One of the most widely accepted hypotheses in cancer research is that the initial event leading to malignancy is a change in a single cell. Following this, there may be a series of events within succeeding generations of cells, finally culminating in many malignant cells which may develop into a mass as a "solid" tumor or may be at once widespread throughout the body, such as occurs in the leukemias.

Carcinomas are the most common form of cancer. They arise from epithelial cells, important as a covering or lining tissue, which exists in numerous forms. Skin is composed of one kind of epithelium; when it becomes cancerous, it is designated as skin carcinoma. Other carcinomas arise from the glandular organs, such as breast, which are composed of different epithelial tissue, as are the smooth, shiny mucous membranes such as those that line the mouth, stomach, and lungs.

Sarcomas occur less frequently. They are a more heterogeneous group of cancers, arising from fibrous (connective) tissue and from muscle, bone, and cartilage. Together, the carcinomas and sarcomas have been classified as "solid" tumors. They generally spread and seed secondary growths in other parts of the body, and are therefore considered systemic diseases, that is, affecting the body as a whole.

fibrosarcoma infiltrating muscle tissuecarcinoma of the breast

These are pictures of mouse cancer cells, magnified under the microscope: left, fibrosarcoma infiltrating muscle tissue; right, carcinoma of the breast.

Diagram of cell shows structures

Diagram of cell shows structures usually visible with the electron microscope. The nucleus contains the master chemical called DNA (deoxyribonucleic acid) in the chromosomes, not shown here. The nucleolus, found in the nucleus, is believed to be a storehouse of DNA. The cytoplasm contains protein-making sites or ribomes; endoplasmic reticulum, the network to which ribosomes adhere; mitochondria, which are believed to be centers for limited production of protein; and the Golgi complex, whose function is as yet unknown. The plasma membrane is a complex structure through which materials pass in and out of the cell. The plasma membrane also acts as a sensory apparatus for the cell.

The generalized forms of malignancy are the leukemias, lymphomas, and multiple myeloma; these are cancers of the blood-forming and lymphoid organs. The majority of leukemias are characterized by an uncontrolled multiplication and accumulation of abnormal white blood cells; the lymphomas by an overproduction of cells found in the lymphoid organs, such as lymph nodes; and multiple myeloma by an abnormal growth of plasma cells in the bone marrow. Multiple myeloma may remain localized in the bone for some time and is sometimes classified as a type of bone cancer.

If not removed or destroyed, cancers multiply without regard to the regulatory controls of the body; they compress, invade, and destroy normal tissue; they break away from the site of origin of the malignant change and, transported through the blood or lymph, start new growths known as metastases; and they eventually kill the host in which they are growing.

Thirty years ago a classic laboratory experiment demonstrated that a single leukemic cell transplanted into a susceptible mouse could initiate fatal disease in the animal. One approach to development of a scientific rationale for curative drug treatment is based on a concept of selective destruction of all cancer cells wherever they may occur in the body, without harm to normal cells. This research involves knowledge of the nature of cancer cells, particularly how they differ from normal cells in growth characteristics, metabolism, and biochemical composition. Accumulation of such knowledge will hopefully one day permit tailoring of curative drugs with specific action on a vulnerable system unique to cancer cells.