Cancer

Cancer refers to disease caused by the malignant growth of a certain type of cells in vertebrates: epithelial cells. This growth usually results in a mass called a tumor, when rapidly dividing malignnt cells form a solid mass, but also can involve a change in the composition of the blood or other body fluids, when malignant cells circulate in blood, or cerebral spinal fluid. In cases, such as leukemia, for example, tumors are not ordinarily present but many aspects of the disease are the same as those for which malignant tumors are the primary and most obvious manifestation ofn cancer. That's because, in all cases of cancer, there is unbridled growth of malignant cells, and that means that much of the body's nutrition goes towards supporting abnormal rather than normal cells, that some or all of the functions of the normal body parts that have become cancerous are lost, and that the general resistance and vitality of a person is compromised by the excessive demands of the abnormal cells. When cancers are "early", they usually involve only a relatively small amount of abnormal growth, when they are both late and widely dissemated, they generally involve a much greater portion of the body. That's one reason why early and limited cancers less obviously impair health. Since it is these cancers that are most successfully treated, by and large, much of modern medicine has been devoted to finding means to diagnose malignancies early, and public health campaigns have promoted education of citizens to early warning signs of the various kinds of cancer.

Although the origin of cancer in an epithelial tissue is strictly required for the most technically accurate use of the term, cancer, the word is often used, even by physicians and hospitals, to include all the diseases directly caused by malignant tumors and cells. For example, malignant tumors called sarcomas arise from abnormal muscle-type cells, and since these tissues are derived from mesothelium rather than from epithelium, that word sarcoma is used rather than cancer to name the tumor. Still, cancer centers and specialists include treatment of sarcoma in practice, despite the fact that sarcomas and cancers differ in the medical classification of malignant tumors. In other words, all types of malignant tumors are sometimes referred to as cancers, and one type of these malignancies:epithelial malignancies, are, more strictly, also called cancers.

This introductory article will give a brief description of what a malignancy is, and how cells are thought to become malignant. That understanding is important as a basis to comprehending the medical and surgical treatment of cancers, and effective practices in their prevention. After a general introduction to malignancies, major types of cancers (epithelial malignancies) are surveyed, with links provided for further information. The clinical emphasis is on human cancers, but references to cancers in other species of animals is also made. Although the frequencies and the aggresiveness of the various types of cancers vary according to species, generally, the basic biology of cancers is true for all species, including humans and domestic animals.