There two main steps in cancer progression: the initial growth of the cancer and the subsequent spread via metastasis. Solid tumors are subject to the physiological constraints of biological systems: Without nutrients and oxygen, they will die. Therefore a solid tumor is initially limited in size to no larger than 1 to 2 millimeters in diameter (about the size of a small pea).
For a tumor to become aggressive, it needs to be able to nourish the cells at the center of its mass that are too far away from blood vessels. This is achieved by angiogenesis. Through mutation, a few cancer cells may gain the ability to produce angiogenic growth factors. These growth factors are proteins that are released by the tumor into nearby tissues, where they stimulate new blood vessels to grow into the tumor. This allows the tumor to rapidly expand in mass and invade surrounding tissue. It also provides a route for the cancer cells to escape into the new blood vessels and circulate throughout the body, where they can lodge in other organs forming metastases.
The most common way for a cancer to metastasize is through the lymphatic system. The lymphatic system is a network of channels throughout the body that carry a tissue fluid called lymph.
When a primary neoplasm metastasizes to another location, its cell type does not change. If leukemia metastasizes to the liver and develops a tumor, the tumor will display the characteristics of the leukemia, not those of a liver cancer. In some cases this can help physicians determine the original site of a tumor.
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