Causes
Cigarette Smoke. Smoking causes 87% of all cases of lung cancer, accounting for 28% of all cancer deaths. Cigarettes, nicotine, or both may contribute to lung cancer in one or more of the following ways:
- The smoke is the most dangerous component of the cigarette. Chemicals formed during smoking trigger genetic mutations that lead to cancer. When people inhale cigarette smoke, they bring into their lungs tar that includes over 4,000 chemicals, some of which are carcinogenic (cancer-causing). Other inhaled chemicals in cigarette smoke that may increase the risk for cancer include cyanide, benzene, formaldehyde, methanol (wood alcohol), acetylene (the fuel used in torches), and ammonia. Smoke also contains nitrogen oxide and carbon monoxide, both of which are harmful gases.
- Nicotine itself may be a hazard. A 2000 laboratory study suggested that the human body might be converting inhaled nicotine into a chemical called aminoketone, which has been linked to the formation of tobacco-related lung cancer. A 2001 study reported that nicotine triggered new blood vessel growth, which could, in theory, promote growth of any existing tumors. Whether or not these studies apply to long-term use of nicotine replacement products (such as patches), or to cigarette smoking, is still unclear. The studies should certainly not discourage people from using nicotine replacement methods for quitting. However, these studies may indicate that people should not use these devices on a long-term basis.
Radon. Radon is a gas produced naturally by the breakdown of uranium. It is often present in the soil and in water and can seep into any dwelling. Radon may be responsible for between 10% and 14% of lung cancer deaths, making it, after smoking, the second leading cause of this cancer.
Other Contributors. Toxic particles leading to precancerous changes in the lung are also found in marijuana. In one study, 53.8% of cigarette smokers, 66.7% of marijuana smokers, and all of those subjects who smoked both substances showed evidence of precancerous changes in the lungs.
There is considerable debate over the lung cancer risk posed by depleted uranium used in military weapons (e.g., in the Gulf and Balkan conflicts). A 2001 study estimated that it would cause an additional 8 deaths from lung cancer out of every 10,000 people or soldiers who were highly exposed to this substance. The study was based on a mathematical model, however, and the issue is not settled.
Other lung carcinogens include asbestos, arsenic, certain petrochemicals (materials made from crude oil or natural gas), and other airborne (carried through the air) byproducts of various mining and manufacturing processes.
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Genetic Mutations
- Genetic mutations that cause cancer generally occur in two types of genes: Tumor-suppressor genes prevent cells from endlessly copying themselves
- Proto-oncogenes encourages cells to keep making copies of themselves (when a proto-oncogene changes [becomes mutated], it is then called an oncogene)
Damage to either type of gene can cause a mutation that results in an uncontrolled division of cells. This uncontrolled division forms tumors.
It is unlikely that a single specific abnormality causes all cases of lung cancer. It probably takes a variety of mutations to start the devastating chain of events leading to cancer. The following mutations are among those under investigation:
- BPDE-caused mutations: The chemical BPDE, a byproduct of tobacco smoke, is involved with a number of genetic mutations, including those to an oncogene called K-ras, and to three tumor-suppressor genes known as p53, PPP2R1B, and p16. When normal, the tumor-suppressor genes are involved in cell repair and healthy copying of the cell. When they are damaged or blocked, out of control cell production can occur, leading to cancer. About 10% of the population may carry a gene that protects against lung cancer, by reducing levels of BPDE.
- Chemotherapy resistance genes: Tumors that contain the p53 mutation may also be more resistant to chemotherapy.
- Rb Mutations: Another important contributor to lung cancer is a genetically defective protein called retinoblastoma (Rb), which is associated with very aggressive tumors. Low levels of the normal Rb gene may sometimes predict aggressive cancer, especially in patients with small cell lung cancer.
- Mutations to the FHIT gene: Another potentially important mutation may be an abnormality in the FHIT gene. This mutation causes the cells lining the lung to become more vulnerable to the effects of tobacco smoke and other carcinogens.
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