Introduction:
Imagine a healthy tissue containing thousands of cells. Each cell serves the greater good, which is the continuation of a person’s life. Each cell is programmed so that when the cell is old or no longer needed, it dies a peaceful and timely death. This death is called apoptosis. All cells are in communication, which allows for the smooth repair and replacement of tissues and other aspects of cell behavior. Communication takes place either indirectly, via exchange of messenger compounds such as hormones and growth factors, or directly, via cell-to-cell contact. Contact allows cells to respond to the “feel” of neighboring cells, via cell adhesion molecules, and to exchange messenger molecules through cell-to-cell portals called gap junctions. With the help of proper communication, appropriate cells proliferate when new cells are needed, and when enough new cells have been produced, cell division stops.
The elderly are more susceptible to cancer than other groups because it is a multistage process that may take 15-45 years before clinical signs are evident. Cancerous growth forms as the result of a sequence of events over a period of time. The events are associated with the exposure of DNA to harmful molecules originating from within or outside of the body. Researchers suggest that an average of five or six chemical insults to susceptable genes will lead to cancer. Some people are genetically susceptible to cancer because enzymes which help to prevent cancer growth are not efficient or enzymes that may contribute to cancer formation are very active.
A cancer is an abnormal growth of cells (usually derived from a single cell). The cells have lost normal control mechanisms and thus are able to expand continuously, invade adjacent tissues, migrate to distant parts of the body, and promote the growth of new blood vessels from which the cells derive nutrients. Cancerous (malignant) cells can develop from any tissue within the body.
As cancerous cells grow and multiply, they form a mass of cancerous tissue—called a tumor—that invades and destroys normal adjacent tissues. The term tumor refers to an abnormal growth or mass. Tumors can be cancerous or noncancerous. Cancerous cells from the primary (initial) site can spread throughout the body (metastasize).
Types of Cancer:
Cancerous tissues (malignancies) can be divided into those of the blood and blood-forming tissues (leukemias and lymphomas) and “solid” tumors, often termed cancer. Cancers can be carcinomas or sarcomas.
Leukemias and lymphomas:
Leukemias and lymphomas are cancers of the blood and blood-forming tissues and cells of the immune system. They often harm the body by crowding out normal blood cells in the bone marrow and bloodstream, so that normal functioning cells are gradually replaced by cancerous blood cells. They expand lymph nodes, producing large masses in the arm pit, groin, abdomen, or chest.
Carcinomas:
Carcinomas are cancers of epithelial cells, which are cells that cover the surface of the body, produce hormones, and make up glands. Examples of carcinomas are cancer of the skin, lung, colon, stomach, breast, prostate, and thyroid gland. Typically, carcinomas occur more often in older than in younger people.
Sarcomas:
Sarcomas are cancers of mesodermal cells, which are the cells that form muscles and connective tissue. Examples of sarcomas are leiomyosarcoma (cancer of smooth muscle that is found in the wall of digestive organs) and osteosarcoma (bone cancer). Typically, sarcomas occur more often in younger than in older people.
Development and Spread:
Cancerous cells develop from healthy cells in a complex process called malignant transformation.
Initiation:
The first step in cancer development is initiation, in which a change in the cell's genetic material primes the cell to become cancerous. The change in the cell's genetic material may occur spontaneously or be brought on by an agent that causes cancer (a carcinogen). Carcinogens include many chemicals, tobacco, viruses, radiation, and sunlight. However, not all cells are equally susceptible to carcinogens. A genetic flaw in a cell may make it more susceptible. Even chronic physical irritation may make a cell more susceptible to carcinogens.
Promotion:
The second and final step in the development of cancer is promotion. Agents that cause promotion, or promoters, may be substances in the environment or even some drugs (such as barbiturates). Unlike carcinogens, promoters do not cause cancer by themselves. Instead, promoters allow a cell that has undergone initiation to become cancerous. Promotion has no effect on cells that have not undergone initiation. Thus, several factors, often the combination of a susceptible cell and a carcinogen, are needed to cause cancer.
Some carcinogens are sufficiently powerful to be able to cause cancer without the need for promotion. For example, ionizing radiation (which is used in x-rays and is produced in nuclear power plants and atomic bomb explosions) can cause various cancers, particularly sarcomas, leukemia, thyroid cancer, and breast cancer.
Spread:
Cancer can grow directly into surrounding tissue or spread to tissues or organs, nearby or distant. Cancer can spread through the lymphatic system. This type of spread is typical of carcinomas. For example, breast cancer usually spreads first to the nearby lymph nodes, and only later does it spread to distant sites. Cancer can also spread via the bloodstream. This type of spread is typical of sarcomas.
These events invariably occur naturally over time as we are exposed to man-made environmental contaminants and toxins that occur naturally in our food, in minerals, and in our bodies as oxidative by-products of metabolism.
Examples of naturally occurring carcinogens include aflatoxin, a by-product of mold found in peanut butter; isocyanate, found in brown mustard; estragole from basil;nitrosamines from cooked bacon; and benzo(a)pyrenes found in charred or barbecued foods.
Excess consumption of fat or salt, tobacco use, and alcohol consumption can raise susceptibility to chemical insult resulting in disease.
