Lung cancer is a malignant tumor of the lungs. Most commonly it is bronchogenic carcinoma (about 90%). Lung cancer is the most lethal malignant tumor worldwide, causing up to 3 million deaths.
Exposure to carcinogens, such as those present in tobacco smoke, immediately causes small changes to the tissue lining the bronchi of the lungs (the bronchial mucous membrane). This effect is cumulative, and over time with continued exposure more and more tissue gets damaged until a tumor develops. If the tumor grows inwards it may obstruct the air passageway, causing breathing difficulties. The lungs may then collapse and infections can develop, leading to lung abscess. The patient here would start to cough up blood-stained material. However, if the tumor grows outwards in to the lung it may not even be noticed by the patient before it starts to spread to other parts of the body. Common symptoms include: coughing up blood or lung-material bad, chronic, cough, wheezing, chest pains, weight loss or loss of appetite, shortness of breath.
Types of lung cancer
There are two main types of lung cancer categorized by the size of the cancerous cells seen under a microscope: small and non-small cell lung cancer.
1. Small cell types:
Small cell carcinoma (also called oat cell carcinoma) is the less common form of lung cancer, making up 20% of cases. It tends to start in the larger breathing tubes and grows rapidly becoming quite large.
2. Non-small cell types:
Epidermoid carcinoma (or squamous cell carcinoma) also starts in the larger breathing tubes but grows slower meaning that the size of these tumors varies when on diagnosis. Adenocarcinoma (or for slower growing forms alveolar cell cancer) is a form which starts near the surface of the lung. Large cell carcinoma is a fast-growing form that grows near the surface of the lung.
These types add up to 90% of all cases of lung cancer. Other forms include carcinoid, cylindroma, mucoepidermoid and malignant mesothelioma.
Causes of lung cancer
There are four major reasons why people get lung cancer (and actually cancer in general):
Carcinogens such as those contained in cigarretes
- Radiation
- Genetic susceptibility
- viruses
- Smoking, particularly of cigarettes, is believed to be by far the main cause of cancer, which in at least in theory makes it one of the easiest diseases to prevent. An estimated 80% of lung cancers result from smoking, due to the hundreds of known carcinogens, such as benzene, present in cigarette smoke. The length of time that a person continues to smoke as well as the amount smoked increases there chances of contracting lung cancer. However if someone stops smoking then these chances steadily decrease as the damage to their lungs is repaired.
- Passive smoking, whereby exhaled smoke is taken up by other people, has recently been identified as a much larger cause of lung cancer in non-smokers than previously believed. The US Environmental Protection Agency (EPA) in 1993 concluded that about 3,000 lung-cancer related deaths were caused by passive smoking every year, however the true extent is still being contested among scientists.
- Asbestos is another popularly known carcinogen mainly for mesothelioma (affecting the mesothelium lining lungs, abdomen or heart.) Often this exposure happens to people unavoidably through their work.
- Radon is a colourless and odorless gas derived from the breakdown of radioactive radium, which in turn is the decayed product of uranium, found in the earth's crust. It is the second major cause of lung cancer after smoking. This radiation ionises genetic material, causing mutations that sometimes become cancerous. Radon gas levels vary over where you live. In areas such as Cornwall in the UK, radon-gas is a major problem, and fans have to be installed to drive out the gas. In the US the EPA estimated that 1 in 15 homes have radon levels above the recommended standard.
- Oncogenes are genes which is believed make people more susceptible to cancer. Proto-oncogenes are believed to turn in to oncogenes when exposed to particular carcinogens. Viruses are suspected to cause cancer in humans, as this link has already been proven in animals.
Staging of lung cancer
Treatment of lung cancer
Treatments for lung cancer depends on the specific form, how far it has developed and on other details of the patient (eg. age). Common treatments include surgery, chemotherapy and radiation therapy.
Susceptibility to lung cancer
The group most likely to develop lung cancer are the over-fifties who also have a history of smoking. Lung cancer is the second most commonly occurring form of cancer in most western countries, however it is the leading cancer-related cause of death for men and women. It is expected that in 2001 there will be 169,500 new cases of lung cancer; 90,700 in men and 78,000 in women. Although the rate of men dying from lung cancer is declining in western countries, it is actually increasing for women due to the increased take up of smoking in this group.
Prevention of lung cancer
Prevention plans are the most cost-effective means of fighting lung cancer on national and global scale. While in most countries industrial and domestic carcinogens have been identified and banned, tobacco smoking still is quite widespread. Fighting tobacco smoking should be primary goal for lung cancer prevention.
Because prognosis depends heavily on early detection there have been several attempts at secondary prevention. Regular chest radiography and sputum examination programs were not effective in early detection of this cancer and did not resulted in reduction of mortality. However in September 2003 one study published in Lancet is quite promising. The study showed that spiral computed tomography and positron emission tomography were effective in detecting early stages of lung cancer in high risk population of heavy smokers.
Reference:
Early lung-cancer detection with spiral CT and positron emission tomography in heavy smokers: 2-year results Lancet 2003; 362: 593-97.
