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 after World War II was at first ascribed to better diagnostic methods, but by 1956 it had become clear that the rate of increase was too great to be accounted for in this way. At that time the first epidemiological studies began to indicate that a long history of cigarette smoking was associated with a great increase in risk of death from lung cancer. By 1965 cancer of the lung and bronchus accounted for 43 percent of all cancers in the United States in men, an incidence nearly three times greater than that of the second most common cancer (of the prostate gland) in men, which accounted for 16.7 percent of cancers. The 1964 Report of the Advisory Committee to the Surgeon General of the Public Health Service (United States) concluded categorically that cigarette smoking was causally related to lung cancer in men. Since then, many further studies in diverse countries 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 to be explained by the later development of heavy cigarette smoking in women compared with men, who greatly increased their cigarette consumption during World War II. By 1988 there was evidence suggesting that the peak incidence of lung cancer due to cigarette smoking in men may have been passed. The incidence of lung cancer mortality in women, however, is increasing.

   The reason for the carcinogenicity of tobacco smoke is not known. Tobacco smoke contains many carcinogenic materials, and although it is assumed that the "tars" in tobacco smoke probably contain a substantial fraction of the cancer-causing condensate, it is not yet established which of these is responsible. In addition to its single-agent effects, cigarette smoking greatly potentiates the cancer-causing proclivity of asbestos fibres, increases the risk of lung cancer due to inhalation of radon daughters (products of the radioactive decay of radon gas), and possibly also increases the risk of lung cancer due to arsenic exposure. Cigarette smoke may be a promoter rather than an initiator of lung cancer, but this question cannot be resolved until the process of cancer formation is better understood. Recent data suggest that those who do not smoke but who live or work with 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, and because it may spread beyond the lungs at an early stage, the first symptoms noted by the patient vary from blood staining of the sputum, to a pneumonia that does not resolve fully with antibiotics, to shortness of breath due to a pleural effusion; the physician may discover distant metastases to the skeleton, or in the brain that cause symptoms unrelated to the lung. Lymph nodes may be involved early, and enlargement of the lymph nodes in the neck may lead to a chest examination and the discovery of a tumour. In some cases a small tumour metastasis in the skin may be the first sign of the disease. Lung cancer may develop in an individual who already has chronic bronchitis and who therefore has had a cough for many years. The diagnosis depends on securing tissue for histological examination, although in some cases this entails removal of the entire neoplasm 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 been attempted, and large-scale trials of routine sputum examination for the detection of malignant cells have been conducted, but neither screening method appears to have a major impact on mortality. Therefore, attention has been turned to prevention by every means possible. Foremost among them are efforts to inform the public of the risk and to limit the advertising of cigarettes. Steps have been taken to reduce asbestos exposure, both in the workplace and in public and private buildings, and to control air pollution. The contribution of air pollution to the incidence of lung cancer is not known with certainty, though there 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 be confined 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 increasingly efficient insulation of houses, radon daughters may reach concentrations high enough to place the occupants at risk for lung cancer. A recent survey of houses in the United States indicated that about 2 percent of all houses had a level of radon daughters that posed some risk to the occupants. Major regional variations in the natural distribution of radon occur, and it is not yet possible to quantify precisely the actual magnitude of the risk. In some regions of the world (such as the Salzburg region of Austria) levels are high enough that radon daughters are believed to account for the majority of cases of lung cancer in nonsmokers.

   Workers exposed to arsenic in metal smelting operations, and the community around the factories from which arsenic is emitted, have an increased risk for lung cancer. Arsenic is widely used in the electronics industry in the manufacture of microchips, 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 affects men and women in equal proportion and is not related to cigarette smoking. Pulmonary adenocarcinoma of the lung also has a more equal sex incidence than other types, and although its incidence is increased in smokers, it may also be caused by other factors.

   It is common to feel intuitively that one should be able to apportion cases of lung cancer among discrete causes, on a percentage basis. But in multifactorial disease, this is not possible. Although the incidence of lung cancer would probably be far lower without cigarette smoking, the contribution of neither this factor nor any of the other factors mentioned can be precisely quantified.