Risk Factors for Cancer
Non-Ionizing Radiation
Non-ionizing radiation is defined as a low frequency, long wavelength energy, which includes AM and FM radio signals, microwave ovens, power lines, heat lamps, visible light, and ultraviolet radiation.
Non-ionizing radiation is not energetic enough to penetrate deeply or to create free radicals. It only penetrates single-celled organisms and the superficial cell layers of multicellular organisms. Its energy is sufficient to boost the energy of electrons in molecules. Nucleotides in DNA absorb this energy (maximum mutagenesis occurs at 234 nm). Excitation of nucleotides can induce abnormalities such as formation of bulky thymine dimers that cause mutations by interfering with proper base pairing during replication.
Ultraviolet radiation from sun exposure and tanning salons is a major cause of skin cancer, accounting for 50-90% of all skin cancers, according to the World Health Organization. Ultraviolet radiation penetrates the superficial layers of the skin and causes mutations when the radiant energy is absorbed and disrupts bonds within DNA. Ultraviolet light also causes premature aging of skin and eye damage, including cataracts. For more information on the health effects of ultraviolet radiation, see the module on Sun Exposure.
Skin Cancers
The National Institutes of Health (NIH) has a good web site providing an array of information about sun damage and skin cancer. See NIH Senior Health.
The images below illustrate the appearance of the three major forms of skin cancer.
The next set of images illustrate the characteristics of melanoma, and the changes that occur during its evolution.
Source: http://www.webmd.com/melanoma-skin-cancer/abcds-of-melanoma-skin-cancer
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Do cell phones cause cancer? From National Cancer Institute:
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Ionizing Radiation
Protons, neutrons, x-rays, & gamma rays have highly energetic wavelengths, frequency which penetrate deeply into tissues. Lead shields block these forms of radiation.
Ionizing radiation can damage DNA with a direct hit, but it more frequently damages DNA indirectly by stripping away electrons when they strike a molecule, thereby creating a highly reactive free radicals. The resulting free radicals can damage other molecules by stealing their electrons, and they also can break phosphate bonds in DNA. Breaks in a single strand can be repaired easily, but sometimes double strand breaks are not. Ionizing radiation is very dangerous because its high energy allows it to penetrate deeply into tissue leaving a trail of free radicals in its path. Ionizing radiation has a cumulative mutagenic effect.
Examples:
- Cancers caused by nuclear fallout (Chernobyl; Hiroshima & Nagasaki)
- "The Radium Girls" - Cancers of the jaw and tongue caused by exposure to radium in factory workers who painted radium-containing glow in the dark watch dials in the 1920s. These workers hand painted the dials with tiny brush that they licked in order to maintain a fine point to the brush. The tongue was therefore exposed to radium, and some of the radium was swallowed and then it was absorbed and circulated and eventually ended up accumulating in bone..
- Radon is a naturally occurring gas produced by the radioactive decay of the element radium. This gas rises from the ground, the site of the decay, and gets into the home. Residential radon is a primary source of exposure to ionizing radiation in most countries, and is a major source of lung cancer.
Chemical Carcinogens
A chemical carcinogen is any discrete chemical compound, which has been shown to cause cancer. There are several classes of chemicals that tend to be carcinogenic. Within these classes, some commonly encountered chemicals include formaldehyde, chloroform, asbestos, arsenic, and aflatoxin. Carcinogens may enter the body through skin absorption, ingestion, or inhalation attacking many organs. Dose, or the amount and length of exposure, directly affect the amount of damage obtained. To reduce exposure to these common carcinogens use proper ventilation and hygienic habits.
Tobacco use accounts for at least 30% of cancer deaths and 87% of lung cancer deaths. Tobacco instigates cancer of the lung, oral cavity, nasal cavity, larynx, oropharynx, hypopharynx, esophagus, stomach, liver, bladder, ureter, kidney, cervix, and myeloid leukemia. Interestingly enough, lung cancer is the most preventable form of cancer in our society today. Cigarette smoke contains more than 60 carcinogens. Therefore, the cessation of cigarette smoking would immensely decrease many cancer occurrences.
For more information on chemical carcinogens, please see:
Heterocyclic Amines & Polycyclic Aromatic Hydrocarbons
Heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) are a family of carcinogenic chemicals formed from the cooking of muscle meats (beef, pork, fowl, and fish); other sources of protein have not been associated with HCAs (e.g. eggs, tofu). HCAs form when amino acids and creatine (a chemical found in muscles) react at high cooking temperatures. Frying, broiling, and barbecuing produce the largest amounts of HCAs because the meats are cooked at very high temperatures. They have been associated with an increased risk of stomach, colorectal, and pancreatic cancer.
For more information, see http://www.cancer.gov/cancertopics/factsheet/Risk/cooked-meats.
Viruses
Viruses are estimated to cause 15 – 20% of all cancers in humans. Some factors that influence the progression to cancer development include the host's genetic susceptibility, mutations, exposure to other carcinogens, and immune system deficiencies. Oncoviruses include the Epstein –Barr virus, hepatitis B virus, human papilloma virus (HPV), herpes virus, and hepatitis C virus.
