CANCER - TYPES, CAUSES AND RISK FACTORS
Revised on August 23, 2013
Cancer is a disease of cells, abnormal growth of cells which tend to proliferate in an uncontrolled way. In the other words,
cancer is a condition in which abnormal cells divide without control and are able to invade other tissues. Cancer cells are
able to spread to other parts of the body through the blood and lymph systems. Cancer diseases named for the organ or
type of cell in which they start. In colon cancer, cancer that begins in the colon; in melanoma, cancer that begins in skin
Cancers are grouped in categories such as carcinoma, sarcoma, leukemia, lymphoma, and central nervous system
cancers. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Subtypes of
carcinoma include adenocarcinoma, basal cell carcinoma, squamous cell carcinoma, and transitional cell carcinoma.
Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
Leukemia is a 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 are cancers that begin in the cells of the immune
system. And finally, central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord.
ORIGIN OF CANCER
Normal cells grow and divide in a controlled way to produce more cells for keeping the body healthy. When cells get old or
damaged, they die and are replaced with new cells. However, when the genetic material (DNA) of a cell becomes damaged
or changed, producing mutations that affect normal cell growth and division. When this happens, old cells do not die and
new cells continue to form but the body does not need them. Then the extra cells form a mass of tissue called a tumor.
Tumors can be benign (non-cancerous) or malignant (cancerous). Benign tumors can often be removed. Cells in benign
tumors do not spread to other parts of the body. While, cells in these malignant 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. Finally,
some cancers do not form a mass of tissue, e.g. leukemia.
CANCERS WITH GREATER FREQUENCY
Cancers that are diagnosed with the greatest frequency in the United States include bladder cancer, breast cancer, colon
and rectal cancer, endometrial cancer, kidney (renal cell) cancer, leukemia, lung cancer, melanoma, non-Hodgkin
lymphoma, pancreatic cancer, prostate cancer and thyroid cancer.
CAUSES OF CANCER, RISK FACTORS OF CANCER
The risk factors of cancer development can be grouped into chemical / environment, food, genetics, hormones, infectious
agents, radiation, tobacco and personal reasons.
Chemical Environment and Sun Light
Exposure to asbestos may increase the risk of asbestosis, lung cancer, mesothelioma, and potentially other diseases, whle
exposure to certain pesticides may have an increased risk of developing one or more types of cancer. Formaldehyde and
certain chemicals in early hair dye formulations are known to cause cancers. The sources of formaldehyde in the home
include pressed-wood products, cigarette smoke, and fuel-burning appliances. Prolonged exposure to sun light causes
damage through ultraviolet (UV rays), potentially leading to skin cancers.
Food - Acrylamide, Alcohol, Tobacco, Artificial Sweeteners, Fluoride, HCAs and PAHs
Acrylamide, artificial sweeteners, fluoridated water, certain chemicals in meat, and cooked at high temperatures are known
to be related to cancer occurrence. Acrylamide has been found in potato chips, French fries, and other food products
produced by high-temperature cooking. Food and cigarette smoke are the major sources of exposure to acrylamide. To
avoid high levels of acrylamide in some foods one can decrease cooking time, blanch potatoes before frying, and post-dry
(drying in a hot air oven after frying).
There is a strong scientific consensus of an association between alcohol drinking and several types of cancer. The types of
cancer related to alcohol consumption are head and neck cancer, esophageal cancer, liver cancer, breast cancer and
colorectal cancer. Alcohol consumption may also lead to cancers of the pancreas, ovary, prostate, stomach, uterus, and
bladder. Based on data from 2009, an estimated 3.5 percent of all cancer deaths in the United States (about 19,500
deaths) were alcohol related. Further, epidemiologic research shows that people who use both alcohol and tobacco have
much greater risks of developing cancers of the oral cavity, pharynx (throat), larynx, and esophagus than people who use
either alcohol or tobacco alone. Alcohol is being converted to a toxic chemical called acetaldehyde by alcohol
dehydrogenase (ADH) in our body. Many individuals of Chinese, Korean, and especially Japanese descent carry a version
of the gene for ADH that codes for a "superactive" form of the enzyme. This superactive ADH enzyme speeds the
conversion of alcohol (ethanol) to toxic acetaldehyde, and people who have this superactive ADH have a higher risk of
pancreatic cancer than those with the more common ADH form. Aldehyde dehydrogenase 2 (ALDH2), an enzyme,
metabolizes toxic acetaldehyde to non-toxic substances. Some people, particularly those of East Asian descent, carry a
variant of the gene for ALDH2 that codes for a defective form of the enzyme. When these people drink alcohol,
acetaldehyde builds up. The accumulation of acetaldehyde leads to facial flushing and heart palpitations, this unpleasant
feeling makes most people who have inherited the ALDH2 variant are unable to consume large amounts of alcohol.
