Metastasis is the process where cancer cells detach from their original tumor and spread to other parts of the body, forming new tumors. These new tumors, called metastatic or secondary tumors, are composed of cells identical to the primary cancer. This spread can lead to health complications by interfering with distant organ function.
How Cancer Cells Spread
Cancer cells spread through several biological steps. First, they detach from the primary tumor, being less “sticky” than normal cells. They then invade surrounding healthy tissues, often by producing enzymes like matrix metalloproteases (MMPs) that break down the extracellular matrix.
Once free, circulating tumor cells (CTCs) can enter the bloodstream or lymphatic system via intravasation. They travel through these systems, sometimes forming clumps with platelets to survive immune attacks. However, most CTCs do not survive this journey due to immune detection and harsh conditions.
Surviving cells then undergo extravasation, exiting vessels at a distant site. They invade the surrounding tissue of the new organ. To establish a new tumor, these cells must adapt to the new microenvironment and grow, often by inducing angiogenesis to secure their own blood supply.
Common Locations of Metastasis
Cancer cells can metastasize to almost any part of the body, but certain organs are more frequently affected. Common sites for metastatic tumors include bones, liver, lungs, and brain. These sites are often susceptible due to their rich blood supply, making it easier for CTCs to settle.
The specific organs where cancer spreads also depend on the primary cancer type. For example, prostate cancer often metastasizes to bones, while colorectal cancer frequently spreads to the liver. Breast cancer commonly metastasizes to bones, lungs, liver, and sometimes the brain. This organ-specific pattern is explained by the “seed and soil” theory, suggesting certain cancer cells (“seeds”) thrive in particular organ environments (“soil”).
Impact on Diagnosis and Treatment
Metastasis significantly impacts cancer diagnosis and treatment. When cancer spreads to distant parts of the body, it is classified as Stage IV cancer. This indicates advanced disease and often correlates with a less favorable prognosis compared to localized cancer.
Diagnosing metastasis often involves imaging techniques like PET-CT scans, though secondary tumors may not cause immediate symptoms. Treatment for metastatic cancer typically shifts from localized approaches, such as surgery or radiation, to systemic therapies. These systemic treatments, including chemotherapy, targeted therapy, and immunotherapy, reach cancer cells throughout the body.
While metastatic cancer is often not curable, treatments can help manage symptoms, slow tumor growth, and improve quality of life. Research explores new strategies for targeting metastatic cells and their microenvironments, with recent advances in immunotherapy showing promising results for some metastatic cancers, like melanoma and lung cancer.