How Cancer Develops and Spreads

Cancerous cells develop from healthy cells in a complex process called transformation. The first step in the process is initiation, in which a change in the cell's genetic material (in the DNA and sometimes in the chromosome structure) 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.

The second and final step in the development of cancer is called promotion. Agents that cause promotion are called promoters. 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 noninitiated cells. 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 a variety of cancers, particularly sarcomas, leukemia, thyroid cancer, and breast cancer.

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; only later does it spread more extensively throughout the body. Cancer can also spread via the bloodstream. This type of spread is typical of sarcomas.

See the table Some Carcinogens.