Risks for cancer

DEFINITION: Risk is the probability that exposure to a hazard will result in a negative outcome, such as cancer. Although cancer is one of the most feared diseases in the modern world, the majority of cancer risks can be reduced through simple lifestyle choices.

Concepts of risk: There are several concepts of risk. Absolute risk is the probability of an event occurring 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 specific 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 result from 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 much less significant than an increase from 20 to 40 percent, yet both could be reported in the media as a 50 percent increase in risk.

Risk perception: 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 details 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 differently, which may be related to how much power individuals feel 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 the 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.

Age: The incidence of cancer in individuals under twenty is about twenty-five per 100,000 people, but in individuals over sixty, the rate increases to 1,000 per 100,000 people. Across all cancer types, over half of diagnoses occur in individuals over sixty-five. The increase in incidence is linked to the natural cell damage that occurs over time, longer exposure times to environmental hazards like the sun, viruses, and chemicals, and less effective cellular repair abilities.

Tobacco use: Tobacco use is responsible for about one-third of all cancer deaths. Smokers are fifteen to thirty times more likely to develop lung cancer than individuals who have never smoked. Lung cancer is not the only tobacco-related cancer. It also greatly increases the risk for cancer of the larynx, mouth, nose, 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. 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:Obesity is estimated to be responsible for 14 to 20 percent of cancer deaths in the United States. It is linked to a high risk of postmenopausal breast, colorectal, endometrial, esophageal, kidney, 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 colon, rectum, stomach, and esophagus cancers. Some researchers have found associations between fat intake and increased risk. Similarly, while fiber intake was once thought to protect against colon cancer, evidence is inconclusive.

High fat intake is a major risk factor for cancers. In the Nurses’ Health Study, which followed more than 87,000 women for up to twenty-four years, researchers compared the occurrence of breast cancer 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 cancer risk.

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 an increased risk of stomach and colorectal cancers associated with eating processed meats, pork, and red meat. Preparing meat at high temperatures (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 established.

A poor diet increases the risk of overweight and obesity, which are 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 lowers 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 over 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 further reduced their risk. Losing weight also benefits men. Among almost seventy thousand men in the Cancer Prevention Study II, those who lost at least eleven pounds over ten years 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: Research indicates that physically active people have a lower risk of certain types of cancer than sedentary people. However, it is unknown if physical activity is the reason for this. Since regular physical activity helps prevent obesity, an indirect association between physical activity and cancer risk has been hypothesized. 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 the risk of breast cancer by 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 cancer risk 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. 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 commonly associated with increased cancer risk. Experts assert this risk is very low for the average person.

Aflatoxins are naturally occurring toxins produced by certain fungus species. They are found in 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 the current risk from potentially carcinogenic chemicals, such as pesticides or cleaning agents, is unacceptably high. Many Americans avoid contact with chemicals and chemical products in everyday life to help lower their exposure to cancer-causing agents.

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 and pancreatic cancer. Hereditary forms of colon cancer, melanoma, pancreatic cancer, and brain cancers are also known. Genetic testing is available for many genetic markers that indicate cancer risk.

Both low and high birth weight are risk factors for testicular cancer. Among men, being relatively short at birth is associated with an increased risk of colorectal cancer in adulthood. Because studies that follow people from birth for sixty to eighty years are expensive and logistically difficult, little research has focused on other possible risk factors present at birth.

Viral and bacterial infections:Virus-related cancers account for about 15 percent of cancer deaths. Several 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 on the stomach lining, increases the risk of 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 more significant for individuals with chronic, untreated infections, so screening for viruses to detect them early is essential 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—reducing the body’s ability to absorb vitamins, raising hormone levels, or suppressing the immune system. There may be some association with diet since heavy drinkers tend to have poorer diets than abstainers. All types of alcohol, such as 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 reduce the body's ability to prevent cancer from forming.

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

Bibliography

Amado-Garzon, Sandra B., et al. "Elevated Vitamin B12, Risk of Cancer, and Mortality: A Systematic Review." Cancer Investigation, 2024, pp. 1-12. doi.org/10.1080/07357907.2024.2366907.

"Cancer Facts & Figures 2024." American Cancer Society, www.cancer.org/research/cancer-facts-statistics/all-cancer-facts-figures/2024-cancer-facts-figures.html. Accessed 20 July 2024.

"Cancer Prevention Overview." National Cancer Institute, 23 Oct. 2023, www.cancer.gov/about-cancer/causes-prevention/patient-prevention-overview-pdq. Accessed 20 July 2024.

Colditz, Graham A., and Cynthia J. Stein. Handbook of Cancer Risk Assessment and Prevention. Jones and Bartlett, 2004.

Rauh, Stefan. Survivorship Care for Cancer Patients: A Clinician’s Handbook. Springer, 2021.

United States Congress Office of Technology. Cancer Risk: Assessing and Reducing the Dangers in Our Society. Routledge, 2019.

Wilcox, Nicholas S., et al. "Cardiovascular Disease and Cancer: Shared Risk Factors and Mechanisms." Nature Reviews Cardiology, 2024, pp. 1-15. doi.org/10.1038/s41569-024-01017-x.