Научная Петербургская Академия

Реферат: Respiration and Respiratory Systems

Реферат: Respiration and Respiratory Systems

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.



(C) 2009