Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary lung cancers, are carcinomas that derive from epithelial cells. Worldwide, lung cancer is the most common cause of cancer-related death in men and women, and is responsible for 1.3 million deaths annually, as of 2004.^[1] The most common symptoms are shortness of breath, coughing (including coughing up blood), and weight loss.^[2]

The main types of lung cancer are small-cell lung cancer (SCLC), also called oat cell cancer, and non-small-cell lung cancer (NSCLC). The most common cause of lung cancer is long-term exposure to tobacco smoke.^[3] Nonsmokers account for 15% of lung cancer cases,^[4] and these cases are often attributed to a combination of genetic factors,^[5][6] radon gas,^[7] asbestos,^[8] and air pollution^[9][10][11] includingsecondhand smoke.^[12][13]

Lung cancer may be seen on chest radiograph and computed tomography (CT scan). The diagnosis is confirmed with a biopsy. This is usually performed by bronchoscopy or CT-guided biopsy. Treatment and prognosis depend on the histological type of cancer, the stage(degree of spread), and the patient's general wellbeing, measured by performance status. Common treatments include surgery,chemotherapy, and radiotherapy. NSCLC is sometimes treated with surgery, whereas SCLC usually responds better to chemotherapy and radiation therapy. This is partly because SCLC often spreads quite early, and these treatments are generally better at getting to cancer cells that have spread to other parts of the body.^[14]

Survival depends on stage, overall health, and other factors, but overall 14% of people diagnosed with lung cancer survive five years after the diagnosis.^[2]

Signs and symptoms

Symptoms that may suggest lung cancer include:^[15]

If the cancer grows in the airway, it may obstruct airflow, causing breathing difficulties. The obstruction can lead to accumulation of secretions behind the blockage, and predispose topneumonia. Many lung cancers have a rich blood supply. The surface of the cancer may be fragile, leading to bleeding from the cancer into the airway. This blood may subsequently be coughed up.

Depending on the type of tumor, so-called paraneoplastic phenomena may initially attract attention to the disease.^[16] In lung cancer, these phenomena may include Lambert-Eaton myasthenic syndrome (muscle weakness due to auto-antibodies), hypercalcemia, or syndrome of inappropriate antidiuretic hormone (SIADH). Tumors in the top (apex) of the lung, known as Pancoast tumors,^[17] may invade the local part of the sympathetic nervous system, leading to changed sweating patterns and eye muscle problems (a combination known as Horner's syndrome) as well as muscle weakness in the hands due to invasion of the brachial plexus.

Many of the symptoms of lung cancer (bone pain, fever, and weight loss) are nonspecific; in the elderly, these may be attributed to comorbid illness.^[14] In many patients, the cancer has already spread beyond the original site by the time they have symptoms and seek medical attention. Common sites of metastasis include the brain, bone, adrenal glands, contralateral (opposite) lung, liver, pericardium, and kidneys.^[18] About 10% of people with lung cancer do not have symptoms at diagnosis; these cancers are incidentally found on routine chest radiograph.^[2]

Causes

The main causes of any cancer 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.^[14]

Smoking

Smoking, particularly of cigarettes, is by far the main contributor to lung cancer.^[19] Cigarette smoke contains over 60 known carcinogens,^[20]including radioisotopes from the radon decay sequence, nitrosamine, and benzopyrene. Additionally, nicotine appears to depress the immune response to malignant growths in exposed tissue.^[21] Across the developed world, 91% of lung cancer deaths in men during the year 2000 were attributed to smoking (71% for women).^[22] In the United States, smoking is estimated to account for 87% of lung cancer cases (90% in men and 85% in women).^[23] 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 nonsmokers: 1.3% in men and 1.4% in women.^[24]

Radon gas

Radon is a colorless and odorless 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 in the general population, after smoking^[7]with the risk increasing 8–16% for every 100 Bq/m³ increase in the radon concentration.^[36] 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 picocuriesper liter (pCi/L) (148 Bq/m³).^[37] 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.^[38][39]

Asbestos

Asbestos can cause a variety of lung diseases, including lung cancer. There is a synergistic effect between tobacco smoking and asbestos in the formation of lung cancer.^[8] In the UK, asbestos accounts for 2–3% of male lung cancer deaths.^[40] Asbestos can also cause cancer of thepleura, called mesothelioma (which is different from lung cancer).