Lung cancer is a disease where tissue in the lung grows out of control. This leads to invasion of adjacent tissue and infiltration beyond the lungs (metastasis). Lung cancer, the most common cause of cancer-related death in men and the second most common in women, is responsible for 1.3 million deaths worldwide annually. The most common symptoms are shortness of breath, cough (including coughing up blood), and weight loss.

The main types of lung cancer are small cell lung cancer and non-small cell lung cancer. This distinction is important because non-small cell lung cancer is sometimes treated with surgery, while small cell cancer is not. Also, small cell lung cancer usually responds better to chemotherapy.

The most significant risk factor for developing lung cancer is long-term exposure to inhaled carcinogens, especially tobacco smoke. The occurrence of lung cancer in non-smokers, who account for less than 10% of cases, appears to be due to a combination of genetic factors. Radon gas, asbestos, and air pollution may also contribute to the development of lung cancer.

Lung cancer may be seen on chest x-ray and CT scan. The diagnosis is confirmed with a biopsy. This is usually performed via bronchoscopy or CT-guided biopsy.

Treatment and prognosis depend upon the histological type of cancer, the stage (degree of spread), and the patient's performance status. Possible treatments include surgery, chemotherapy, and radiotherapy. Even with treatment, the overall five-year survival rate is 14%.

Causes

The main causes of lung cancer (and cancer in general) include carcinogens (such as those in tobacco smoke), ionizing radiation, and viral infection. This exposure causes cumulative changes to the DNA in the tissue lining the bronchi of the lungs (the bronchial epithelium). As more tissue becomes damaged, eventually a cancer develops.

Smoking

The incidence of lung cancer is highly correlated with smoking. Source: NIH.

Smoking, particularly of cigarettes, is by far the main contributor to lung cancer. In the United States, smoking is estimated to account for 87% of lung cancer cases (90% in men and 85% in women). Among male smokers, the lifetime risk of developing lung cancer is 17.2%. Among female smokers, the risk is 11.6%. This risk is significantly lower in non-smokers: 1.3% in men and 1.4% in women. Cigarette smoke contains over 60 known carcinogens including radioisotopes from the radon decay sequence, nitrosamine, and benzopyrene. Additionally, nicotine appears to depress the immune response to malignant growths in exposed tissue. The length of time a person continues to smoke as well as the amount smoked increases the person's chance of developing lung cancer. If a person stops smoking, this chance steadily decreases as damage to the lungs is repaired and contaminant particles are gradually removed. Across the developed world, almost 90% of lung cancer deaths are caused by smoking. In addition, there is evidence that lung cancer in never-smokers has a better prognosis than in smokers, and that patients who smoke at the time of diagnosis have shorter survival than those who have quit.

Passive smoking

the inhalation of smoke from another's smoking— is a cause of lung cancer in non-smokers. Studies from the U.S. (1986, 1992, 1997, 2001, 2003), Europe (1998), the UK (1998), and Australia (1994) have consistently shown a significant increase in relative risk among those exposed to passive smoke. Recent investigation of sidestream smoke suggests it is more dangerous than direct smoke inhalation.

The Philip Morris tobacco company attempted to delay the release of the 1997 IARC study, to affect the wording of its conclusions, to neutralise its negative results for their business, and to counteract its impact on public and policymakers' opinion.

Asbestos

Asbestos can cause a variety of lung diseases. It increases the risk of developing lung cancer. There is a synergistic effect between tobacco smoking and asbestos in the formation of lung cancer. Asbestos can also cause cancer of the pleura, called mesothelioma (which is different from lung cancer).

Radon gas

Radon is a colorless and odourless gas generated by the breakdown of radioactive radium, which in turn is the decay product of uranium, found in the earth's crust. The radiation decay products ionize genetic material, causing mutations that sometimes turn cancerous. Radon exposure is the second major cause of lung cancer after smoking.

Radon gas levels vary by locality and the composition of the underlying soil and rocks. For example, in areas such as Cornwall in the UK (which has granite as substrata), radon gas is a major problem, and buildings have to be force-ventilated with fans to lower radon gas concentrations. The United States Environmental Protection Agency (EPA) estimates that one in 15 homes in the U.S. has radon levels above the recommended guideline of 4 picoCuries per liter (pCi/L). Iowa has the highest average radon concentration in the United States; studies performed there have demonstrated a 50% increased lung cancer risk with prolonged radon exposure above the EPA's action level of 4 pCi/L.

Viruses

Viruses are known to cause lung cancer in animals and recent evidence suggests similar potential in humans. Implicated viruses include human papillomavirus, JC virus, simian virus 40 (SV40), BK virus and cytomegalovirus.

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