Lung Cancer

Lung Cancer, malignancy of the lungs that is the leading cause of cancer deaths for both men and women in the United States and Canada. About 90 percent of all lung cancer occurs in current or former smokers. The American Cancer Society estimates that 164,000 new cases of lung cancer are diagnosed annually in the United States and an estimated 157,000 people die from the disease each year. According to the Canadian Cancer Society, 20,600 new cases of lung disease are diagnosed in Canada annually, and the disease causes 17,700 deaths a year.



Cancerous Human Lung This dissection of human lung tissue shows light-colored cancerous tissue in the center of the photograph. At bottom center lies the heart. While normal lung tissue is light pink in color, the tissue surrounding the cancer is black and airless, the result of a tarlike residue left by cigarette smoke. Most lung cancer begins in the cells lining the main air passages, or bronchi. In their cancerous state, these cells lack the cilia that normally catch and eliminate foreign particles inhaled into the lung. Mucous ordinarily cleared by bronchial cilia becomes trapped, blocking air passages. Lung cancer accounts for the largest percentage of cancer deaths in the United States, and cigarette smoking is directly responsible for the majority of these cases.Phototake NYC/Martin Rotker

There are four main types of lung cancer, which tend to arise in different parts of the lung and differ in characteristics and behavior. Squamous cell carcinoma accounts for 20 percent of all lung cancer. It frequently begins toward the central part of the lung in the bronchial tubes, which are the major airways that lead into the lungs. About 40 percent of lung cancers are adenocarcinomas, cancers that originate on the outer edges of the lungs and under the lining of the bronchial tubes. Small-cell carcinoma spreads more rapidly than any other type of lung cancer. It is characterized by small cells that may be shaped like either oat grains or rods and makes up about 20 percent of lung cancer cases. Ten percent of all cases are large-cell carcinomas, cancers characterized by large, abnormal cells that usually begin along the outer edges of the lung.

Lung cancer often spreads, or metastasizes, through the bloodstream or lymphatic system to other tissues. More than 50 percent of people newly diagnosed for lung cancer already show signs of cancer elsewhere in the body. Conversely, some cancers detected in the lung are actually cancers that have metastasized from other parts of the body to the lungs. These are not considered lung cancers. For example, if breast cancer spreads to the lung, it is still considered breast cancer. It does not look or behave like a cancer that starts in the lung.


RISK FACTORS

Cigarette smoking is the single greatest cause of lung cancer. Up to 90 percent of lung cancer patients are smokers, and most of the remainder have been exposed to secondhand smoke. The great majority of lung cancer cases could be prevented and thousands of lives could be saved each year if people quit smoking.

Other environmental factors linked to a higher incidence of lung cancer include prolonged exposure to certain workplace substances, such as arsenic, asbestos, and high doses of ionizing radiation, such as that emitted by uranium (see Radiation Effects, Biological). Long-term exposure to air pollution, which is largely composed of the remains of burned petroleum and coal, may also increase the risk of lung cancer. One constituent of indoor air pollution, naturally occurring radon gas, a colorless, odorless gas that silently seeps from the earth in some regions, poses a special danger to cigarette smokers.

Not all heavy smokers develop lung cancer, suggesting that heredity may also play a role in lung cancer development. Some people seem to inherit an inability to break down certain types of cancer-causing chemicals, making them more susceptible to disease. Exposure to tobacco smoke, for example, may cause key cancer prevention genes to malfunction in these people. For instance, a tumor suppressor gene called FHIT prevents the growth and spread of tumors. Many people with lung cancer inherit a mutated form of this gene that fails to perform this job. The tumor suppressor gene p53 also prevents tumor growth, and researchers have learned that a mutated form of the p53 gene fails to block the growth of cancerous tumors.

Some studies indicate that women are twice as likely as men to develop lung cancer. Researchers have found a gene called GRPR that causes abnormal cell growth in people exposed to cigarette smoke. The gene is more active in women than in men and it may explain why females are more susceptible to lung cancer.


WHAT CAUSES LUNG CANCER?

