Metastatic cancer refers to cancer that has spread from its original site in the body to other, distant locations. This process, known as metastasis, is a more advanced stage of the disease. Understanding how cancer cells travel and establish new tumors elsewhere is important for comprehending cancer progression.
How Cancer Spreads
Cancer spreads when cells detach from the primary tumor. These cells acquire the ability to invade surrounding normal tissue, a process called local invasion. During this invasion, cancer cells break through the basement membrane, a thin layer of tissue that separates different cell types.
After invading local tissues, cancer cells enter nearby blood vessels or lymphatic vessels, a step known as intravasation. This allows them to travel throughout the body. Cancer cells can circulate as individual cells or as small clusters.
During circulation, many cancer cells perish due to the harsh conditions of the bloodstream or attacks from the immune system. However, a small proportion of these cells survive and eventually arrest in small blood vessels at a distant site. They then exit these vessels and invade new tissues, a process called extravasation.
Once extravasated, these cancer cells begin to grow and form new tumors in the distant location, a process called colonization. Colonization is a complex step of metastasis, requiring cancer cells to adapt to and thrive in a new microenvironment. This can involve evading local immune defenses and stimulating the growth of new blood vessels to supply the developing secondary tumor.
Where Metastasis Occurs and Its Symptoms
Cancer can spread to almost any part of the body, but certain organs are more common sites for metastasis. These include the bones, lungs, liver, and brain. Other possible sites include lymph nodes, skin, and adrenal glands.
Specific types of cancer tend to metastasize to particular organs. For instance, breast cancer commonly spreads to the bones, brain, liver, and lungs. Lung cancer often metastasizes to the adrenal glands, bone, brain, liver, and other parts of the lung.
Symptoms of metastatic cancer vary depending on the location and size of the secondary tumors. When cancer spreads to the bones, it can cause pain and increase the risk of fractures. Lung metastasis may lead to symptoms such as coughing, chest pain, or shortness of breath.
If cancer metastasizes to the liver, individuals might experience jaundice (a yellowing of the skin or eyes) or swelling in the abdomen. Brain metastases can manifest as headaches, seizures, dizziness, vision problems, or difficulties with speech. General symptoms that can occur with advanced cancer, regardless of the specific metastatic site, include fatigue, unexplained weight loss, and night sweats.
Detecting and Treating Metastatic Cancer
Detecting metastatic cancer involves a combination of diagnostic methods. Imaging scans identify secondary tumors throughout the body. These include computed tomography (CT) scans, which provide detailed cross-sectional images, and magnetic resonance imaging (MRI) scans, which are particularly useful for examining the brain and spinal cord.
Positron emission tomography (PET) scans, often combined with CT (PET/CT), detect cancer cells by identifying areas of increased metabolic activity, as cancer cells absorb more radioactive sugar than normal cells. Bone scans are specifically used to check for cancer spread to the bones, involving a radioactive substance that accumulates in areas of rapid bone growth or repair.
Biopsies, where a tissue sample is removed for laboratory testing, confirm the presence of cancer cells and determine their type. Blood tests can also detect circulating tumor cells or specific tumor markers, offering less invasive ways to monitor for cancer spread. These diagnostic tools help medical teams understand the extent of the disease and plan treatment.
Treatment for metastatic cancer is systemic, meaning it targets cancer cells throughout the body, rather than just at a single site. Common approaches include chemotherapy, which uses powerful drugs to kill or slow the growth of cancer cells. Targeted therapy focuses on specific molecular features of cancer cells that drive their growth, while immunotherapy helps the body’s immune system recognize and destroy cancer cells more effectively.
Radiation therapy can be used to shrink tumors and alleviate symptoms in specific metastatic sites. Surgery may also be an option for removing some metastatic tumors, especially if they are causing symptoms or are limited in number. The aim of these treatments is to control the disease, manage symptoms, and extend life, as a complete cure is not always possible for many metastatic cancers.