Lung cancer is the result of malignant cells forming in the tissues of the lung, usually in the cells lining the air passages. The cells in our bodies are constantly dividing and reproducing. Usually, there's an orderly pattern to this reproduction as cells develop and specialize to meet particular needs. Occasionally, however, a cell becomes damaged. There's a mutation in its DNA, and rather than maturing and dying as is normal, it continues to reproduce unchecked. In essence, this is cancer – uncontrolled reproduction and growth of abnormal cells in the body.

Most lung cancers are believed to start in the epithelial lining of the lungs – the linings of the large and small airways that perform the task of extracting oxygen from the air. Because of this, lung cancer is sometimes called bronchogenic carcinoma – cancer arising from the bronchia. A smaller percentage of lung cancers begin in the pleura – the thin tissue sac that surrounds the lungs. These cancers are called mesothelioma. The most common form of mesothelioma is linked to asbestos exposure.

Cancer of the lungs is one of the deadliest forms of cancer. While it may take a period of years to develop, the cancer often goes undetected until late in the process. In addition, it tends to metastasize (migrate to other parts of the body) early, which leaves fewer opportunities to fight the mutated cells with surgery or radiation. Once the lung cancer does metastasize, it quickly spreads to the most vulnerable and important organs of the body, particularly the adrenal glands, the liver, the brain and the bones.

There are two primary forms of lung cancer – Small Cell Lung Cancer (SCLC) and Non-Small Cell Lung Cancer (NSCLC). Small Cell Lung Cancer is less common, though far more deadly. It's directly linked to cigarette smoking – less than 1% of SCLC is diagnosed in non-smokers. It's also extremely aggressive and fast-moving, metastasizing rapidly to other organs, and often undiscovered until it's already widespread.

Non-Small Cell Lung Cancer, on the other hand, is far more common, accounting for nearly 80% of all diagnosed lung cancers. There are three main types of non-small cell lung cancer, generally characterized by the size, shape, and chemical composition of the cells that form the cancer:

Squamous Cell Carcinoma (also referred to as Epidermoid Carcinoma): accounts for around 25% – 30% of all lung cancers, and is associated with a history of smoking. This cancer is nearly always found in the central chest area, near the bronchus.

Adenocarcinoma (also referred to as Bronchioloalveolar Carcinoma): accounts for around 40% of all lung cancers, and is found in the external region of the lung. Treatment for this form of lung cancer often leads to a more successful outcome than that of other lung cancers.

Large-Cell Undifferentiated Carcinoma: accounting for only 10% – 15% of lung cancers, this form may show up in any area of the lung. It tends to spread quickly, and often results in a poor prognosis.

It's also possible for lung cancer to be a combination of Non-Small Cell Lung Cancer types.

There are other, less common types of lung cancer. For instance, bronchial carcinoids are small tumors often found in people under 40 years of age. They tend to grow slowly, and be amenable to treatment. Carcinoid tumors account for approximately 5% of lung tumors. Some are non-cancerous. The others are generally slow-growing and can be successfully treated with surgery.

Finally, some cancers discovered in the lungs aren't lung cancers at all. Since the lungs are prone to metastatic cancers from other sites, it's not uncommon for tumors from other primary cancers to find their way to the lungs. When this occurs, the tumors are often discovered in the peripheral tissues of the lungs rather than in the central tissues.

Please note that the information provided in this article is for information purposes only. It should not be used during a medical emergency or for the diagnosis or treatment of lung cancer. Such situations should always involve the expertise of a physician or health care provider.