Therefore, most people with the defective form of ALDH2 have a low risk of developing alcohol-related cancers.
Artificial sweeteners, also called sugar substitutes, are substances that are used instead of sucrose (table sugar) to
sweeten foods and beverages. Studies in laboratory rats during the early 1970s linked saccharin with the development of
bladder cancer. Subsequently, studies in rats showed an increased incidence of urinary bladder cancer at high doses of
saccharin. However, the bladder tumors seen in rats are due to a mechanism not relevant to humans, accharin was delisted
in 2000 from the U.S. National Toxicology Program’s Report on Carcinogens. Aspartame, is distributed under several trade
names (e.g., NutraSweet® and Equal®). In 2005, a laboratory study found more lymphomas and leukemias in rats fed very
high doses of aspartame (equivalent to drinking 8 to 2,083 cans of diet soda daily). However, there were some
inconsistencies in the findings. Thus, it is believed that increasing consumption of aspartame-containing beverages was not
associated with the development of lymphoma, leukemia, or brain cancer. While for other sweeteners, such as acesulfame
potassium, sucralose, and neotame, FDA has reviewed more than 100 safety studies that were conducted on each
sweetener, including studies to assess cancer risk. The results of these studies showed no evidence that these sweeteners
cause cancer or pose any other threat to human health.
It was found that people who lived where drinking water supplies had naturally occurring fluoride levels of approximately 1
part fluoride per million parts water or greater (>1.0 ppm) had fewer dental caries (cavities) than people who lived where
fluoride levels in drinking water were lower. Water fluoridation is the process of adding fluoride to the water supply so the
level reaches approximately 0.7 ppm, or 0.7 milligrams of fluoride per liter of water; this is the optimal level for preventing
tooth decay. In 1993, the Subcommittee on Health Effects of Ingested Fluoride of the National Research Council, part of the
National Academy of Sciences, conducted an extensive literature review concerning the association between fluoridated
drinking water and increased cancer risk. The review included data from more than 50 human epidemiological studies and
six animal studies. The Subcommittee concluded that none of the data demonstrated an association between fluoridated
drinking water and cancer.
Heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) are chemicals formed when muscle meat,
including beef, pork, fish, and poultry, is cooked using high-temperature methods, such as pan frying or grilling directly
over an open flame. Studies have shown that exposure to HCAs and PAHs can cause cancer in animal models. In many
experiments, rodents fed a diet supplemented with HCAs developed tumors of the breast, colon, liver, skin, lung, prostate,
and other organs. Rodents fed PAHs also developed cancers, including leukemia and tumors of the gastrointestinal tract
and lungs. However, the doses of HCAs and PAHs used in these studies were very high—equivalent to thousands of times
the doses that a person would consume in a normal diet.
BRCA1 and BRCA2 are human genes that belong to a class of genes known as tumor suppressors. Mutation of these
genes has been linked to hereditary breast and ovarian cancer. A woman’s lifetime risk of developing breast and/or ovarian
cancer is greatly increased if she inherits a harmful mutation in BRCA1 or BRCA2. Such a woman has an increased risk of
developing breast and/or ovarian cancer at an early age (before menopause) and often has multiple, close family members
who have been diagnosed with these diseases. Harmful BRCA1 mutations may also increase a woman’s risk of developing
cervical, uterine, pancreatic, and colon cancer. Harmful BRCA2 mutations may additionally increase the risk of pancreatic
cancer, stomach cancer, gallbladder and bile duct cancer, and melanoma.
