Respiration and Respiratory Systems

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.