Lung cancer is called “primary” if the cancer originates in the lungs and “secondary” if it originates elsewhere in the body but has metastasized to the lungs. These two types are considered different cancers from diagnostic and treatment perspectives.
In 2007, about 15% of all cancer diagnoses and 29% of all cancer deaths were due to lung cancer. It is the number one cause of death from cancer every year and the second most diagnosed after breast and prostate cancers (for women and men, respectively). Lung cancer is usually found in older persons because it develops over a long period of time.
How is lung cancer classified?
Lung cancer can be broadly classified into two main types based on the cancer’s appearance under a microscope: non-small cell lung cancer and small cell lung cancer. Non-small cell lung cancer (NSCLC) accounts for 80% of lung cancers, while small cell lung cancer accounts for the remaining 20%.
NSCLC can be further divided into four different types, each with different treatment options:
- Squamous cell carcinoma or epidermoid carcinoma. As the most common type of NSCLC and the most common type of lung cancer in men, squamous cell carcinoma forms in the lining of the bronchial tubes.
- Adenocarcinoma. As the most common type of lung cancer in women and in nonsmokers, adenocarcinoma forms in the mucus-producing glands of the lungs.
- Bronchioalveolar carcinoma. This type of lung cancer is a rare type of adenocarcinoma that forms near the lungs’ air sacs.
- Large-cell undifferentiated carcinoma. A rapidly growing cancer, large-cell undifferentiated carcinomas form near the outer edges or surface of the lungs.
Small cell lung cancer (SCLC) is characterized by small cells that multiply quickly and form large tumors that travel throughout the body. Almost all cases of SCLC are due to smoking.
What causes cancer?
Cancer is ultimately the result of cells that uncontrollably grow and do not die. Normal cells in the body follow an orderly path of growth, division, and death. Programmed cell death is called apoptosis, and when this process breaks down, cancer begins to form. Unlike regular cells, cancer cells do not experience programmatic death and instead continue to grow and divide. This leads to a mass of abnormal cells that grows out of control.
Lung cancer occurs when a lung cell’s gene mutation makes the cell unable to correct DNA damage and unable to commit suicide. Mutations can occur for a variety of reasons. Most lung cancers are the result of inhaling carcinogenic substances.
Carcinogens are a class of substances that are directly responsible for damaging DNA, promoting or aiding cancer. Tobacco, asbestos, arsenic, radiation such as gamma and x-rays, the sun, and compounds in car exhaust fumes are all examples of carcinogens. When our bodies are exposed to carcinogens, free radicals are formed that try to steal electrons from other molecules in the body. These free radicals damage cells and affect their ability to function and divide normally.
About 87% of lung cancers are related to smoking and inhaling the carcinogens in tobacco smoke. Even exposure to second-hand smoke can damage cells so that cancer forms.
Cancer can be the result of a genetic predisposition that is inherited from family members. It is possible to be born with certain genetic mutations or a fault in a gene that makes one statistically more likely to develop cancer later in life. Genetic predispositions are thought to either directly cause lung cancer or greatly increase one’s chances of developing lung cancer from exposure to certain environmental factors.