What are risks for cancer?

Concepts of risk: There are several concepts of risk. Absolute risk
is defined as the probability that an event will occur over a defined period.
Age-specific lifetime risk estimates are a type of absolute risk. For example, a
woman may have a cumulative 12.5 percent lifetime risk of breast cancer but only
have a 2 percent chance of developing the disease in the next five years. Risk is
frequently described in epidemiological studies (studies of population-wide
patterns of disease) using a ratio known as a relative risk (RR), which compares
the incidence of disease in people who have a certain risk factor, such as family
history, to those who do not have the risk factor (control group). Many studies of
cancer risk factors have found higher or lower relative risks that are so slight
that they could be the result of random chance. These are referred to as
statistically insignificant. When assessing risk, it is also important to know the
effect size, rather than just a percentage increase in risk. For example, an
increase in risk from 1 to 2 percent is a much less significant finding than an
increase in risk from 20 to 40 percent, yet both increases in risk could be
reported in the media as a 50 percent increase in risk.

Risk perception: Risk means different things to different people.
Experts often see risk differently from the average person, because experts tend
to evaluate risk based on statistics and technical information. The average
person, however, may judge risks based on technical information and many other
factors, such as how familiar the risk is, whether the risk can be controlled, the
catastrophic potential of the outcome, and the voluntariness of the exposure. Men
and women also tend to perceive risk very differently, which may be related to how
much power individuals feel they have over their exposure to hazards. Many people
are willing to tolerate a higher level of risk from activities seen as beneficial
or enjoyable, such as smoking and drinking; the perceived risk goes down as the
perceived benefit goes up.

Because cancer is so common, represents such a large burden on society, and is a
highly dreaded disease, public health researchers are very interested in
understanding risk factors for cancer. One of the most important messages coming
out of this research since the early 1980s is that the majority of risks for
cancer can be mitigated through lifestyle changes.

Tobacco use: Tobacco use is responsible for about one-third of all
cancer deaths, according to the American Cancer Society. A 2014 US Surgeon General
Report stated that smokers are approximately twenty-five times more likely to
develop lung cancer than individuals who have never smoked. Lung cancer is not the
only tobacco-related cancer; tobacco use also greatly increases
the risk for cancer of the larynx, mouth, noses and sinuses,, esophagus, pharynx,
colon, ovaries, bladder, kidney, pancreas, stomach, uterus, and cervix. Heavy
smokers are at a higher risk of developing lung cancer than light smokers; for
example, a person who smokes forty cigarettes a day for twenty years is at higher
risk than one who smokes twenty a day for forty years. No matter how long or how
much a person has used tobacco, it is never too late to quit: The risk of cancer
begins to decrease as soon as tobacco use stops. Smokers should discuss
smoking
cessation with their physician.

Cigars and pipes are often seen as less harmful than cigarettes. However, even if
cigar and pipe smokers do not inhale, they are at increased risk for cancer of the
oral cavity and lungs. Pipe smokers also are at increased risk for lip cancers in
areas where the pipe stem rests. Using chewing tobacco and dry snuff increases the
risk of cancer in the cheek, gums, and lips.

Diet and obesity and overweight: Overweight and obesity are
estimated to be responsible for 14 to 20 percent of cancer deaths in the United
States. Obesity is linked to a right risk of postmenopausal breast cancer,
colorectal cancer, endometrial cancer, esophageal cancer, kidney cancer, and
pancreatic cancer. Diets low in vegetables, fruits, whole grains, and beans and
high in animal protein and fat have been convincingly linked to higher risk for
many cancers, including those of the colon, rectum, stomach, and esophagus. Some
researchers have found associations with fat intake and increased risk, while
others have not. Similarly, while fiber intake was once thought to
protect against colon cancer, the evidence now is inconclusive.

High fat intake is a major risk factor for cancers. In the large, well-known
Nurses’ Health Study, which followed more than 87,000 women for up to twenty-four
years, researchers compared the occurrence of breast cancers in women who ate the
most animal fat to women who ate the lowest amount and found a statistically
significant relative risk of 1.33 (33 percent increased risk). Plant fats, such as
those from avocados and walnuts, do not appear to increase the risk of cancer.

Processed meats, such as bacon, hot dogs, and sausage, may also carry a cancer
risk. In a study that followed nearly 200,000 men and women for seven years,
people who consumed the most processed meats increased their risk of pancreatic
cancer by 68 percent over those who consumed few or no meat products. Other
studies have shown increased risk of stomach and colorectal cancers associated with eating
processed meats, pork, and red meat. Preparing meat at high temperatures (for
example, grilling or using a wok) can create higher levels of cancer-causing
substances in the meat. These studies suggest that eating red meat, processed
meats, and pork should be limited to two to three times a week at most,
particularly in childhood, when eating habits are being established.

A poor diet increases the risk of overweight and obesity, which are thought to be
responsible for about 14 percent of cancer deaths in men and 20 percent in women.
Researchers have identified three major ways that excess weight especially in the
midsection may increase cancer risk. One is that body fat secretes substances that
seem to promote inflammation throughout the body, increasing the chance of
deoxyribonucleic acid (DNA) damage that allows cancer to start. The second is that
being overweight can lead to higher blood levels of insulin and insulin-related
growth factors, which promote the development of some cancers. Third, excess body
fat also changes the levels of several reproductive hormones, such as estrogen and
testosterone.

