Lung cancer is the uncontrolled growth of abnormal cells that start off in one or both lungs; usually in the cells that line the air passages. The abnormal cells do not develop into healthy lung tissue, they divide rapidly and form tumors.
As tumors become larger and more numerous, they undermine the lung’s ability to provide the bloodstream with oxygen. Tumors that remain in one place and do not appear to spread are known as “benign tumors”.
Malignant tumors, the more dangerous ones, spread to other parts of the body either through the bloodstream or the lymphatic system. Metastasis refers to cancer spreading beyond its site of origin to other parts of the body. When cancer spreads it is much harder to treat successfully.
Primary lung cancer originates in the lungs, while secondary lung cancer starts somewhere else in the body, metastasizes, and reaches the lungs. They are considered different types of cancers and are not treated in the same way.
According to the National Cancer Institute, by the end of 2015 there will have been 221,200 new lung cancer diagnoses and 158,040 lung-cancer related deaths in the USA.
According to the World Health Organization (WHO), 7.6 million deaths globally each year are caused by cancer; cancer represents 13% of all global deaths. As seen below, lung cancer is by far the number one cancer killer.
Total deaths worldwide caused by cancer each year
- Lung cancer - 1,370,000 deaths
- Stomach cancer - 736,000 deaths
- Liver cancer - 695,000 deaths
- Colorectal cancer - 608,000 deaths
- Breast cancer - 458,000 deaths
- Cervical cancer - 275,000 deaths
The American Cancer Society says that lung cancer makes up 14% of all newly diagnosed cancers in the USA today. It adds that annually, more patients die from lung cancer alone than prostate, breast and colon cancers combined (in the USA).
An American man’s lifetime risk of developing lung cancer is 1 in 13; for a woman the risk is 1 in 16. These risk figures are for all US adults, including smokers, ex-smokers and non-smokers. The risk for a regular smoker is dramatically higher.
Most lung cancer patients are over the age of 60 years when they are diagnosed. Lung cancer takes several years to reach a level where symptoms are felt and the sufferer decides to seek medical help.
Female lung cancer rates set to rise rapidly
Over the next three decades, female lung cancers will increase thirty-five times faster than male lung cancers, scientists from King’s College London reported in October 2012.
The scientists estimate that in the UK, female lung cancer deaths will reach 95,000 annually in 2040, from 26,000 in 2010 – a rise of more than 350%. Male annual lung cancer deaths will increase by 8% over the same period, to 42,000 in 2040 from 39,000 in 2010.
The authors of the report say that lung cancer will continue being the largest cancer killer over the next thirty years. Twice as many people will be living with lung cancer in 2040 compared to 2010. The main reason for the increase will be longer lifespans - the older you are, the higher your risk of cancer is, including lung cancer.
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.
How does lung cancer develop? - video
A short video explaining how lung cancer develops. Video by eHow.
On the next page we look at the symptoms of lung cancer and how lung cancer is classified and diagnosed. On the final page we discuss treatments for lung cancer.