LUNG CANCER
Lung cancer is a malignant disease in which lung cells become abnormal, characterized by uncontrollable, unlimited growth. These cells can then invade nearby normal tissue and destroy organ structure, a process called "invasion." Lung cancer cells can also break down lung tissue structure and enter the bloodstream or lymphatic system and thus spreads to distant organs in other parts of the body, a process called metastasis. Clinically, lung cancer can be classified into two groups according to its cell types under microscopy: non-small cell lung cancer and small cell lung cancer. Non-small cell lung cancer includes cancers of three cell types: squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. Small cell lung cancer, also called oat cell cancer, is a less common cancer that grows faster, and is more likely to spread to other parts of the body than non-small cell lung cancer.
Lung cancer is a highly lethal disease in the United States and worldwide. According to Parkin et al. (1999), lung cancer was the most frequent cancer in 1990, worldwide, with 1.04 million new cases (771,800 in men and 265,100 in women). It is the most common cancer in men and the fifth most frequent cancer in women. Lung cancer is the leading cause of cancer deaths worldwide, with a total of 921,000 deaths per year (692,600 in men and 228,400 in women) in 1990. In the United States, it was estimated that 169,500 new lung cancer patients (90,700 men and 78,800 women) would be diagnosed and 157,400 (90,100 men and 67,300 women) would die of lung cancer in 2001. The five-year survival rate of lung cancer is 13.7 percent in the United States, 7.8 percent in developing countries, 7 percent in Eastern Europe, 7.9 percent in China, and 6.7 percent in India.
The changes (increase or decrease) of lung cancer incidence corresponds to the alterations of prevalence of smoking in the population twenty to thirty years earlier, representing a latent period between tobacco exposure and the occurrence of lung cancer. A significant decrease in the incidence of lung and bronchus cancer in males in North America started in the late 1980s. Between 1990 and 1996 there was a 2.6 percent decline in incidence per year. Incidence rates of lung and bronchus cancer in females are stabilizing in the United States. Although the death rate from lung cancer in males is decreasing, it is increasing among females, and it has now exceeded the breast cancer death rate among females.
Tobacco smoking is a major cause of lung cancer. Over 4,000 chemical compounds have been identified in the tobacco leaf. Carcinogens in tobacco smoke can damage the cells in the lungs, which may lead to the development of lung cancer. More than fifty chemical compounds in tobacco smoke have been recognized as known or probable human carcinogens, some of which may be formed during combustion (or smoking) and some which may exist naturally in tobacco. Several groups of carcinogens in tobacco smoke are related to lung cancer, including polycyclic aromatic hydrocarbons (PAHs), aromatic amines, benzene, hydrazine, and vinyl chloride. Smoking results in damage to the bronchial and lung epithelium, which leads to lung cell proliferation and finally to lung cancer. Animal studies confirm the carcinogenic potential of tobacco smoke in tissues having smoke contact: in these studies smoke exposure leads to laryngeal tumors and pulmonary adenomas. In humans, cigarette smokers have increased levels of tobacco carcinogen DNA adducts in the lung and bronchus when compared with nonsmokers.
A very strong association between cigarette smoking and lung cancer has been consistently observed in studies done since the early 1950s. These studies have shown that cigarette smoking precedes lung cancer occurrence. It has been estimated that cigarette smokers have a ten-fold higher risk of lung cancer, in comparison with nonsmokers. With the increased number of cigarettes smoked per day, the risk is increased—heavy smokers are at greater risk of lung cancer than moderate smokers; and moderate smokers are at higher risk than light smokers and nonsmokers. The risk for individuals who smoke two or more packs per day is about twenty times that of nonsmokers, and longer smoking duration has a stronger effect on the risk of lung cancer. Beginning to smoke at an early age is also related to an increased risk, and the lung cancer risk declines with an increased duration of cessation. The percentage of reduction in risk after quitting smoking depends on the duration of exposure to smoking. The observed relationship between cigarette smoking and the risk of lung cancer is consistent with different study designs and in studies of different populations all over the world. Over eighty-five percent of deaths from lung cancer can be attributed to cigarette smoking. It is estimated that tobacco smoking accounts for over ninety percent of male lung cancer deaths and seventy-nine percent of female lung cancer deaths in the United States.
Smoking of other tobacco products, such as cigar and pipe smoking, is also associated with an increased risk of lung cancer. Like cigarette smoking, the risk of lung cancer is increased with the frequency and years of cigar and pipe smoking. Environmental tobacco smoke (ETS), also known as secondhand smoke, increases the risk of lung cancer among nonsmokers. It is estimated that ETS may lead to 3,000 new cases of lung cancer per year in nonsmokers in the United States. Other risk factors for lung cancer include race, occupational exposures (e.g., arsenic, asbestos, chromium, mustard gas, PAHs), residential radon exposure, radiation, air pollution, and nutritional factors. The host susceptibility factors for lung cancer include inheritance of different polymorphic genotypes that may interact with tobacco smoke in determining the risk of lung cancer.
Smoking cessation or lifelong abstinence from smoking offer the best opportunities to reduce lung cancer incidence and death rates. Reducing the prevalence of smoking will lead to a dramatic decrease in the incidence of lung cancer in the general population. According to the Centers for Disease Control and Prevention (CDC), cigarette smoking is the single most preventable cause of premature death in the United States. More than 400,000 people die from causes attributable to cigarette smoking each year, including 276,000 men and 142,000 women. The promotion of smoking cessation is the most cost-effective tool against lung and other smoking-related cancers and diseases.
Control of other risk factors, such as workplace exposures associated with the increased risk of lung cancer, environmental tobacco smoke, and radon exposure in residences, may also lead to a reduced risk of lung cancer. Sputum cytology and chest radiographs are not recommended for lung cancer screening because no favorable impact of the screening on lung cancer mortality has been demonstrated. Recent developments have pointed out that the molecular genetic alterations associated with progression toward lung cancer, such as p53 mutations in sputum samples, may help to identify high-risk individuals for early detection and chemoprevention.
BIBLIOGRAPHY
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