Smoking is the number one cause of lung cancer. Lung cancer may also be the most tragic cancer because in most cases, it might have been prevented -- 87% of lung cancer cases are caused by smoking. Cigarette smoke contains more than 4,000 different chemicals, many of which are proven cancer-causing substances, or carcinogens. Smoking cigars or pipes also increases the risk of lung cancer.
The more time and quantity you smoke, the greater your risk of lung cancer. But if you stop smoking, the risk of lung cancer decreases each year as normal cells replace abnormal cells. After ten years, the risk drops to a level that is one-third to one-half of the risk for people who continue to smoke. In addition, quitting smoking greatly reduces the risk of developing other smoking-related diseases, such as heart disease, stroke, emphysema and chronic bronchitis.
Many of the chemicals in tobacco smoke also affect the nonsmoker inhaling the smoke, making "secondhand smoking" another important cause of lung cancer. It is responsible for approximately 3,000 lung cancer deaths annually.
Radon is considered to be the second leading cause of lung cancer in the U.S. today. Radon gas can come up through the soil under a home or building and enter through gaps and cracks in the foundation or insulation, as well as through pipes, drains, walls or other openings. Radon causes between 15,000 and 22,000 lung cancer deaths each year in the United States -- 12 percent of all lung cancer deaths are linked to radon.
Radon problems have been found in every state. The EPA estimates that nearly 1 out of every 15 homes in the U.S. has indoor radon levels at or above the level at which homeowners should take action -- 4 picocuries per liter of air (pCi/L) on a yearly average. Radon can be a problem in schools and workplaces, too.
Because you cannot see or smell radon, the only way to tell if you are being exposed to the gas is by measuring radon levels. Exposure to radon in combination with cigarette smoking greatly increases the risk of lung cancer. That means for smokers, exposure to radon is an even greater health risk.

Another leading cause of lung cancer is on-the-job exposure to cancer-causing substances or carcinogens. Asbestos is a well-known, work-related substance that can cause lung cancer, but there are many others, including uranium, arsenic, and certain petroleum products.
There are many different jobs that may involve exposure. Some examples are working with certain types of insulation, working in coke ovens, and repairing brakes. When exposure to job-related carcinogens is combined with smoking, the risk of getting lung cancer is sharply increased.

A recent study published in the Journal of American Medical Association suggests that particulate matter pollution can cause lung cancer.
Lung cancer takes many years to develop. But changes in the lung can begin almost as soon as a person is exposed to cancer-causing substances. Soon after exposure begins, a few abnormal cells may appear in the lining of the bronchi (the main breathing tubes). If a person continues to be exposed to the cancer-causing substance, more abnormal cells will appear. These cells may be on their way to becoming cancerous and forming a tumor.


SYMPTOMS AND DIAGNOSIS

The symptoms of lung cancer are not usually apparent during early stages of the disease. The first symptoms to appear are often similar to those of other, nonmalignant respiratory ailments. The patient may develop a persistent cough, or find that a chronic smoker’s cough is worsening. Other symptoms include chest pain, shortness of breath, hoarseness, bloody sputum (fluid coughed up from the respiratory tract), and frequent bouts of bronchitis or pneumonia. Sometimes, the first symptoms of lung cancer are bone pain, headaches, dizziness, or other signs that the disease has metastasized.

A physician who suspects lung cancer may use a chest X ray to view the lungs and locate any tumors. Computed tomography, a series of X-ray images combined by a computer, may be used to pinpoint the tumor’s approximate size and location. To further evaluate cancerous cells, the physician must examine them under a microscope. In a procedure known as a biopsy, the physician removes a small tissue sample from the tumor. Sometimes a microscopic analysis of cells contained in the sputum will also help confirm a diagnosis.

A diagnostic technique called bronchoscopy enables the physician to visually examine the patient’s bronchial tubes. In this procedure, the physician feeds a bronchoscope, a thin, fiber optic tool equipped with a light and magnifying device, through the patient’s mouth and down into the trachea and bronchial tubes to view the lungs. The bronchoscope can be fitted with a tiny cutting tool, enabling the physician to obtain samples of cells for later microscopic examination


TREATMENT

The course of treatment depends on the type of lung cancer, the size and location of the tumor, the stage of disease, and the age and general health of the patient. If there is no evidence that the disease has spread beyond the lung, the tumor can be removed using one of several surgical procedures. In a resection, only a small part of the lung is removed. The left lung is composed of two lobes, the right lung of three; the removal of an entire lobe is called a lobectomy. Removal of an entire lung is called a pneumonectomy. Following surgery, radiation treatment and chemotherapy (anticancer drugs) may be used to ensure that the entire tumor is eliminated. Unfortunately, fewer than 50 percent of lung cancer patients are candidates for surgery because their cancer has spread to other parts of the body. In these cases, patients may receive chemotherapy or radiation treatment or a combination of both, depending on the severity of the illness.

Lung cancer patients must be monitored regularly following treatment, even if no signs of cancer persist. Second lung cancers develop in a significant number of survivors.

Gene therapy—the use of genes to replace defective genes or supersede their actions—is an emerging treatment for lung cancer and other diseases with genetic components. Preliminary results from experiments using gene therapy for lung cancer, first reported in 1996, have found that replacing defective p53 tumor suppressor genes with normal p53 genes shrank or stopped the growth of some lung tumors.