Overview
Information about the prevention of cancer and the science of screening appropriate individuals at high-risk of developing cancer is gaining interest. Physicians and individuals alike recognize that the best “treatment” of cancer is preventing its occurrence in the first place or detecting it early when it may be most treatable.
Lung cancer is the most common cancer in the world and is the leading cause of cancer death, with 160,000 deaths in the U.S. annually. There are three main types of lung cancer. The most common are non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC), which together account for over 90% of all lung cancers. Malignant mesothelioma is a rare cancer that develops in the tissues that comprise the lining of the lung.
The chance of an individual developing cancer depends on both genetic and non-genetic factors. A genetic factor is an inherited, unchangeable trait, while a non-genetic factor is a variable in a person’s environment, which can often be changed. Non-genetic factors may include diet, exercise, or exposure to other substances present in our surroundings. These non-genetic factors are often referred to as environmental factors. Some non-genetic factors play a role in facilitating the process of healthy cells turning cancerous (i.e. the correlation between smoking and lung cancer) while other cancers have no known environmental correlation but are known to have a genetic predisposition. A genetic predisposition means that a person may be at higher risk for a certain cancer if a family member has that type of cancer.
Heredity or Genetic Factors
While lung cancer has mainly been associated with environmental factors, there is emerging evidence that indicates that some individuals have a genetic predisposition for the disease. For example, individuals with chronic obstructive pulmonary disease appear to have an increased risk of developing lung cancer regardless of whether or not they smoke. Research is ongoing to further define the genetic factors associated with lung cancer.
Environmental or Non-Genetic Factors
Lung cancer is primarily associated with environmental factors, especially smoking. Several environmental agents have been associated with the development of lung cancer. Some are more easily avoidable than others.
Cigarettes: There is a large body of research that indicates that individuals who smoke are at a significantly increased risk of developing lung cancer. Approximately 85% of the lung cancer cases diagnosed each year can be associated with smoking. Over time, carcinogens in cigarettes damage the cells in the lungs and eventually, these damaged cells may become cancerous. Several factors contribute to a smoker’s risk of developing lung cancer, such as the age at initiation of smoking, the number of years smoked, the depth of inhalation, and the number of cigarettes smoked daily.
Cigars and Pipes: Individuals who smoke cigars or pipes appear to be at an increased risk of developing lung cancer, although this risk appears lower than that of cigarette smokers. Again, the number of years smoked, the amount smoked and the depth of inhalation all play a role in whether or not lung cancer will develop. Researchers have speculated that the reason for the decreased risk among cigar and pipe smokers when compared with cigarette smokers may be that cigar and pipe smokers do not inhale as deeply as cigarette smokers because cigar smoke tends to be more irritating. Cigar smokers also tend to smoke less each day, as it takes about 1-2 hours to completely smoke a cigar, whereas it takes less than 10 minutes to smoke a cigarette.
Environmental Tobacco Smoke (ETS): Exposure to environmental tobacco smoke (also called secondhand smoke) is often referred to as involuntary or passive smoking. Prolonged exposure to ETS has been associated with an increased risk for developing lung cancer. Like smokers, nonsmokers who are exposed to ETS absorb nicotine and other carcinogens, however these carcinogens are less concentrated than if they were directly inhaled. Still, the carcinogens from ETS pose health risks for nonsmokers. Researchers have estimated that ETS is responsible for approximately 3,000 lung cancer deaths per year among nonsmokers in the U.S. In addition, children of smokers appear to have reduced lung function and more respiratory tract infections than children of nonsmokers.
Lung Diseases: Some lung diseases, including tuberculosis (TB), have been associated with an increased risk of developing lung cancer. Lung diseases leave scarred tissue on the lungs that appear to increase susceptibility to lung cancer.
Air Pollution: Although the relationship between lung cancer and air pollution has not been clearly defined, some research indicates that exposure to certain air pollutants may be associated with lung cancer. These air pollutants include by-products of fossil fuel combustion, exhaust from motor vehicles and diesel engines, and emissions from power plants and industrial centers.
Asbestos: Asbestos is the general name applied to a group of naturally occurring minerals that form fibers. These asbestos fibers have been used in a variety of applications such as textiles, cement, paper, wicks, ropes, floor and roofing tiles, water pipes, wallboard, fireproof clothing, gaskets and brake linings.
Asbestos fibers easily break into particles. When inhaled, these dust particles can lodge in the lungs and cause damage that leads to an increased risk of lung cancer. It is estimated that since the beginning of World War II, approximately 8 million people have been exposed to asbestos in the workplace. Approximately 5% of the lung cancer cases diagnosed each year are caused by asbestos.
Clinical studies have demonstrated that individuals who are exposed to large amounts of asbestos have a 5 times greater risk of developing lung cancer than individuals who have not been exposed. In addition, individuals who smoke and are exposed to asbestos have an even greater risk. Studies show that smokers who are exposed to asbestos are 90 times more likely to develop lung cancer than individuals who do not smoke and have not been exposed to asbestos.
