There are over 100 different types of cancer. It can affect almost any organ in the body from the skin to the colon. The most common forms of cancer in North America are lung cancer, breast cancer, and prostate cancer. There are four major groups of cancers:
- carcinomas are tumors that start in the exterior or interior linings of the internal organs (called epithelial tissue) and on the exterior surface of the body
- leukemias are cancers of the blood-forming tissues
- lymphomas are tumors that originate in the lymphatic system
- sarcomas are tumors that originate in connective tissue, such as muscle, bones, and cartilage
As someone's body grows, certain cells divide and multiply to create new tissue, while other cells (like muscle or nerve cells) do not divide and multiply. The body has specific genes called oncogenes that control the ability of cells to divide and grow. Genes called tumor suppression genes tell the cells to stop dividing. Cancer occurs when either the oncogenes are "turned on" when they aren't supposed to be, or the tumor suppression genes are "turned off" when they're supposed to be on. This results in excess growth in the form of tumors.
Cancer cells go through different stages as they divide and multiply to form a tumor. At first, normal cells divide faster than they should and the total number of cells increases. This is called hyperplasia. At the second stage, called dysplasia, the new cancer cells look misshapen. The cancer cells then form a growing ball of cells, called a primary tumor. The tumor begins to push and squash the cells around it. As the tumor grows bigger, it burrows and invades into surrounding cells - this process is called invasion. When cancerous cells spread into a blood vessel or a lymph node, they can travel in the blood or lymph fluid to other parts of the body where they start to divide once again. This process is called metastasis, which means that the cancer has spread to other areas of the body.
Cancer causes more fear than any other disease. However, many cancers can now be treated and put into remission. This means traces of cancer are no longer found in the body following treatment. For example people with prostate, bladder, skin, uterine, or breast cancer have at least an 80% chance of being disease-free (without cancer) 5 years after being diagnosed with cancer, assuming the cancer was detected and treated at an early stage.
The exact cause of cancer is not known, but various factors are likely at play. Although genetic factors have been linked to certain types of cancers, less than 10% of cancers are inherited. Less than 10% of breast cancers are associated with mutated genes known as BRCA1 and BRCA2. These two inherited genes account for about 50% of the inherited forms of breast cancer.
Most forms of cancer are due to genetic mutations of cells that occur within a person's life as a result of environmental factors such as cigarette smoke or exposure to radiation. Exposure to the following environmental factors can cause cancer:
- tobacco smoking: Smoking causes lung cancer and is also associated with an increased risk for cancers of the mouth (oral cancers), larynx, esophagus, bladder, and cervix.
- chemicals: Exposure to industrial dyes, asbestos, and benzene is linked to cancer.
- ionizing radiation: A connection between ionizing radiation and cancer has been made, but the exact amount of radiation exposure that increases the risk of cancer is not known.
- viruses: Certain viruses, such as the human immunodeficiency virus (HIV, which causes AIDS) are associated with an increased risk of liver cancer, lymphomas, and sarcomas. The human papillomavirus (HPV, which causes venereal warts) is associated with an increased risk of oral, anal, and cervical cancer.
- sunlight: Prolonged exposure (e.g., sun tanning) causes skin damage and may result in skin cancer.