Although most strains of HPV will not remain active in the body for more than several years, a number of aggressive strains are significantly more likely to cause cancer. These strains activate certain oncogenes. When these oncogenes impede factors that limit cell growth, cells divide much more rapidly than normal, and can lead to tumor development.
Chronic infection with hepatitis causes 80% of all primary liver cancers worldwide. The most common risk factor for liver cancer is chronic infection with hepatitis B virus (HBV). HBV attacks the liver, leading to progressive liver damage, which in turn develops into liver cancer. Those infected with HPV are 100 times more likely to develop liver cancers than those who are not infected. There is an effective vaccine against HBV. There are effective therapies to manage chronic hepatitis B infections and help prevent progression to liver cancer.
For further information on hepatitis B, consult the following module: http://www.hepb.org/hepatitisbcd/modules/infectd/id450101/id459101g.html
AIDS-Related Cancers
AIDS is not a direct cause of cancer, but it indirectly increases the risk by impairing immune function. The National Cancer Institute states the following:
"Definition of AIDS-related cancers: Certain cancer types that are more likely to occur in people who are infected with the human immunodeficiency virus (HIV). The most common types are Kaposi sarcoma and non-Hodgkin lymphoma. Other AIDS-related cancers include Hodgkin disease and cancers of the lung, mouth, cervix, and digestive system."
People with AIDS are 310 times more likely to develop Kaposi's sarcoma and 113 times more likely to develop non-Hodgkin's lymphoma (Goedert, J. J., Coté, T. R., Virgo, P., Scoppa, S. M., Kingma, D. W., Gail, M. H., Biggar, R. J. (1998). Spectrum of AIDS-associated malignant disorders. The Lancet, 351(9119), 1833-1839. doi:10.1016/S0140-6736(97)09028-4.)
BMI and Obesity
High body mass index is associated with an increased risk of colon, breast, endometrium, kidney, esophagus, gastric cardia, pancreas, gallbladder and liver cancer.
The National Cancer Institute says the following regarding the possible mechanisms for this association:
"Several possible mechanisms have been suggested to explain the association of obesity with increased risk of certain cancers:
- Fat tissue produces excess amounts of estrogen, high levels of which have been associated with the risk of breast, endometrial, and some other cancers.
- Obese people often have increased levels of insulin and insulin-like growth factor-1 (IGF-1)
in their blood (a condition known as hyperinsulinemia or insulin resistance), which may promote the development of certain tumors. - Fat cells produce hormones, called adipokines, that may stimulate or inhibit cell growth. For example, leptin, which is more abundant in obese people, seems to promote cell proliferation, whereas adiponectin, which is less abundant in obese people, may have antiproliferative effects.
- Fat cells may also have direct and indirect effects on other tumor growth regulators, including mammalian target of rapamycin (mTOR) and AMP-activated protein kinase.
- Obese people often have chronic low-level, or "subacute," inflammation, which has been associated with increased cancer risk."
Limiting fat and calorie intake may decrease risk of some cancers (e.g., breast and colon cancer). In addition, higher consumption of fruits & vegetables is associated with a reduction in cancer risk. The responsible components remain unknown, but many recommend at least 5 servings/day.
Angiogenesis and Dietary Inhibitors of Angiogenesis
Angiogenesis is the formation of new blood vessels. This normally takes place during growth and during healing after a tissue injury. Tumors also need new blood vessels in order to grow, and there is now abundant evidence that inhibitors of angiogenesis can stop or at least slow the growth of tumors. Several angiogenesis inhibitors have been approved for use in cancer treatment by the Food and Drug Administration. (For more information see the following link from the National Institutes of Health:
http://www.cancer.gov/cancertopics/factsheet/Therapy/angiogenesis-inhibitorsIn addition, there is also evidence that there are a number of naturally occurring angiogenesis inhibitors in some vegetables and fruits. To learn more about these, watch the video below from TED.com.
Alcohol Consumption
Alcohol intake is associated with an increased risk of cancer of the mouth, esophagus, pharynx and larynx, colon, and liver. Consumption of alcohol is the primary cause of liver cancer; oral and pharyngeal cancers are more common in alcohol users than in non-alcohol users. Smokers who also drink are at an increased risk of developing cancer. According to the American Cancer Society, drinking in moderation is key to reducing the risk of alcohol-related cancers; men should have no more than two drinks per day and women should have no more than one.
Heredity
Most cancers are sporadic, i.e., with no hereditary predisposition. However, a predisposition to certain types of cancers can be hereditary. Some examples are:
- Retinoblastoma: As noted earlier, retinoblastomas may occur sporadically, although there is also a familial form in which descendants can inherit a defective allele for Rb. This, by itself, does not initiate cancer, because the other allele has the normal code for Rb protein. However, if a mutation disables the second allele, then production of the Rb protein (a tumor suppressor) is knocked out in that cell.
- Breast cancer: Around 85% of breast cancers are sporadic (non-hereditary). However, an increased risk of breast cancer can be inherited via two mutant genes - BRCA1 and BRCA2.
- Colon cancer:
For more information on heredity and cancer, refer to: http://www.cancer.org/Cancer/CancerCauses/GeneticsandCancer/heredity-and-cancer