Hormones - Pregnancy, Contraceptives, and Menopausal Hormone Use
Current use of oral contraceptives (birth control pills) appears to slightly increase the risk of breast cancer, especially
among younger women. Further, oral contraceptive use is also associated with an increased risk of cervical cancer;
however, this increased risk may be because sexually active women have a higher risk of becoming infected with human
papillomavirus, which causes virtually all cervical cancers. On the other hand, women who use oral contraceptives have
reduced risks of ovarian and endometrial cancer.
Menopausal hormone therapy (MHT) is a treatment to relieve common symptoms of menopause and to address long-term
biological changes, such as bone loss, that result from declining levels of the natural hormones estrogen and progesterone
in a woman’s body during and after the completion of menopause. Although menopausal hormone therapy provides short-
term benefits, such as relief from hot flashes and vaginal dryness, several health concerns are associated with its use,
including increased risk for certain cancers. Consequently, the U.S. Food and Drug Administration currently advises women
to use MHT for the shortest time and at the lowest dose possible to control menopausal symptoms.
The hormonal changes that occur during pregnancy may influence a woman’s chance of developing breast cancer later in
life. Breast cancer is the most common cancer in pregnant and postpartum women, occurring in about 1 in 3,000 pregnant
women. The average patient is between 32 to 38 years of age and, with many women choosing to delay childbearing, it is
likely that the incidence of breast cancer during pregnancy will increase. Breast cancer pathology is similar in age-matched
pregnant and nonpregnant women.
Studies have shown that a woman’s risk of developing breast cancer is related to her exposure to hormones that are
produced by her ovaries (endogenous estrogen and progesterone). Reproductive factors that increase the duration and/or
levels of exposure to ovarian hormones, which stimulate cell growth, have been associated with an increase in breast
cancer risk. Pregnancy and breastfeeding both reduce a woman’s lifetime number of menstrual cycles, and thus her
cumulative exposure to endogenous hormones (1). In addition, pregnancy and breastfeeding have direct effects on breast
cells, causing them to differentiate, or mature, so they can produce milk. In mid-1900s, a few retrospective studies
suggested that induced abortion was related to an increased risk of breast cancer. However, these studies had important
design limitations. Later on, studies have repeatedly shown no assoication between induced abortion and breast cancer
risk. In 2009, the Committee on Gynecologic Practice of the American College of Obstetricians and Gynecologists
concluded that “more rigorous recent studies demonstrate no causal relationship between induced abortion and a
subsequent increase in breast cancer risk”
Infectious Agents-HIV Infection, HPV and Helicobacteria
People infected with human immunodeficiency virus (HIV) have a higher risk of some types of cancer than uninfected
people of the same age- Kaposi sarcoma, non-Hodgkin lymphoma, and cervical cancer. In addition, people infected with
HIV are at higher risk of several other types of cancer including anal, liver, and lung cancer, and Hodgkin lymphoma. Some
types of sexually transmitted human papillomaviruses (HPVs) can cause genital warts. Other types, called high-risk or
oncogenic HPVs, cause all cervical cancers. They also cause most anal cancers and some vaginal, vulvar, penile, and
oropharyngeal cancers. In fact, most infections with high-risk HPVs do not cause cancer. Many HPV infections go away on
their own within 1 to 2 years. Helicobacter pylori (H. pylori) is a type of bacterium that is found in the stomach of about two-
thirds of the world’s population. H. pylori infection is a major cause of stomach cancer and is associated with an increased
risk of gastric mucosa-associated lymphoid tissue lymphoma. Other risk factors for gastric cancer include chronic gastritis;
older age; male sex; a diet high in salted, smoked, or poorly preserved foods and low in fruits and vegetables; tobacco
smoking; pernicious anemia; a history of stomach surgery for benign conditions; and a family history of stomach cancer.
