Cancer Glossary & Scientific Reference

Lung Cancer

Smoking, Asbestosis, Radon, Mesothelioma

compiled & edited by Healing Cancer Naturally from material © Encyclopædia Britannica, Inc.

Lung cancer, a malignant tumor of the lung, was first described by doctors in the mid-1800s. In the early 20th century it was considered relatively rare up to the time of World War II. 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. Risk factors also include occupational chemicals, metals etc.

See Cigarettes and tobacco smoke and Risk factors.

Types of Lung Cancer

There are two basic kinds of lung cancer: small-cell carcinoma and non-small-cell carcinoma. The latter consists primarily of three types of tumour: squamous-cell carcinoma, adenocarcinoma, and large-cell carcinoma.

Small-cell carcinoma, also called oat-cell carcinoma, accounts for about 20 to 25 percent of all lung cancers. It is characterized by cells that are small and round, oval, or shaped like oat grains. Small-cell carcinoma is the most aggressive type of lung cancer; it tends to spread quickly, before symptoms are apparent.

Some 25 to 30 percent of primary lung cancers are squamous-cell carcinomas, also called epidermoid carcinomas. This tumour is characterized by flat, scalelike cells, and it often develops in the larger bronchi of the central part of the lung. Squamous-cell carcinoma tends to remain localized in the lung longer than other types of cancer and thus is generally more responsive to treatment.

Worldwide, adenocarcinoma accounts for some 25 to 30 percent of lung cancers; it is the most common type of lung cancer in the United States. Cells of adenocarcinoma are cube- or column-shaped, and they form structures that resemble glands and are sometimes hollow. Tumours often originate in the smaller, peripheral bronchi; symptoms at the time of diagnosis often reflect invasion of the lymph nodes, pleura, or the other lung or metastasis to other organs.

About 20 percent of all lung cancers are large-cell carcinomas. There is some dispute as to whether it is a distinct type of cancer or merely a group of atypical squamous-cell carcinomas and adenocarcinomas. Large-cell carcinomas generally originate in the peripheral areas of the lung.

Lung Cancer Genesis & Symptoms

Lung cancers arise in the epithelium lining the bronchi (the branching complex of air passages), by which air passes to the lungs, or in the fine air sacs at the periphery. The most common forms arise in bronchial glandular epithelium that has been altered by long exposure to cigarette smoke to form less specialized squamous cells, which eventually evolve into squamous-cell carcinomas.

Structurally, unaltered glandular epithelia of bronchi may also undergo malignant transformation to give rise to adenocarcinomas, but these tumours do not appear to be related to cigarette smoking.

In the earliest stages of lung cancer there are often no symptoms. In later stages symptoms may include coughing, chest pain, shortness of breath, blood in the sputum, and repeated episodes of pneumonia; weight loss, loss of appetite, and weakness may also accompany the disease. Sometimes the first symptoms result from the metastasis of the tumour to other parts of the body.

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.

Cancers in different organs and individual carcinomas of the same cell type may have different doubling times (i.e., the time it takes for the tumour mass to double in the number of cells or in size). Doubling times observed for lung cancer, for instance, may vary widely, with ranges from eight days to more than 700 days. The doubling time is a reflection not only of the rate of tumour cell growth but also of the loss of cells through spontaneous death.

Lung Cancer Risk Factors

Risk factors include cigarettes and tobacco smoke, exposure to radon gas and to asbestos; smokers exposed to these substances run a greater risk of developing lung cancer than do nonsmokers. Uranium and pitchblende miners, workers in chromium and nickel refining, welders, and workers exposed to haloethers also have an increased incidence of lung cancer, as do some workers in hydrocarbon-related processing, such as coal processors, tar refiners, and roofers.

See Occupational chemicals, metals etc.

Cigarettes and Tobacco Smoke

The deleterious health effects of tobacco smoking have long been recognized. As early as the 19th century, isolated reports were circulated that cigar and pipe smoking caused cancer of the mouth. It was not until the 20th century--in fact, not until 1950--that firm evidence was established that lung cancer was directly related to cigarette smoking.

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.

By the late 20th century lung cancer was the leading cause of cancer-related death among men in some 28 developed countries, including the United States. During the 1980s it surpassed breast cancer as the leading cause of death from cancer among women in the United States.

