The growth of a tumor from a single genetically altered cell is a stepwise progression. The process described below is applicable for a solid tumor such as a carcinoma or a sarcoma. Blood cell tumors go through a similar process but since the cells float freely, they are not limited to one location in the body.
Hyperplasia– The altered cell divides in an uncontrolled manner leading to an excess of cells in that region of the tissue. The cells have a normal appearance but there are too many of them!
Dysplasia– Additional genetic changes in the hyperplastic cells lead to increasingly abnormal growth. The cells and the the tissue no longer look normal. The cells and the tissue may become disorganized.
Carcinoma in situ– Additional changes make the cells and tissues appear even more abnormal. The cells are now spread over a larger area and the region of the tissue involved primarily contains altered cells. The cells often ‘regress’ or become more primitive in their capabilities. An example would be a liver cell that no longer makes liver-specific proteins. Cells of this type are said to be de-differentiated or anaplastic. A key facet of in situ growths is that the cells are contained within the initial location and have not yet crossed the basal lamina to invade other tissues. Cancers of this type are often totally curable by surgery since the abnormal cells are all in one location.
Tumors of this type have not yet invaded neighboring tissue. Based on information about patients with similar growths and microscopic examination, these growths are often considered to have the potential to become invasive and are treated asmalignant growths.
Cancer (Malignant tumors)– These tumors have the ability to invade surrounding tissues and/or spread (metastasize) to areas outside the local tissue. These metastatic tumors are the most dangerous and account for a large percentage of cancer deaths. The next few pages will go into some detail on the changes and capabilities that allow cancer cells to form large tumors and to metastasize to other parts of the body.
Some tumors do not progress to the point where they invade distant tissues. Such tumors are said to be benign. Because they do not spread beyond their initial location, they are not considered to be cancerous. Benign tumors are less often lethal than malignant tumors, but they can still cause serious health problems. Large benign tumors can put pressure on organs and cause other problems. In the case of brain tumors, the limited space within the skull means that a large growth in the brain cavity can be fatal.
More information on this topic may be found in Chapters 13 and 14 of The Biology of Cancer by Robert A. Weinberg.