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.