Radiation - Electric and magnetic fields, Cell Phones, Nuclear Power Plants, Radon, CT Scan
Electric and magnetic fields (EMF) are areas of energy that surround any electrical device. There is limited evidence that
magnetic fields cause childhood leukemia, and there is inadequate evidence that these magnetic fields cause other
cancers in children. Studies of magnetic field exposure from power lines and electric blankets in adults show little evidence
of an association with leukemia, brain tumors, or breast cancer. Probably, electric fields are easily shielded or weakened by
walls and other objects. Cell phones emit radiofrequency energy. Studies so far have not shown a consistent link between
cell phone use and cancers of the brain, nerves, or other tissues of the head or neck. Ionizing radiation is a form of energy
that is present naturally in the environment at low levels. Natural sources of ionizing radiation include radioactive minerals in
the earth and cosmic rays from outer space. Ionizing radiation is also given off, or emitted, by medical x-ray machines, by
some other man-made devices, and by radioactive isotopes produced in nuclear reactors and explosions of nuclear
weapons. At high doses, ionizing radiation can cause immediate damage to a person’s body, including radiation sickness
and death. Lower doses may cause cancer, usually many years later. The cancer risk depends on the amount of radiation,
the type of radiation, and the parts of the body exposed. Radon is a radioactive gas released from the normal decay of the
elements uranium, thorium, and radium in rocks and soil.Radioactive particles from radon can damage cells that line the
lungs and lead to lung cancer. A study shows that radiation exposure from computed tomography scans (CT scans) in
childhood results in very small but increased risks of leukemia and brain tumors in the first decade after exposure. The
study focused on leukemia and brain tumors because bone marrow and the brain are particularly sensitive to radiation.
Children are also more sensitive to radiation than adults because of their rapidly dividing cells and growing bodies.
Obesity is associated with increased risks of cancers of the esophagus, breast (postmenopausal), endometrium (the lining
of the uterus), colon and rectum, kidney, pancreas, thyroid, gallbladder, and possibly other cancer types. Obese people
are also at higher risk of coronary heart disease, stroke, high blood pressure, diabetes, and a number of other chronic
Psychological stress alone has not been found to cause cancer, but psychological stress that lasts a long time may affect a
person’s overall health and ability to cope with cancer.
RISK FACTORS FOR CERTAIN CANCERS
Risk factors for bladder cancer include smoking cigarettes, being exposed to certain dyes, or textiles, working as a dry
cleaner, taking Aristolochia fangchi (a Chinese herb), drinking water with high levels of arsenic, having history of kidney or
bladder stones, past treatment with certain anti-cancer drugs, or radiation therapy to the pelvis, having a kidney transplant
and having hereditary nonpollyposis colon cancer.
The strongest risk factor for breast cancer is age. A woman’s risk of developing this disease increases as she gets older.
Other risk factors for a woman developing breast cancer include inherited changes in certain genes, a history of breast
cancer, having dense breasts, beginning to menstruate before age 12, starting menopause after age 55, having a first full-
term pregnancy after age 30, never having been pregnant, obesity after menopause, and alcohol use.
Colon and Rectal Cancer
Most colon cancers are adenocarcinomas (cancers that begin in cells that make and release mucus and other fluids). The
risk factor for colon cancer include a family history of cancer of the colon or rectum, certain hereditary conditions, a history
of ulcerative colitis or Crohn disease, a personal history of cancer of the colon, rectum, ovary, endometrium, or breast, a
personal history of polyps (small areas of bulging tissue) in the colon or rectum. Colon cancer risk may also be related to
red meat consumption. The risk factors for rectal cancer includes Being aged 40 or older.
The risk factors for rectal cancer include - having certain hereditary conditions, such as familial adenomatous polyposis
(FAP) and hereditary nonpolyposis colon cancer, having a history of colorectal cancer, polyps in the colon or rectum,
cancer of the ovary, endometrium, or breast, or having a parent, brother, sister, or child with a history of colorectal cancer
Endometrial cancer is a cancer that forms in the tissue lining the uterus. Most endometrial cancers are adenocarcinomas.
Risk factors include postmenopausal estrogen therapy, obesity, a high-fat diet, reproductive factors like nulliparity, early
menarche and late menopause, polycystic ovarian syndrome, and tamoxifen use. Women with hereditary nonpolyposis
colorectal cancer syndrome have a markedly increased risk of endometrial cancer compared with women in the general
Kidney cancer is the cancer that forms in tissues of the kidneys. Kidney cancer includes renal cell carcinoma (cancer that
forms in the lining of very small tubes in the kidney that filter the blood and remove waste products) and renal pelvis
carcinoma (cancer that forms in the center of the kidney where urine collects). It also includes Wilms tumor, which is a type
of kidney cancer that usually develops in children under the age of 5.
