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In cancer, metastasis occurs when cells break away from the primary tumor and are transported through the body to other organs and tissues where they attach and grow a new, secondary, tumor. Usually, the metastatic cells squeeze into the blood or lymphatic systems, so the secondary tumors can occur far from the original cancer. A primary tumor can usually be treated with patients often being cured. If metastasis of the tumor has occurred, the prognosis is a lot bleaker. Many cancer-related deaths are actually caused by the secondary tumors resulting from metastasis.
Tumors can occur in two different states, benign and malignant. A tumor that is benign does not grow as aggressively or as quickly as a malignant one, it does not invade surrounding tissues, and it does not metastasize. Benign tumors are often harmless to the health of the individual, and usually not classified as a type of cancer. Malignant tumors, or cancers, are much more aggressive and can be lethal. One common characteristic of a malignant cancer is that it can go through a stage of metastasis.
For metastasis of a tumor to occur, a series of complex steps must take place. Cancer researchers have discovered that angiogenesis is critical for a primary cancer to metastasize. Angiogenesis is the development of a new blood vessel system. When discussing cancer, the new blood vessel system occurs within the tumor. As the new blood vessels are within the tumor itself, it is much easier for the cancerous cells to break away and then be transported through the circulatory system to a new site.
Metastasis is more common with some types of cancer than others. Lung, breast, colon, kidney, and skin melanomas are common sites for a primary tumor to produce metastatic cells. There are also common areas in the body where secondary tumors are more likely to occur, such as the adrenal glands, bones, brain, and liver. Certain types of cancer also tend to spread to a particular area. For example, prostate cancer usually creates metastatic bone cancer, and stomach cancer in women tends to metastasize to the ovaries.
When cancer is diagnosed in a patient, a doctor can usually easily determine if the tumor is a primary or secondary one based on the type of cells making up the tumor. Primary tumors are made up of abnormal cells of the surrounding tissue. If the tumor is a secondary tumor, it is made up of abnormal cells of the original tumor, not the surrounding tissue where it exists. For example, lung cancer would consist of only abnormal lung cells if it were a primary tumor. If the tumor was actually caused by metastasis of a skin melanoma, the cells in the tumor in the lung would actually be skin cells and not lung cells.