Although lung cancer accounts for only 15% of all newly-diagnosed cancers in the United States, it is the leading cause of cancer death in U.S. men and women. It is more deadly than colon, breast, and prostate cancers combined. About 160,000 patients die from lung cancer each year. Death rates have been declining in men over the past decade, and they have about stabilized in women.
The lungs are two spongy organs surrounded by a thin moist membrane called the pleura. Each lung is composed of smooth, shiny lobes: the right lung has three lobes, and the left has two. About 90% of the lung is filled with air. Only 10% is solid tissue.
- Air is carried from the trachea (the windpipe) into the lung through flexible airways called bronchi.
- Like the branches of a tree, the bronchi in turn divide into over a million smaller airways called bronchioles.
- The bronchioles lead to grape-like clusters of microscopic sacs called alveoli.
- In each adult lung, there are about 300 million of these tiny alveoli. A thin membrane makes up the alveoli sacs. Oxygen and carbon dioxide pass through this membrane to and from capillaries.
- Capillaries, the smallest of our blood vessels, carry blood throughout the body.
The major features of the lungs include the bronchi, the bronchioles, and the alveoli. The alveoli are the microscopic blood vessel-lined sacks in which oxygen and carbon dioxide gas are exchanged.
Lung cancer develops when genetic mutations (changes) occur in a normal cell within the lung. As a result, the cell becomes abnormal in shape and behavior, and reproduces endlessly. The abnormal cells form a tumor that, if not surgically removed, invades neighboring blood vessels and lymph nodes and spreads to nearby sites. Eventually, the cancer can spread (metastasize) to locations throughout the body.
The two major categories of lung cancer are small cell lung cancer and non-small cell lung cancer. Most lung cancers are non-small cell cancer, the subject of this report. Less common cancers of the lung are known as carcinoids, cylindromas, and certain sarcomas (cancer in soft tissues). Some experts believe all primary lung cancers come from a single common cancerous (malignant) stem cell. As it copies itself, that stem cell can develop into any one of these cancer types in different people.
In addition, cancers in the lung may have spread from other sites, such as the breast, thyroid, or colon. In these cases, doctors name the cancer after its original location, such as "breast cancer with lung metastases."
Non-Small Cell Lung Cancers
Non-small cell lung cancers are categorized into three types:
- Squamous cell carcinoma (also called epidermoid carcinoma)
- Large cell carcinoma
These separate types are grouped together because, in the early stages before the cancers have spread, they all can be treated with surgery.
Squamous Cell Carcinoma. Squamous cells are formed from reserve cells. These are round cells that replace injured or damaged cells in the lining (the epithelium) of the bronchi, the major airways. Tumors formed from squamous cells are usually found in the center of the lung, either in a major lobe or in one of the main airway branches. They may grow to large sizes and form cavities in the lungs.Click the icon to see an image of squamous cell carcinoma.
When squamous cell cancer spreads, it may travel to the bone, adrenal glands, liver, small intestine, and brain.
Squamous cell carcinoma is nearly always caused by smoking, and it used to be the most common cancer. It still makes up 25 - 30% of all lung cancers.
Adenocarcinoma. Adenocarcinomas usually start from the mucus-producing cells in the lung. About two-thirds of adenocarcinomas develop in the outer regions of the lung, while one-third develop in the center of the lung.
In 1965, 12% of lung cancers were adenocarcinomas. They are now estimated to account for 40% of all lung cancers and are the most common lung cancers in many countries. They are also the most common lung cancers in women, and their rates are increasing dramatically in men. Until recently, adenocarcinoma was only weakly linked to smoking. Experts now suggest, however, that the dramatic increase in this lung cancer type in recent decades may be due to low-tar, filtered cigarettes. People who smoke them draw tiny particles deeper into their lungs.
The course of this cancer varies widely. Most often, it develops slowly and causes few or no symptoms until it is far advanced. In some cases, however, it can be extremely aggressive and rapidly fatal. In 50% of cases in which this cancer spreads, it spreads only to the brain. It also can spread to the other lung, liver, adrenal glands, and bone.Click the icon to see an image of adenocarcinoma.
Bronchoalveolar Lung Cancer. Bronchoalveolar lung cancer is actually a subtype of adenocarcinoma. It develops as a layer of column-like cells on the lung and spreads through the airways, causing great volumes of sputum. This cancer also is increasing in incidence.
Large Cell Carcinoma. Large cell carcinoma, which makes up about 10 - 15% of lung cancers, includes cancers that cannot be identified under the microscope as squamous cell cancers or adenocarcinomas.Click the icon to see an image of large cell carcinoma.
Small Cell Lung Cancer
Small cell lung cancer may, like squamous cells, originate from reserve cells or other cells in the epithelium. It causes 10 - 15% of all lung cancers. Without chemotherapy, it is very aggressive and usually rapidly fatal. It requires a different treatment approach from non-small cell lung cancer, so it is not discussed in this report.Click the icon to see an image of small cell carcinoma.