Transformation is the word used to describe the 2-step process by which a normal, healthy cell changes into a cancer cell. The first step in transformation is called initiation. During initiation, a cell's genetic material changes (mutates) and the growth inhibiting factor is reduced or lost. This change can occur spontaneously but it can also be caused by the presence of a substance (called a carcinogen) that encourages the mutation. Known carcinogens include chemicals, viruses, tobacco, radiation, and even sunlight. Not all cells are equally at risk when exposed to carcinogens. In some, a genetic flaw may make the cell more likely to mutate; in other cases, a cellular factor may make it more resistant to a specific carcinogen. Other factors may also contribute to initiation. Physical irritation over a long period can make a cell more vulnerable to carcinogens.

Promotion is the second step in the transformation of a healthy cell into a cancerous cell. Promotion is caused by agents known as promoters. The promoters stimulate the growth of the cell. With a damaged or lost growth inhibiting factor, the cell grows uncontrollably. Promotion cannot affect a cell that has not been initiated. That is why promoters do not, all by themselves, cause cancer. Substances found in the environment and certain drugs act as promoters.

Certain carcinogens are powerful enough to cause cancer on their own; they both initiate and promote cancer development. On the other side, not every carcinogen that an animal encounters will cause cancer. The DNA (genes) of animals, just like the DNA in humans, includes built-in repair mechanisms. An animal's DNA can protect the animal from the changes that cause cancer. Only when the protective mechanisms fail or are overwhelmed will an animal develop cancer.

Once a cell has turned cancerous, it starts growing and spreading. Because there is no control on the growth of these cells, the cancer will often take advantage of any available path to spread and find new places to grow. This process is called metastasizing. The 2 most common paths for cancerous cell spread are the lymphatic system and the bloodstream. The lymphatic system is made up of small vessels that collect the fluid surrounding cells and return that fluid to the bloodstream. Carcinomas are the malignant tumors formed of the epithelial cells lining the internal and external surfaces of the body. Carcinomas usually spread through the lymphatic system. Sarcomas are malignant tumors formed from bone, cartilage, fat, or other connective tissues. Sarcomas usually spread through the bloodstream.

Last full review/revision July 2011