Causes of cancer
[10] Aging has been repeatedly and consistently regarded as an important aspect to consider when evaluating the risk factors for the development of particular cancers.
Many of the cancer syndrome cases are caused by mutations in tumor suppressor genes that regulate cell growth.
[21] Decades of research has demonstrated the link between tobacco use and cancer in the lung, larynx, head, neck, stomach, bladder, kidney, esophagus and pancreas.
[24][25] The other five toxicants, acetaldehyde, cadmium, ethylene oxide, formaldehyde and isoprene act through various mechanisms including direct interaction with DNA.
[32] Non-fibrous particulate materials that cause cancer include powdered metallic cobalt and nickel, and crystalline silica (quartz, cristobalite, and tridymite).
[32] Usually, physical carcinogens must get inside the body (such as through inhaling tiny pieces) and require years of exposure to develop cancer.
[32] Common occupational carcinogens include:[33] Many different lifestyle factors contribute to increasing cancer risk.
[39] In particular, alcohol use has been shown to increase the risk of developing cancers of the mouth, esophagus, pharynx, larynx, stomach, liver, ovaries, and colon.
[40] The main mechanism of cancer development involves increased exposure to acetaldehyde, a carcinogen and breakdown product of ethanol.
[42] Other mechanisms have been proposed, including alcohol-related nutritional deficiencies, changes in DNA methylation, and induction of oxidative stress in tissues.
[45] Several risk factors for the development of colorectal cancer include high intake of fat, alcohol, red and processed meats, obesity, and lack of physical exercise.
[55] The current understanding regarding the mechanism of cancer development in obesity relates to abnormal levels of metabolic proteins (including insulin-like growth factors) and sex hormones (estrogens, androgens and progestogens).
[58] Some treatments and prevention approaches leverage this cause by artificially reducing hormone levels, and thus discouraging hormone-sensitive cancers.
[58] Perhaps the most familiar example of hormonal therapy in oncology is the use of the selective estrogen-receptor modulator tamoxifen for the treatment of breast cancer.
In addition, the oncoproteins independently induce genomic instability in normal human cells, leading to an increased risk of cancer development.
[66] The mechanism by which H. pylori causes cancer may involve chronic inflammation or the direct action of some of the bacteria's virulence factors.
[67] Parasitic infections strongly associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).
[71] Chronic inflammation can lead to DNA damage over time and the accumulation of random genetic alterations in cancer cells.
[72] Inflammation can contribute to proliferation, survival, angiogenesis and migration of cancer cells by influencing tumor microenvironment.
[9] Unlike chemical or physical triggers for cancer, ionizing radiation hits molecules within cells randomly.
[74] Even if the radiation particle does not strike the DNA directly, it triggers responses from cells that indirectly increase the likelihood of mutations.
Non-ionizing radio frequency radiation from mobile phones, electric power transmission, and other similar sources have been described as a possible carcinogen by the World Health Organization's International Agency for Research on Cancer.
[79] Some people, such as those with nevoid basal cell carcinoma syndrome or retinoblastoma, are more susceptible than average to developing cancer from radiation exposure.
[74] Children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect.
Radiotherapy deliberately deliver high doses of radiation to tumors and surrounding tissues as a form of disease treatment.
It is estimated that 0.4% of cancers in 2007 in the United States are due to CTs performed in the past and that this may increase to as high as 1.5–2% with rates of CT usage during this same time period.
[74] Radiation is a more potent source of cancer when it is combined with other cancer-causing agents, such as radon gas exposure plus smoking tobacco.
[81] There have also been reports of Kaposi's sarcoma occurring after transplantation due to tumorous outgrowth of virus-infected donor cells.
[83] However, repeated injuries to the same tissues might promote excessive cell proliferation, which could then increase the odds of a cancerous mutation.
In the United States, approximately 3,500 pregnant women have a malignancy annually, and transplacental transmission of acute leukemia, lymphoma, melanoma and carcinoma from mother to fetus has been observed.
