Non-small cell lung cancer is more common than small cell lung cancer, and it generally grows and spreads more slowly. There are three main types of non-small cell lung cancer. They are named for the type of cells in which the cancer develops: squamous cell carcinoma, adenocarcinoma, and large cell carcinoma.

Small cell lung cancer, sometimes called oat cell cancer, is less common than non-small cell lung cancer. This type of lung cancer grows more quickly and is more likely to spread to other organs in the body.

Lung tissue also has epidermal growth factor receptors (EGFR), which when activated induces cancer development and progression.  Such EGFR is a target for effective treatment against lung cancer (e.g., erlotinib, an EGFR inhibitor).

Signs and Symptoms

Lung cancer takes many years to develop. However, changes in the lung can begin almost immediately upon exposure to carcinogens (cancer-causing substances). Soon after exposure begins, microscopic examination of the tissue lining the bronchi (the main breathing tubes) will reveal a few unusual cells. With continued exposure, more abnormal cells appear. These cells may be on their way to becoming cancerous and forming a tumor.

The symptoms of the cancer vary depending on several factors, including where in the lung the tumor is found. If the cancer is located in one of the bronchi, it can irritate the lining of the bronchus (one of the main airways that branches off of the trachea or windpipe) and cause a chronic cough. The cancerous area may bleed when a person coughs. If the tumor grows larger, it may gradually fill the bronchus so that air can't pass in or out. A locked bronchus may also cause repeated lung infections or pneumonia.

A tumor located in the outer part of the lung may not produce any symptoms until it is fairly large. Sometimes the first sign may be chest pain from the tumor growing into the lining of the lungs or the ribs and muscle of the chest wall.

A person's lungs have extensive networks of blood and lymph vessels. Cancer cells may grow into these vessels and be carried by the blood or lymph to circulate through the body. The cancer cells may then be deposited in other organs of the body. The spreading of cancer to other parts of the body is called metastasis.

The first site of tumor metastasis is usually the lymph nodes within the lungs and the mediastinum (the space between the two lungs in the middle of the chest).

It is possible for cancer cells that begin in other organs to spread to the lungs. These cases are very different medical problems, however. Depending on the organ of origin, such cases might be termed "primary breast cancer, metastatic to the lungs," or "primary kidney cancer, metastatic to the lungs." These cancers are less often treated with surgery or radiation therapy, but with chemotherapy.
 
Common signs and symptoms of lung cancer include:

• A cough that doesn't go away and gets worse over time
• Constant chest pain
• Coughing up blood
• Shortness of breath, wheezing, or hoarseness
• Repeated problems with pneumonia or bronchitis
• Swelling of the neck and face
• Loss of appetite or weight loss
• Fatigue

These symptoms may be caused by lung cancer or by other conditions. It is important to check with a doctor.

Risk Factors

Researchers have discovered several causes of lung cancer -- most are related to the use of tobacco.

• Cigarettes. Smoking cigarettes causes lung cancer. Harmful substances, called carcinogens, in tobacco damage the cells in the lungs. Over time, the damaged cells may become cancerous. The likelihood that a smoker will develop lung cancer is affected by the age at which smoking began, how long the person has smoked, the number of cigarettes smoked per day, and how deeply the smoker inhales. Stopping smoking greatly reduces a person's risk for developing lung cancer.

• Cigars and Pipes. Cigar and pipe smokers have a higher risk of lung cancer than nonsmokers. The number of years a person smokes, the number of pipes or cigars smoked per day, and how deeply the person inhales all affect the risk of developing lung cancer. Even cigar and pipe smokers who do not inhale are at increased risk for lung, mouth, and other types of cancer.

• Environmental Tobacco Smoke. The chance of developing lung cancer is increased by exposure to environmental tobacco smoke (ETS) -- the smoke in the air when someone else smokes. Exposure to ETS, or secondhand smoke, is called involuntary or passive smoking.

• Radon. Radon is an invisible, odorless, and tasteless radioactive gas that occurs naturally in soil and rocks. It can cause damage to the lungs that may lead to lung cancer. People who work in mines may be exposed to radon and, in some parts of the country, radon is found in houses. Smoking increases the risk of lung cancer even more for those already at risk because of exposure to radon.
 
• Asbestos. Asbestos is the name of a group of minerals that occur naturally as fibers and are used in certain industries. Asbestos fibers tend to break easily into particles that can float in the air and stick to clothes. When the particles are inhaled, they can lodge in the lungs, damaging cells and increasing the risk for lung cancer. Studies have shown that workers who have been exposed to large amounts of asbestos have a risk of developing lung cancer that is 3 to 4 times greater than that for workers who have not been exposed to asbestos. This exposure has been observed in such industries as shipbuilding, asbestos mining and manufacturing, insulation work, and brake repair. The risk of lung cancer is even higher among asbestos workers who also smoke. Asbestos workers should use the protective equipment provided by their employers and follow recommended work practices and safety procedures.

• Pollution. Researchers have found a link between lung cancer and exposure to certain air pollutants, such as by-products of the combustion of diesel and other fossil fuels. However, this relationship has not been clearly defined, and more research is being done.

• Lung Diseases. Certain lung diseases, such as tuberculosis (TB), increase a person's chance of developing lung cancer. Lung cancer tends to develop in areas of the lung that are scarred from TB.

• Personal History. A person who has had lung cancer once is more likely to develop a second lung cancer compared with a person who has never had lung cancer. Quitting smoking after lung cancer is diagnosed may prevent the development of a second lung cancer.

Researchers continue to study the causes of lung cancer and to search for ways to prevent it. We already know that the best way to prevent lung cancer is to quit (or never start) smoking. The sooner a person quits smoking the better. Even if you have been smoking for many years, it's never too late to benefit from quitting.

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