Asbestos also increases the risk of developing mesothelioma, a rare type of lung cancer that develops in the lining of the lung. Mesothelioma can result from neighborhood or environmental exposure to asbestos, occupational exposure, and household contact with asbestos dust. The rates of mesothelioma peaked for individuals born around 1910 and have steadily declined over time. Currently, there are about 2,000 cases of mesothelioma each year. This is primarily an asbestos-related cancer; only a fraction of mesothelioma cases are unrelated to asbestos exposure. Individuals involved in shipbuilding during World War II have an increased risk of mesothelioma. In fact, several shipbuilding areas such as Virginia, England, Wales and Japan, have higher rates of mesothelioma. In addition, workers who are heavily exposed to asbestos bring the dust into their homes on their clothing, hair and skin, thus exposing their family members and placing them at a higher risk of developing mesothelioma. Smoking does not appear to increase the risk of developing mesothelioma.
Radon: Radon is a carcinogen that can cause damage to the lungs that may eventually lead to lung cancer. It is estimated that radon is responsible for about 10% of the lung cancer cases diagnosed each year. Individuals who smoke and are exposed to radon are at an even higher increased risk of developing lung cancer. Most of the cancer deaths associated with radon occur among smokers.
Radon is a naturally occurring, invisible radioactive gas. It is present in soil and rocks and can seep into homes and other buildings. In situations where ventilation is restricted, radon can accumulate in the atmosphere and lead to higher levels of exposure. Individuals who work in mines are often exposed to higher concentrations of radon than the general population. However, radon can also be present in many homes. Some geographic areas have higher concentrations of radon, but radon can also seep into homes in geographic areas that are not considered high risk. When ventilation is restricted in basements in order to conserve energy, radon can seep from the soil into the basement and accumulate in the poorly ventilated area.
Prevention
Cancer is largely a preventable illness. Two-thirds of cancer deaths in the U.S. can be linked to tobacco use, poor diet, obesity, and lack of exercise. All of these factors can be modified. Nevertheless, an awareness of the opportunity to prevent cancer through changes in lifestyle is still under-appreciated.
Research is ongoing to determine the causes of lung cancer and find ways to prevent it. Avoiding the environmental risk factors may help to prevent lung cancer. In particular, refraining from smoking is an important factor in the prevention of lung cancer.
Radon: The only way to determine if a home has elevated radon levels is to perform a test. Homeowners can purchase kits designed to test the radon levels in their homes. Geographic location is not a good indicator of radon levels. Homes that are located in the same area often have different indoor radon levels. Several factors contribute to increased radon levels including soil composition under and around the house and ventilation conditions. In addition, barometric pressure and precipitation can cause radon levels to vary over time.
Because of the variances in radon levels, homeowners can utilize both short and long term tests to measure the radon levels in their homes. The tests are simple and inexpensive. Usually, a local or state radon official can assist homeowners in choosing the appropriate test.
If elevated radon levels are detected in a home, the homeowner will need to take steps to reduce the radon levels. Once a radon problem is addressed and corrected, the risk is permanently eliminated.
Asbestos: Nearly everyone has been exposed to asbestos at some point. Asbestos is widely used and small amounts may be found in a variety of products. In addition, the breakdown of asbestos products as well as natural deposits of the fiber can cause the release of asbestos particles into the environment.
Three agencies are responsible for regulating the use of asbestos products and the exposure caused to the general public. The Environmental Protection Agency (EPA) regulates asbestos in buildings, drinking water and the environment. The Consumer Product Safety Commission (CPSC) regulates asbestos in consumer products. The Food and Drug Administration (FDA) monitors asbestos contamination in food, drugs and cosmetics.
Still, individuals may be exposed to asbestos at work or in other environments. The Occupational Safety and Health Administration (OSHA) has issued regulations for employers regarding the treatment of asbestos exposure in the workplace. In addition, the Mine Safety and Health Administration (MSHA) regulates mine safety. Often, workers are provided with protective equipment and instructions regarding work practices and safety procedures. Individuals concerned about asbestos exposure at work may wish to discuss safety precautions with their employers or request information from OSHA.Individuals who are or have been exposed to asbestos should refrain from smoking.
Diet: Diet is a fertile area for immediate individual and societal intervention to decrease the risk of developing certain cancers. Numerous studies have provided a wealth of often-contradictory information about the detrimental and protective factors of different foods.
There is convincing evidence that excess body fat substantially increases the risk for many types of cancer. While much of the cancer-related nutrition information cautions against a high-fat diet, the real culprit may be an excess of calories. Studies indicate that there is little, if any, relationship between body fat and fat composition of the diet. These studies show that excessive caloric intake from both fats and carbohydrates lead to the same result of excess body fat. The ideal way to avoid excess body fat is to limit caloric intake and/or balance caloric intake with ample exercise.
It is still important, however, to limit fat intake, as evidence still supports a relationship between cancer and polyunsaturated, saturated and animal fats. Specifically, studies show that high consumption of red meat and dairy products can increase the risk of certain cancers. One strategy for positive dietary change is to replace red meat with chicken, fish, nuts and legumes.
