cancer cells and the destination organ determine if a metastatic tumor will form.
Diagnosing Metastatic Cancer
Detecting metastatic cancer involves imaging tests and laboratory analyses. Imaging tests like CT scans, PET scans, and MRIs create detailed pictures of organs and tissues to look for new tumors. Bone scans are used specifically to detect cancer that has spread to the bones.
A biopsy provides a definitive diagnosis. A small tissue sample is removed from the suspected metastatic tumor and examined under a microscope. The appearance of the cells will match those of the primary tumor, confirming the cancer has spread.
Tumor markers can also aid in diagnosis and monitoring. These are substances, often proteins, found in blood, urine, or body tissues. Elevated levels of certain tumor markers can indicate the presence and growth of specific cancers and are used with other diagnostic tools to assess the disease.
Treating Cancer That Has Spread
Treatment for metastatic cancer is systemic, designed to reach cancer cells throughout the body. This approach is the primary way to manage the disease because it may have spread to multiple locations, some of which are too small to detect. Common systemic therapies include:
- Chemotherapy uses drugs to kill fast-growing cells. These drugs travel through the bloodstream to reach cancer cells anywhere in the body, with the specific drugs depending on the primary cancer type.
- Targeted therapy focuses on specific genetic mutations or proteins in cancer cells. These drugs interfere with the molecules that cancer cells use to grow and spread, offering a more precise approach than chemotherapy.
- Immunotherapy helps the body’s immune system recognize and attack cancer cells. Treatments can boost the immune response generally or be tailored to identify specific features of the cancer.
- Hormone therapy is used for hormone-sensitive cancers, like certain breast and prostate cancers. It works by blocking hormone production or interfering with their effects on cancer cells.
While systemic treatments are the main focus, local therapies like radiation and surgery may also be used. These treatments can help manage symptoms, such as pain, or treat a limited number of metastatic tumors in a specific area. The combination of treatments is tailored to the individual based on the cancer type, its location, and the person’s overall health.