Leukemia is a 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 bloodstream. Adult acute lymphoblastic leukemia (ALL) is a type of cancer in
which the bone marrow makes too many lymphocytes (a type of white blood cell). Previous chemotherapy and exposure to
radiation may increase the risk of developing ALL. Possible risk factors for ALL include (1) being male, (2) being white, (3)
being older than 70, (4) last treatment with chemotherapy or radiation therapy, (5) being exposed to radiation from an
atomic bomb, and (6) having certain genetic disorders, such as Down syndrome.
The two main types are small cell lung cancer and non-small cell lung cancer. These types are diagnosed based on how
the cells look under a microscope. Cigarette smoking is the major cause of lung cancer.
Melanoma is form of cancer that begins in melanocytes (cells that make the pigment melanin). It may begin in a mole (skin
melanoma), but can also begin in other pigmented tissues, such as in the eye or in the intestines. Risk factors for
melanoma include the following: (1) having a fair complexion, (2) being exposed to natural sunlight or artificial sunlight over
long periods of time, (3) being exposed to certain factors in the environment (in the air, your home or workplace, and your
food and water). Some of the environmental risk factors for melanoma are radiation, solvents, vinyl chloride, and PCBs, (4)
having a history of many blistering sunburns, especially as a child or teenager, (5) having several large or many small
moles, (6) having a family history of unusual moles (atypical nevus syndrome), (7) having a family or personal history of
melanoma, (8), being white, (9) having a weakened immune system, and (10), having certain changes in the genes that are
linked to melanoma.
Non-Hodgkin Lymphoma is any of a large group of cancers of lymphocytes (white blood cells).
Risk factors for pancreatic cancer include smoking, being very overweight, having a personal history of diabetes or chronic
pancreatitis, having a family history of pancreatic cancer or pancreatitis. having certain hereditary conditions, such as,
multiple endocrine neoplasia type 1 (MEN1) syndrome, hereditary nonpolyposis colon cancer (HNPCC; Lynch syndrome),
von Hippel-Lindau syndrome, Peutz-Jeghers syndrome, hereditary breast and ovarian cancer syndrome, familial atypical
multiple mole melanoma (FAMMM) syndrome.
[Further, fried foods contain various amount of acrylamide, a chemical has caused cancer in laboratory rats. Processed
meats such as hot dogs and sausages raises the risk of pancreatic cancer. Researchers found that consumption of
processed meats 70 grams a day or more could raise the risk as much as 50%.]
Prostate cancer incidence escalates dramatically with increasing age. Approximately 15% of men with a diagnosis of
prostate cancer will be found to have a first-degree male relative (e.g., brother, father) with prostate cancer, compared with
approximately 8% of the U.S. population. Ecological studies have found a correlation between serum levels of testosterone,
especially dihydrotestosterone DHT, and overall risk of prostate cancer among African American, white, and Japanese
males. The risk of developing and dying from prostate cancer is dramatically higher among blacks, is of intermediate levels
among whites, and is lowest among native Japanese. Ecologic studies have demonstrated a direct relationship between a
country’s prostate cancer-specific mortality rate and average total calories from fat consumed by the country’s population.
Fat of animal origin seems to be associated with a high risk for developing prostate cancer. [High blood cholesterol can
make prostate tumors grow faster, researchers injected human prostate cancer cells into mice and fed the mice a high-
cholesterol diet, then it was found cholesterol accumulated in the outer membranes of the tumor cells.]
Four main types of thyroid cancer are papillary, follicular, medullary, and anaplastic thyroid cancer. Risk factors for thyroid
cancer include being between 25 and 65 years old, being female, being exposed to radiation to the head and neck as a
child or being exposed to radiation from an atomic bomb. The cancer may occur as soon as 5 years after exposure, having
a history of goiter (enlarged thyroid), having a family history of thyroid disease or thyroid cancer, having certain genetic
conditions such as familial medullary thyroid cancer (FMTC), multiple endocrine neoplasia type 2A syndrome, and multiple
endocrine neoplasia type 2B syndrome, and being Asian.
STATEMENT AND ACKNOWLEDGEMENT
Recently, I found, a website from National Cancer Institute at the National Institutes of Health (NIH) provides a detailed
information about cancers. I really like that site. If you would like to have detailed information about cancers, please, visit
http://www.cancer.gov/. On the other hand, please, click the HOME key below to read other articles.
Cancer Prevention Brain Cancer A Special Case for Cancer Development