This rapid increase in the incidence of lung cancer was due mostly to the increased use of cigarettes that began after World War I.

The worldwide incidence in the disease was expected to increase in future decades as a result of the spread in cigarette smoking, particularly among women and in the developing countries.

Lung cancer is now the most common cause of cancer deaths in men of most Western countries. Because cigarette smoking became a popular and fairly widespread habit among men during World War II and because this disease has a long latency period, the rise in the incidence of lung cancer during the second part of the 20th century was expected.

In the late 20th century, in many countries, as many as one-third of all cancer deaths in men and 10 percent of those in women are attributed to cigarette smoking. As the current trend of males to give up smoking continues, the male percentage will decline. Less optimistically, however, increasingly more women picked up the habit in the 1950s and '60s; the proportion of women dying from a tobacco-related cancer thus was increasing.

In general, the risk of developing a tobacco-related cancer depends on the intensity of the habit as determined by duration of the smoking habit, number of cigarettes smoked per day, tar content of the cigarette, and the depth of inhalation. Hence heavy smokers have a greater likelihood of developing the disease than do light smokers. The risk is also greater for those who started smoking at a young age.

Cigarette smoking not only is related to the development of lung cancer but also affects the development of cancer of the bladder, oral cavity, and esophagus. As noted, the risk of developing one of these cancers increases with the number of cigarettes smoked per day.

Studies have also shown that various cancer sites are affected differently by different tobacco products, as well as by different intensities. In general, however, those sites that come into direct contact with tobacco smoke--the lungs, oral cavity, and larynx--are those that are the most adversely affected by exposure to smoke.

Smoking is also a prime risk factor in cardiovascular system diseases. Nicotine is known to contract the blood vessels and to release hormones that raise the blood pressure. Both effects could have an adverse effect on the heart.

Smokers have distinctly higher levels of carbon monoxide in their blood than nonsmokers. Carbon monoxide readily combines with hemoglobin, causing many physiological effects.

One is a decrease in the amount of hemoglobin available to carry oxygen and a resulting increase in the affinity for oxygen of the hemoglobin that is available.

This in turn reduces the availability of oxygen to the tissues. It has been shown that even minute amounts of carbon monoxide decrease the exercise ability of patients with known coronary artery disease.

Another important piece of epidemiological evidence linking smoking to disease is that as individuals give up smoking, the risk of lung cancer, coronary artery disease, chronic bronchitis, emphysema, and other tobacco-related diseases declines.

The speed and degree of this decline depends, as would be expected, on the duration and intensity of the smoking habit. Among those who have smoked over 20 cigarettes a day for over 20 years, a minimum of three years must elapse after quitting before a decreased risk for cancer is evident; more than 10 years of abstinence is necessary before the degree of risk approaches that for those who have never smoked.

Nonsmokers’ Risk

Passive smoking--i.e., a nonsmoker's inhalation of smoke produced by smokers in an enclosed space--also appears to heighten the risk of developing lung cancer. Several studies have found that, over the long term, the nonsmoking spouses of smokers experience a lung cancer risk that is almost double that of spouses neither of whom smoke.

In the early 1990s it was estimated that passive smoking caused some 2,500-3,300 lung cancer deaths each year in the United States, or about 2 percent of all U.S. lung cancer deaths.
Compare Radon.

It should be emphasized, however, that smokers continue to have a much higher lung-cancer risk; the lung-cancer mortality risk for a heavy smoker is 20 to 30 times greater than that of a nonsmoker.

Carcinogenicity of Tobacco Smoke

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, see Radon), 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.

Occupational Chemicals, Metals etc.

Fumes inhaled by workers during coke-oven operations and in refineries have been associated with high incidences of lung cancer. Various metals have been implicated as carcinogens for the lung and several other body sites among copper-ore miners and smelters of nickel and cobalt ores.

Asbestos has been established as a carcinogen for the lung and for the mesothelium (membrane) that lines body cavities. Workers chronically exposed to dust containing asbestos fibres have an incidence of lung cancer 10 times the normal rate.

(It is noteworthy that the risk of cancer is increased 90-fold in asbestos workers if they also smoke. This is an excellent example of two agents acting synergistically to induce cancer at a higher incidence and often in a shorter time than either agent does alone.)

