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Реферат: Respiration and Respiratory Systems
LUNG CANCER
Up to the time of World War II, cancer of the lung was a relatively rare condition. The increase in its incidence in Europe afterWorld War II was at first ascribed to better diagnostic methods, but by 1956 it had become clear that the rate of increase wastoo great to be accounted for in this way. At that time the first epidemiological studies began to indicate that a long history ofcigarette smoking was associated with a great increase in risk of death from lung cancer. By 1965 cancer of the lung andbronchus accounted for 43 percent of all cancers in the United States in men, an incidence nearly three times greater than thatof the second most common cancer (of the prostate gland) in men, which accounted for 16.7 percent of cancers. The 1964Report of the Advisory Committee to the Surgeon General of the Public Health Service (United States) concludedcategorically that cigarette smoking was causally related to lung cancer in men. Since then, many further studies in diversecountries have confirmed this conclusion.
The incidence of lung cancer in women began to rise in 1960 and continued rising through the mid-1980s. This is believed tobe explained by the later development of heavy cigarette smoking in women compared with men, who greatly increased theircigarette consumption during World War II. By 1988 there was evidence suggesting that the peak incidence of lung cancerdue to cigarette smoking in men may have been passed. The incidence of lung cancer mortality in women, however, isincreasing.
The reason for the carcinogenicity of tobacco smoke is not known. Tobacco smoke contains many carcinogenic materials, andalthough it is assumed that the "tars" in tobacco smoke probably contain a substantial fraction of the cancer-causingcondensate, it is not yet established which of these is responsible. In addition to its single-agent effects, cigarette smokinggreatly potentiates the cancer-causing proclivity of asbestos fibres, increases the risk of lung cancer due to inhalation of radondaughters (products of the radioactive decay of radon gas), and possibly also increases the risk of lung cancer due to arsenicexposure. Cigarette smoke may be a promoter rather than an initiator of lung cancer, but this question cannot be resolved untilthe process of cancer formation is better understood. Recent data suggest that those who do not smoke but who live or workwith smokers and who therefore are exposed to environmental tobacco smoke may be at increased risk for lung cancer,eloquent testimony to the power of cigarettes to induce or promote the disease.
Because lung cancer is caused by different types of tumour, because it may be located in different parts of the lung, andbecause it may spread beyond the lungs at an early stage, the first symptoms noted by the patient vary from blood staining ofthe sputum, to a pneumonia that does not resolve fully with antibiotics, to shortness of breath due to a pleural effusion; thephysician may discover distant metastases to the skeleton, or in the brain that cause symptoms unrelated to the lung. Lymphnodes may be involved early, and enlargement of the lymph nodes in the neck may lead to a chest examination and thediscovery of a tumour. In some cases a small tumour metastasis in the skin may be the first sign of the disease. Lung cancermay develop in an individual who already has chronic bronchitis and who therefore has had a cough for many years. Thediagnosis depends on securing tissue for histological examination, although in some cases this entails removal of the entireneoplasm before a definitive diagnosis can be made.
Survival from lung cancer has improved very little in the past 40 years. Early detection with routine chest radiographs has beenattempted, and large-scale trials of routine sputum examination for the detection of malignant cells have been conducted, butneither screening method appears to have a major impact on mortality. Therefore, attention has been turned to prevention byevery means possible. Foremost among them are efforts to inform the public of the risk and to limit the advertising ofcigarettes. Steps have been taken to reduce asbestos exposure, both in the workplace and in public and private buildings, andto control air pollution. The contribution of air pollution to the incidence of lung cancer is not known with certainty, thoughthere is clearly an "urban" factor involved.
Persons exposed to radon daughters are at risk for lung cancer. The hazard from exposure was formerly thought to beconfined to uranium miners, who, by virtue of their work underground, encounter high levels of these radioactive materials.However, significant levels of radon daughters have been detected in houses built over natural sources, and with increasinglyefficient insulation of houses, radon daughters may reach concentrations high enough to place the occupants at risk for lungcancer. A recent survey of houses in the United States indicated that about 2 percent of all houses had a level of radondaughters that posed some risk to the occupants. Major regional variations in the natural distribution of radon occur, and it isnot yet possible to quantify precisely the actual magnitude of the risk. In some regions of the world (such as the Salzburgregion of Austria) levels are high enough that radon daughters are believed to account for the majority of cases of lung cancerin nonsmokers.
Workers exposed to arsenic in metal smelting operations, and the community around the factories from which arsenic isemitted, have an increased risk for lung cancer. Arsenic is widely used in the electronics industry in the manufacture ofmicrochips, and careful surveillance of this industry may be needed to prevent future disease.
Some types of lung cancer are unrelated to cigarette smoking. Alveolar cell cancer is a slowly spreading condition that affectsmen and women in equal proportion and is not related to cigarette smoking. Pulmonary adenocarcinoma of the lung also has amore equal sex incidence than other types, and although its incidence is increased in smokers, it may also be caused by otherfactors.
It is common to feel intuitively that one should be able to apportion cases of lung cancer among discrete causes, on apercentage basis. But in multifactorial disease, this is not possible. Although the incidence of lung cancer would probably be farlower without cigarette smoking, the contribution of neither this factor nor any of the other factors mentioned can be preciselyquantified.