Cancer is not one single disease but a complex of many diseases. About two hundred distinct types of cancer have been recognized. These can be grouped into four main types. they are:
1. Carcinomas. Carcinomas are tumors made up principally of epithelial cells of ectodermal or endodermal origin. The solid tumors in nerve tissue and in tissues of body surfaces, or their attached glands are examples of carcinomas. These include cervical, breast, skin and brain carcinomas. About 85%of cancers are carcinomas.
2. Sarcomas. Sarcomas are tumors made up principally of connective tissue cells, which are of mesodermal origin. They are solid tumors growing from connective tissue, cartilage, bone and muscle. Although they account for most of the cancers studied in laboratory animals, they constitute only about 2% of human cancers.
3. Lymphomas. Lymphomas are cancers in which there is excessive production of lymphocytes by the lymph nodes and spleen. Hodgkin's disease is an example of a lymphoma. Lymphomas constitute about 5%of human cancers.
4. Leukemias. Leukemias are neoplastic growths of leucocytes. (WBC) and are characterized by excessive production of the cells. They constitute about 4%of human cancers.
In addition to these, there may be mixed malignant tumors e.g. tumors arising from both ectodermal and mesodermal tissues.