Although a serious effort has been made to reduce industrial exposure to asbestos, in some parts of the world occupational pollution remains responsible for a debilitating and progressive form of lung disease (asbestosis), as well as cancer, in asbestos workers. Silica and asbestos remain in the lungs for long periods of time, and both produce lung fibrosis.
See Asbestos and Asbestosis.

Some of the hazardous metals commonly encountered in industry have been shown to be carcinogenic, including certain compounds of nickel (linked to lung and nasal cancer), chromium (lung cancer), and arsenic (lung and skin cancer).

In addition to producing skin and lung tumors in humans, arsenic compounds damage many organs, causing skin lesions, decrease in heart contractility, blood vessel damage, and injuries of the nervous system, kidney, and liver. As mentioned, certain nickel and hexavalent chromium compounds, as well as beryllium oxide, are toxic to the lungs and can cause lung cancer.

The increasing use of man-made mineral fibres (as in fibreglass and rock wool) has led to concern that these may also be dangerous when inhaled; present evidence suggests that they do increase the risk of lung cancer in persons occupationally exposed to them.

Many chemicals can damage the lung in high concentration: these include oxides of nitrogen, ammonia, chlorine, oxides of sulfur, ozone, gasoline vapour, and benzene.

Uranium mine workers develop lung cancer to a significantly higher degree than the general population. The cause has been traced to the inhalation of the radioactive gas radon, released from trace amounts of radium in uranium ore.

See Radon.

Asbestos

Any of several minerals that readily separate into long, flexible fibres. The fibre was formerly widely used in brake linings, gaskets, and insulation; and in roofing shingles, floor and ceiling tiles, cement pipes, and other building materials. Asbestos fabrics were used for safety apparel and for such items as theatre curtains and fire stop hangings in public buildings.

The widespread use of asbestos as an insulating material during World War II, and later in flooring, ceiling tiles, brake linings, and as a fire protectant sprayed inside buildings, led to a virtual epidemic of asbestos-related disease 20 years later.

At first only the form of disease known as asbestosis, with radiographic changes and impaired function at an early stage, was recognized. Then it became apparent that exposure to much less asbestos than was needed to cause asbestosis led to thickening of the pleura, and, when both cigarette smoking and asbestos exposure occurred, there was a major increase in the risk for lung cancer.

It is currently believed that the risks from smoking and from significant asbestos exposure are multiplicative in the case of lung cancer.

Finally, a malignant tumor of the pleura known as mesothelioma was found to be caused almost exclusively by inhaled asbestos. Often a period of 20 years or more elapsed between exposure to asbestos and the development of the tumor.

Once these health risks were firmly documented in the 1970s, regulatory agencies in the United States and other developed nations began placing tight restrictions on workers' exposure to asbestos in industrial plants.

In 1989 the U.S. government instituted a gradual ban on the manufacture, use, and export of most products made with asbestos. Since the 1980s various substitutes for asbestos have been developed for use in many products.

As far as is known, all the respiratory changes associated with asbestos exposure are irreversible. Malignant mesothelioma is rare and unrelated to cigarette smoking. Usually the pleural thickenings are not associated with disturbance of function or symptoms, although in occasional cases the pleuritis is more aggressive, in which case both may occur.

It is not yet understood why asbestos causes such devastating changes. Furthermore, not all types of asbestos are equally dangerous; the risk of mesothelioma in particular appears to be much higher if crocidolite, a blue asbestos that comes from South Africa, is inhaled than if chrysotile is inhaled. But exposure to any type of asbestos is believed to increase the risk of lung cancer when associated with cigarette smoking.

Asbestosis

Lung disease that is caused by the prolonged inhalation of asbestos fibres. A type of pneumoconiosis, it is found primarily among workers whose occupations involved asbestos, principally mining, construction, and the manufacture of insulation, fireproofing, cement products, and automobile brakes. The disease is not limited solely to asbestos workers but is also known among people living near mines, factories, and construction sites.

Asbestos fibres that have been inhaled remain in the lungs for years and eventually cause excessive scarring and fibrosis, resulting in a stiffening of the lungs that continues long after exposure ceases.