Preventing weight gain is best, but losing excess body fat also seems to lower
cancer risk. In the Nurses’ Health Study, postmenopausal women who lost twenty-two
pounds or more and kept it off had a 30 percent lower breast cancer risk than
postmenopausal women who did not lose weight. Another major study, the Iowa
Women’s Health Study, followed more than thirty-three thousand women for up to
fifteen years. Women who lost weight after menopause had a 23 percent lower risk
of breast cancer compared with women who gained weight throughout adulthood. Women
who began to lose weight before menopause reduced their risk even more. Losing
weight benefits men as well: Among almost seventy thousand men in the Cancer
Prevention Study II, those who lost at least eleven pounds over a ten-year period
had a 16 to 17 percent overall lower risk of prostate cancer and 42 percent lower
risk of aggressive forms of prostate cancer.

Lack of exercise: Several studies have shown that people who are
physically active have a lower risk of certain types of cancer than people who are
sedentary; however, it is not known if physical activity itself is the reason for
this. Since regular physical activity helps prevent obesity, an indirect
association between physical activity and cancer risk has been hypothesized for
some time. Physical activity also changes the body’s levels of hormones, insulin,
and other growth factors, improves the immune system, and has an anti-inflammatory
effect, all of which may help prevent cancer.

Studies consistently show that after controlling for weight, colon cancer risk
drops 40 to 50 percent with exercise. Similarly, regular moderate exercise may
reduce risk of breast cancer 30 to 40 percent, with greater benefits after
menopause. Exercise may also independently lower the risk of prostate, lung,
endometrial, ovarian, and kidney cancers, although confirming studies are
needed.

Environmental hazards: Environmental hazards that increase the risk
of cancer include radiation, asbestos, certain chemicals (such as
pesticides, pollution, arsenic, and formaldehyde), and aflatoxins. Many of these
hazards are related to occupational exposure. For example, people who work with
herbicides are at increased risk of lymphoma, and construction workers are at
higher risk of lung cancer from asbestos. Asbestos exposure is also related to an
increased laryngeal cancer incidence, and exposure to cement dust raises the risk
for pharyngeal cancer. Radon, a naturally occurring radioactive material, is known
to increase the risk of lung cancer among underground miners exposed to high
levels. Household levels of radon exposure have not been shown to increase lung
cancer risk. Chronic exposure to ultraviolet radiation from the sun is
the leading cause of nonmelanoma skin cancers. Exposure to ionizing radiation from
medical tests such as X-rays, CT scans, fluoroscopy, and nuclear medicine scans is
also associated with increased cancer risk, particularly for leukemia, thyroid
cancer, and breast cancer.

Aflatoxins are naturally occurring toxins produced by
certain species of fungus, which are found on foods such as corn, peanuts, various
other nuts, and cottonseed. High-level aflatoxin exposure is a risk factor for
liver cancer, and infection with hepatitis B increases this risk. Food-borne
aflatoxin exposure is most common in Africa, China, and Southeast Asia.

The average American is very concerned about carcinogens, and believes that the
current risk from potentially carcinogenic chemicals, such as pesticides or
cleaning agents, is unacceptably high. Up to 70 percent of Americans say they try
to avoid contact with chemicals and chemical products in everyday life.

Genetics: About 5 to 10 percent of cancer deaths are related to
genetic
factors and factors present at birth. A family history of any
cancer raises a person’s risk, but the increase varies by type of cancer.
Breast-ovarian cancer syndrome results from mutations in the
BRCA1 or
BRCA2 gene. People with this mutation
have an 80 to 90 percent lifetime risk of breast cancer and a 20 to 60 percent
chance of developing ovarian cancer. Men with this syndrome have an elevated risk
of prostate cancer, and both sexes are at increased risk of melanoma and pancreatic
cancer. Hereditary forms of colon cancer, melanoma, pancreatic cancer, and brain
cancers are also known.

Both low and high birth weight have both been identified as risk factors for
testicular cancer. Among men, but not women, being relatively short at birth is
associated with increased risk of colorectal cancer in adulthood. Because studies
that follow people from birth for sixty to eighty years are expensive and
logistically difficult, very little research has been done on other possible risk
factors that are present at birth.

Viral and bacterial infections:
Virus-related
cancers are thought to account for about 5 percent of cancer
deaths. Several different types of infections with oncogenic
viruses raise a person’s cancer risk. For example, people
infected with hepatitis B virus (HBV) and hepatitis C
virus (HCV) are more susceptible to liver cancer and
lymphoma. Chronic infection with
Helicobacter pylori
, a
bacterium that lives on the lining of the stomach, increases risk for stomach
cancer and lymphoma. Human immunodeficiency virus (HIV) and
Epstein-Barr
virus are linked to lymphoma as well. People with inflammatory bowel
disease have above-average rates of colon cancer. Cervical and anal cancers are
associated with the human papillomavirus (HPV). The risk is
greater for people who have chronic, untreated infections, so screening for
viruses to catch them early is important in preventing cancer.

Alcohol: Alcohol use is associated with increased risk for oral,
esophageal, laryngeal, pharyngeal, breast, colorectal, and liver cancers. Alcohol
may increase cancer risk in several ways: It may reduce the body’s ability to
absorb vitamins, raise the level of hormones in the body, or suppress the immune
system. There may be some association with diet as well, since heavy drinkers tend
to have poorer diets than abstainers. All types of alcohol beer, wine, and liquor
increase risk equally.

Immunosuppressive medications: Immunosuppressive medications, such as
those given to transplant recipients, have also been linked to an increased risk
of cancer. These medications are thought to reduce the body's ability to prevent
cancer from forming.

Screening: Although screening does not, by itself, prevent cancer, it
does reduce the risk of advanced or late-stage cancer. The earlier a case of
cancer is detected, the better the chances of survival. There are simple screening
tests for many common cancers, including breast, prostate, ovarian, colorectal,
and oral.

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