High fruit and vegetable consumption has been associated with a reduced risk for developing at least 10 different cancers. This may be a result of potentially protective factors such as carotenoids, folic acid, vitamin C, flavonoids, phytoestrogens and isothiocyanates. These are often referred to as antioxidants.
There is strong evidence that moderate to high alcohol consumption also increases the risk of certain cancers. One reason for this relationship may be that alcohol interferes with the availability of folic acid. Alcohol in combination with tobacco creates an even greater risk of certain types of cancer.
Exercise: Higher levels of physical activity may reduce the incidence of some cancers. According to researchers at Harvard, if the entire population increased their level of physical activity by 30 minutes of brisk walking per day (or the equivalent energy expenditure in other activities), we would observe a 15% reduction in the incidence of colon cancer.
Screening and Early Detection
For many types of cancer, progress in the areas of cancer screening and treatment has offered promise for earlier detection and higher cure rates. The term screening refers to the regular use of certain examinations or tests in persons who do not have any symptoms of a cancer but are at high risk for that cancer. When individuals are at high risk for a type of cancer, this means that they have certain characteristics or exposures, called risk factors that make them more likely to develop that type of cancer than those who do not have these risk factors. The risk factors are different for different types of cancer. An awareness of these risk factors is important because 1) some risk factors can be changed (such as smoking or dietary intake), thus decreasing the risk for developing the associated cancer; and 2) persons who are at high risk for developing a cancer can often undergo regular screening measures that are recommended for that cancer type. Researchers continue to study which characteristics or exposures are associated with an increased risk for various cancers, allowing for the use of more effective prevention, early detection, and treatment strategies.
No screening procedure for lung cancer has been proven to decrease the number of lung cancer deaths. For this reason, routine screening for lung cancer is not cost-effective. However, despite the fact that they do not reduce the number of lung cancer deaths, some screening procedures are effective at detecting lung cancer at earlier stages. Early detection of lung cancer leads to a better chance of a cure. Therefore, while screening for lung cancer is generally not considered cost-effective, individuals who smoke and/or are at a high risk of developing lung cancer may wish to undergo screening.
There is currently no consensus as to the best way to screen for lung cancer. Historically, physicians have used imaging tests and sputum cytology to detect lung cancer. Although new screening procedures are being evaluated and refined, these methods are still widely used.
Chest X-rays: Historically, chest x-rays have been used as a preliminary method to detect lung cancer. Japanese researchers have had some success improving lung cancer survival rates with the mass use of annual chest x-rays in high-risk populations. Some research indicates that chest x-rays combined with sputum cytology are effective in detecting early lung cancer. Although the tests appear effective in detecting lung cancer, there is not a large body of research that supports the routine use of these procedures, as they do not appear to reduce the lung cancer death rate.
Computed Topography or CT Scan: A CT scan is a technique for imaging body tissues and organs, during which X-ray transmissions are converted to detailed images, using a computer to synthesize X-ray data. A CT scan is conducted with a large machine positioned outside the body that can rotate to capture detailed images of the oranges and tissues inside the body. This method is more sensitive and precise than the chest x-ray.
Sputum Cytology: Sputum cytology is a procedure used to examine mucus that is coughed up from the lungs or breathing tubes. The mucus is examined under a microscope in order to detect cancer cells.
Strategies to Improve Screening and Prevention
Researchers continue to search for effective screening procedures for lung cancer. The potential for earlier detection and higher cure rates increases with the advent of more refined screening techniques. In an effort to provide more screening options and perhaps more effective prevention strategies, researchers continue to explore new techniques for the screening and early detection of cancer.
Spiral CT Scan: A spiral CT scan is a new and better version of the old CT scan. During a spiral CT scan, the patient lies on a table that passes through the CT machine, which rotates around the patient. A computer then combines the images to create a 3D model of the lungs. This allows the physicians to view any potential areas of concern from infinite angles and to detect subtle changes in shape and contour.
In recent studies, spiral CT scans have proven effective in detecting tumors as small as 1-2 millimeters. Research has shown that there is a decrease in cure rate for every millimeter increase in tumor size. Therefore, the use of spiral CT scans to focus on finding lung cancers below 1 centimeter holds promise for diagnosing early stage lung cancer.
In addition, spiral CT scans can detect early emphysema. In a recent clinical study, the Early Lung Cancer Action Program (ELCAP) evaluated the benefits of using low-dose spiral CT scans. The results indicated that spiral CT scans could detect early lung cancer and early emphysema. One unexpected result of these early emphysema diagnoses was that many smokers in the study quit smoking after learning that they had emphysema.
Thus far, low-dose spiral CT scans show potential for the early detection of lung cancer in high-risk people and offer the promise that more people could be cured. Further study will determine the validity of these results. In the meantime, individuals at a high risk of developing lung cancer may wish to undergo spiral CT scans despite the fact that this test is not yet routinely covered by insurance.