Greater effort is needed to make the stiffened lungs expand during breathing, and shortness of breath and inadequate oxygenation of the blood result. Persons with advanced cases of the disease have a dry cough, and the increased cardiac effort needed to expand the lungs may induce a secondary heart disease.

An increased incidence of lung cancer and of malignant mesothelioma (a rare cancer of the membrane lining the lungs) is also associated with asbestos inhalation and asbestosis.

Asbestosis usually results only after at least 10 years' exposure to asbestos, and the first symptoms do not appear until many years after the initial exposure; shipyard workers exposed to asbestos during World War II began developing asbestosis in the 1960s and '70s. (Mesothelioma, by contrast, can develop after relatively little exposure to asbestos.)

It is now known that cigarette smoking seriously aggravates the symptoms of asbestosis. The incidence of asbestosis increased after 1950, probably due to the increasingly widespread industrial use of asbestos. This use declined from the 1970s and had been prohibited in many countries by the 1990s.

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Radon

(Rn), chemical element, a heavy radioactive gas of Group 0 (noble gases) of the periodic table, generated by the radioactive decay of radium. Radon is a colourless, odourless, tasteless gas, 7.5 times heavier than air and more than 100 times heavier than hydrogen.

Radon is rare in nature because its isotopes are all short-lived and because radium, its source, is a scarce element. Exposure to high concentrations of this radon [radon daughters] over the course of many years can greatly increase the risk of developing 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.

By the late 1980s, naturally occurring radon gas had thus come to be recognized as a potentially serious health hazard. The gas, arising from soil and rocks, seeps through the foundations, basements, or piping of buildings and can accumulate in the air of houses that are poorly ventilated.

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 non-smokers.

Among nonsmokers in the United States, radon is now thought to be the single most important cause of lung cancer. Radon levels are highest in homes built over geological formations that contain uranium mineral deposits.

Concentrated samples of radon are prepared synthetically for medical and research purposes. Radon has been used for radiotherapy and radiography.

What You Can Do to Reduce the Radon Levels in Your Home

The British Medical Journal, 2005; 330: 223-7 writes in “RADON: The secret killer in your home, and what you can do about it”:

Think of lung cancer, and you probably instinctively think of cigarette smoking. But there's another major cause, and it's present in most of our homes - the inert gas radon-222, which occurs naturally from the decay of uranium in the earth's crust.

Although concentrations are low outdoors, it's common in our homes, and at relatively high levels. Levels tend to be lower in urban homes than those in rural areas. The underlying rock in urban areas tends to be sedimentary, and more people live upstairs in apartments.

It's high enough, however, says a new study, to be responsible for 9 per cent of all deaths from lung cancer, and that's after allowing for smoking in the home. The study, carried out by Oxford's Radcliffe Infirmary, is based on reports from nine European countries, and involving 7,148 cases of lung cancer.

So what can you do to reduce the radon levels in your home? Improving ventilation and sealing cracks in concrete floors will do a lot. If you have a suspended timber floor, an air brick or fan would help. If you already have an air brick, make sure it's not been covered over by vegetation.

In the worst cases, where radon levels are known to be high, you may need a sump installed, especially if you have a concrete floor. A sump is effectively a small cavity under the floor from which air is extracted.

It's been estimated that the gas 'significantly affects' 100,000 homes in Britain alone - but of those, just 10 per cent could be bothered to do anything to reduce the levels.

Other Types of Lung Cancer

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.

Treatment

Surgery is the preferred treatment [in mainstream or conventional oncology] for patients with non-small-cell carcinoma. Many patients, however, have heart or respiratory diseases that limit their ability to tolerate surgery. Surgery is not the main treatment for patients with cancer in both lungs or for those with metastases to distant organs. It is of no value in most cases of small-cell carcinoma. Small-cell carcinoma responds better to chemotherapy than do other types of lung cancer.

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Prevention

The obvious measure to take of course relates to smoking. The contribution of air pollution to the incidence of lung cancer is not known with certainty, though there is clearly an "urban" factor involved.

Workers exposed to arsenic in metal smelting operations, and the community around the factories from which arsenic is emitted, have an increased risk of 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.

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Cancer Glossary & Scientific Reference