Metastasis describes the process by which cancer cells detach from their original tumor and spread to other areas of the body, establishing new tumors. This phenomenon represents an advanced stage of cancer progression, often called metastatic cancer or stage 4 cancer. It involves cancer cells gaining the ability to migrate and thrive in distant environments.
How Cancer Spreads
The journey of cancer cells from a primary tumor to distant sites involves several steps. The initial stage is invasion, where cancer cells break away from the main tumor and grow into nearby normal tissue. Unlike healthy cells, cancer cells are less “sticky” and produce enzymes that break down surrounding tissue, creating a path for escape.
Following invasion, cancer cells undergo intravasation, penetrating the walls of nearby blood or lymphatic vessels. Once inside these circulatory systems, they travel through the bloodstream or lymphatic fluid. This stage is known as circulation, where the cancer cells, called circulating tumor cells (CTCs), navigate the body’s vascular networks. Most spreading cancer cells do not survive this journey, but some endure.
Upon reaching a new location, CTCs adhere to the inner walls of small blood vessels and then exit into the surrounding tissue, a process termed extravasation. The final step is colonization, where these cells multiply and establish a new tumor, also known as a metastatic or secondary tumor. For this new tumor to grow, it must stimulate the formation of new blood vessels, a process called angiogenesis, to ensure a blood supply.
Common Locations for Metastasis
Cancer can spread to almost any part of the body, though certain types of cancer tend to metastasize to specific organs more frequently. The most common sites for metastatic tumors include the bones, lungs, liver, and brain. The pattern of spread is influenced by factors such as organ proximity to the primary tumor, blood and lymphatic flow patterns, and the distant site’s cellular environment.
For instance, breast cancer often spreads to the bones, lungs, liver, and brain. Colorectal cancer frequently metastasizes to the liver due to its direct connection via the portal vein system. Lung cancer commonly spreads to the brain, bones, and adrenal glands. Prostate cancer often targets the bones, especially the spine and pelvis. Symptoms of metastatic cancer often relate to the organ where the new tumor has formed, such as bone pain from bone metastases or breathing difficulties from lung metastases.
Diagnosing Metastatic Cancer
Detecting metastatic cancer involves a combination of advanced imaging techniques and laboratory tests. These include computed tomography (CT) scans, which provide detailed cross-sectional images, and magnetic resonance imaging (MRI), which uses strong magnetic fields and radio waves to create detailed images of organs and soft tissues.
Positron emission tomography (PET) scans are also used, detecting metabolically active cancer cells throughout the body with a radioactive tracer. Bone scans identify cancer that has spread to the bones. Blood tests for tumor markers can provide clues, though they are not definitive for diagnosis alone. A biopsy of a suspected metastatic tumor, where a tissue sample is examined under a microscope, is often necessary to confirm cancer and determine its origin.
Treating Metastatic Disease
Treating metastatic cancer focuses on controlling the disease, alleviating symptoms, improving quality of life, and extending survival, as a complete cure is often not achievable. Systemic therapies are commonly used because they can reach cancer cells throughout the body. These include chemotherapy, which uses drugs to kill rapidly dividing cells, and targeted therapy, which blocks pathways that promote cancer growth.
Immunotherapy harnesses the body’s own immune system to fight cancer cells, while hormone therapy is used for hormone-sensitive cancers like breast or prostate cancer. Localized therapies may also be employed for specific metastatic sites. Radiation therapy can target and destroy cancer cells in a particular area, such as a bone metastasis causing pain. Surgery may be considered in select cases to remove isolated metastatic tumors, especially if they cause significant symptoms or are amenable to removal.