Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymph systems.
Cancer is not just one disease but many diseases. There are more than 100 different types of cancer. Most cancers are named for the organ or type of cell in which they start – for example, cancer that begins in the colon is called colon cancer; cancer that begins in basal cells of the skin is called basal cell carcinoma.
Cancer types can be grouped into broader categories. The main categories of cancer include:
- Carcinoma – cancer that begins in the skin or in tissues that line or cover internal organs.
- Sarcoma – cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
- Leukemia – cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
- Lymphoma and myeloma – cancers that begin in the cells of the immune system.
- Central nervous system cancers – cancers that begin in the tissues of the brain and spinal cord.
Origins of Cancer
All cancers begin in cells, the body’s basic unit of life. To understand cancer, it’s helpful to know what happens when normal cells become cancer cells.
The body is made up of many types of cells. These cells grow and divide in a controlled way to produce more cells as they are needed to keep the body healthy. When cells become old or damaged, they die and are replaced with new cells.
However, sometimes this orderly process goes wrong. The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division. When this happens, cells do not die when they should and new cells form when the body does not need them. The extra cells may form a mass of tissue called a tumor.
Not all tumors are cancerous; tumors can be benign or malignant.
- Benign tumors aren’t cancerous. They can often be removed, and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body.
- Malignant tumors are cancerous. Cells in these tumors can invade nearby tissues and spread to other parts of the body. The spread of cancer from one part of the body to another is called metastasis.
Some cancers do not form tumors. For example, leukemia is a cancer of the bone marrow and blood.
Risk factors associated with cancer
(Refer to the PDQ summaries on Lung Cancer Prevention; Lung Cancer Screening; and Prevention and Cessation of Cigarette Smoking: Control of Tobacco Use for more information.)
Oncogenic strains of HPV are also linked with cancers of the penis, vagina, anus, and oropharynx. Other examples of infectious agents that cause cancer are hepatitis B and hepatitis C viruses (liver cancer), Epstein-Barr virus (Burkitt lymphoma), and Helicobacter pylori (gastric cancer). (Refer to the PDQ summaries on Cervical Cancer Prevention; Cervical Cancer Screening; Liver (Hepatocellular) Cancer Prevention; and Liver (Hepatocellular) Cancer Screening for more information.)
Radiation is energy in the form of high-speed particles or electromagnetic waves. Exposure to radiation, primarily ultraviolet radiation and ionizing radiation, is a clearly established cause of cancer.
The frequency of use of x-rays such as CT scans is increasing. The radiation doses associated with CT scans are in the range where an increased risk of cancer has been directly observed.
Life-long dietary patterns or dietary intake during specific life stages may be important in inducing or preventing cancer but would not be detected by relatively short-term randomized clinical trials.
In relation to human cancer, diets reflect the sum total of a complex mixture of exposures, as demonstrated by the examples of fruit/vegetable intake and alcohol consumption. No dietary factors appear to be uniformly relevant to all forms of cancer. (Refer to the PDQ summaries on Breast Cancer Prevention; Breast Cancer Screening; Colorectal Cancer Prevention; and Lung Cancer Prevention for more information.)
Environmental Exposures and Pollutants
The relationship between environmental pollutants and cancer risk has been of long-standing interest to researchers and the public. When estimates of the potential burden of cancer have been calculated for different classes of exposure, the factors described earlier, such as cigarette smoking and infections, have represented much greater proportions of the cancer burden than have environmental pollutants. Many other environmental pollutants, such as pesticides, have been assessed for risk with human cancer, but with indeterminate results.
please read risk factor once again to avoid cancer.