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Metastasis is the spread or movement of cancer cells from the primary cancer site to another area of the body. Other than certain white blood cells, this is something most normal cells cannot do, and it is the most deadly characteristic of cancer. During metastasis, tumor cells penetrate the fibrous boundaries that normally separate one tissue from another. The tumor can also infiltrate the walls of blood or lymph vessels and shed cancer cells into the circulation. In the blood, these tumor cells are carried downstream to become lodged in the next capillary bed. Tumor cells shed from colon cancer, for example, are carried by the circulation to the liver, where secondary tumors then arise. Tumor cells from other areas of the body can be carried by the blood through the heart and on to the lungs, where they start metastatic lung tumors. Tumor cells shed into the lymph system often establish themselves in the nearest cluster of lymph nodes, where they grow before spreading to more distant parts of the body. Fewer than 1 in 10,000 cells shed from the primary tumor are thought to survive, but these are enough to spawn secondary tumors elsewhere in the body. About 30 percent of new patients with solid tumors have detectable metastases. About half the remaining patients will be cured by treating the tumor alone; the remainder will have undetectable metastases that will eventually develop into tumors. Tumor staging includes a measure of whether a malignancy has spread beyond the primary tumor. This is a major factor in determining a patient’s prognosis. The goal of early detection is to remove the primary tumor before metastasis has occurred. Unfortunately, some tumors apparently metastasize before they are large enough to be found. The spread of such micrometastases may explain why many women die of breast cancer even after early detection of their primary tumors. Scientists have recently learned that metastasis can occur only after certain genes are turned on. These genes produce the enzymes necessary for the cancer cells to penetrate other tissues and invade blood vessel walls. These enzymes, and receptors for these enzymes, may provide targets for new drugs that can block the process of metastasis.
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