The Causes of Cancer
Tobacco:
In addition to lung cancer, tobacco consumption causes tumors of the larynx, pancreas, kidney, bladder, and in conjunction with alcohol drinking, a high incidence of carcinomas of the oral cavity and the esophagus. In most developed countries, tobacco accounts for as much as 30% of all malignant tumors.
Lung cancer risk is determined by the amount of daily consumption of tobacco, duration of smoking, and the depth of inhalation. For regular smokers, the relative risk of the development of lung cancer is more than 20 times higher than for non-smokers. Environmental tobacco smoke (passive smoking) is also carcinogenic, but the risk is much smaller.
Tobacco smoke contains a great number of chemical carcinogens. The pattern of mutations in the P53 tumor suppressor gene in smoking-associated lung tumors suggests that benzo(a)pyrene metabolites play a major role in the development of lung cancer.
Tobacco & Cancer, The American Cancer Society: http://www.cancer.org/docroot/PED/ped_10.asp?sitearea=PED
Tobaccopedia, an online tobacco encyclopedia: http://tobaccopedia.org/
Alcohol:
Heavy alcohol drinking causes cancer of the oral cavity, pharynx, larynx, esophagus, and liver, and may increase the risk of breast and colorectal cancer. Risk is linearly related to the mean daily consumption.
In the oral cavity, pharynx, larynx, and esophagus, the risk is greatly increased by concurrent smoking.
Occupational Exposures:
Many occupations and some specific chemicals encountered in the workplace are associated with increased risk of cancer.
Occupational cancer most often involved the lung; other sites affected include the skin, urinary tract, nasal cavity, and pleura.
NCI Occupational Epidemiology Branch: http://dceg.cancer.gov/ebp/oeb/
International Programme on Chemical Safety (ICPS): http://www.who.int/pcs/index.htm
Environmental Pollution:
Pollution of air, water, and soil is estimated to account for 1-4% of all cancers.
A small proportion of lung cancer (<5%) is attributable to outdoor air pollution by industrial effluent, engine exhaust products, and other toxins.
Carcinogenic indoor air pollutants include tobacco smoke and cooking fumes.
Chlorofluorocarbons cause destruction of the ozone layer and enhance the risk of skin cancer through increased ultraviolet radiation.
United States Environmental Protection Agency: http://www.epe.gov/
United Nations Environment Programme: http://www.unep.org/
Food Contaminants:
Food may be contaminated by natural or man-made toxins.
Naturally-occurring carcinogens include mycotoxins, particularly aflatoxins, which contribute to the causation of liver cancer.
Food can be contaminated by residual pesticides. Small quantities of heterocyclic amines, which are mutagenic and carcinogenic, can be generated during food processing and cooking.
WHO Food Safety Programme: http://www.who.int/fsf/index.htm
Medicinal Drugs:
Certain drugs used to treat malignant tumors, may rarely cause second primary tumors.
Drugs with hormonal activity or which block hormonal effects may increase risk of some hormonally-responsive cancers, while reducing the risk of others.
IARC Morographs programme, online search facility: http://monographs.iarc.fr
Radiation:
Exposure to ionizing radiation from natural as well as from industrial, medical, and other sources, can cause a variety of neoplasms, including leukemia, breast cancer, and thyroid cancer.
Sunlight is by far the most significant source of ultraviolet irradiation and causes several types of skin cancer.
International Commission for Non-Ionizing Radiation Protection: http://www.icnirp.de
Chronic Infections:
Infectious agents are one of the main causes of cancer, accounting for 18% of cases worldwide, the majority occurring in developing countries.
The most frequently affected organ sites are liver (hepatitis B and C, liver flukes), cervix ueri (human papillomaviruses), lymphoid tissues (Epstein-Barr viruse), stomach (Helicobacter pylori) and the urinary system (Schistosoma haematobium).
The mechanism of carcinogenicity by infectious agents may be direct, e.g. mediated by oncogenic proteins produced by the agent (e.g. human papillomavirus) or indirect, through causation of chronic inflammation with tissue necrosis and regeneration.
Strategies for prevention include vaccination (hepatitis B virus), early detection (cervical cancer) and eradication of the infectious agent (Helicobacter pylori).
Tumors Associated with HIV/AIDS:
Approximately 30-40% of patients with HIV infection are likely to develop malignancies.
Kaposi sarcoma is the most common malignancy in patients with HIV infection.
Non-Hodgkin lymphoma in patients with HIV infection is about two hundred-fold more frequent than expected.
Cervical intraepithelial neoplasia (CIN) has been increasingly diagnosed in HIV-infected women; invasive cervical cancer is currently an AIDS-defining condition.
Testicular cancer appears to be more frequent in HIV-seropopsitive homosexual men.
National Center for Infectious Diseases (USA CDC): http://www.cdc.gov/ncidod/index.htm
WHO infectious disease: http://www.who.int.health_topics/infectious_diseases/en
Diet and Nutrition:
Up to 30% of human cancers are probably related to diet and nutrition.
Excess salt intake causes arterial hypertension and an elevated risk of stomach cancer.
A Western diet (highly caloric food rich in animal fat and protein), often combined with a sedentary lifestyle and hence energy imbalance, increases the risk of colon, breast, prostate, endometrial, and other cancers.
Physical activity, avoidance of obesity, and frequent daily intake of fresh fruit and vegetables reduce the risk of oral cavity, lung, cervix uteri, and other cancers.
NCI Division of Cancer Prevention: Diet, food, nutrition: http://www.cancer.gov/prevention/lifestyle.html#diet
Immunosuppression:
Persistent suppression of the immune system results in an increased cancer risk.
An increased incidence of malignant lymphomas, of which the majority contain the Epstein-Barr virus, is caused by immunosuppressive drugs used to prevent the rejection of organ transplants.
Infectious agents that cause severe immune suppression, such as the human immunodeficiency virus (HIV), are associated with an increased incidence of several tumors, including non-Hodgkin lymphoma and Karposi sarcoma.
International Association of Physicians in AIDS Care: http://www.iapac.org/
The Transplantation Society: http://www.transplantation-soc.org/
Genetic Susceptibility:
Inherited cancer syndromes, usually involving germline mutations in tumor suppressor or DNA repair genes, may account for up to 4% of all cancers.
Inherited mutations of the BRCA1 gene account for a small proportion of all breast cancers, but affected family members have a greater than 70% lifetime risk of developing breast or ovarian cancer.
UICC Familial Cancer and Prevention project: http://www.uicc.org/programmes/epid/familial.shtml
GeneClinics, a clinical information resource: http://www.geneclinics.org/