Cancer is a disease in which cells grow in an abnormal way. Normally, the cells divide in a controlled manner to replace old or damaged cells. If cells keep dividing when new cells are not needed, a mass of tissue called a tumor forms.

Not all tumors are cancer, those that are cancer are called malignant. Cancer tumors invade and damage tissue around them. The cancer cells can also enter the lymph and blood streams, spreading to other parts of the body. Lung cancer is the development of malignant cells from lung tissue.

Cancer Cell Growth

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Normal anatomy and the development of lung cancer

The lungs are two football-sized organs located in the chest cavity. Each lung is divided into sections called lobes. The left lung has 2 lobes and the right lung has 3 lobes. The lungs are surrounded by a membrane called the pleura. The inside layer of the pleura covers the lungs and helps separate the lobes. The outside layer is attached to the chest wall. The fluid that fills the space between the layers serves as a lubricant that allows the lungs to expand and contract in smooth movements. Cancer can develop in any of these areas.

Air moves through the nose or mouth enters the trachea, or windpipe. The trachea divides into tubes called bronchi, which lead into each lung. The bronchi further divide into smaller bronchioles, much like the branches on a tree. The bronchioles end in little air sacs called alveoli. Oxygen from the air passes to the blood through these air sacs. At the same time, carbon dioxide (a waste product) moves from the blood to the lungs so it can be exhaled from the body.

Oxygen Exchange in the Lungs

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Cell division and growth is a normal process in the body to replace old or damaged cells. The inside of the lungs have a higher rate of cell turnover than other areas because of constant usage during normal breathing. Breathing also exposes the lungs to carcinogens (cancer-causing agents) from smoking, pollutants, and other chemicals in the air. These changes may irritate the cells and increase turnover from old to new cells.

Lung cancer can start anywhere in the respiratory tract, but it is most commonly found in the cells that form the lining of the bronchi or alveoli. Tumors that grow in the lungs can cause a persistent, worsening cough or shortness of breath. If it grows beyond the lungs, the cancer can penetrate nearby structures, such as the great blood vessels that lead to and from the heart, tissue in and around the heart, or major nerves. The cancer can also spread to the lymph nodes or blood vessels, which can carry cancer cells to other areas of the body. The most common sites for lung cancer to spread to are the lymph nodes in other parts of the body, the liver, bones, or brain.

Lung Cancer

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Types of lung cancer

Lung cancer is categorized by where tumors start, how they grow, and their appearance under a microscope. Lung cancer types include:

  • Non-small cell —This type generally grows and spreads more slowly. Non-small cell lung cancer accounts for about 85% lung cancer cases. The most common types of non-small cell lung cancer are:
    • Adenocarcinoma—Generally found in the outer areas of the lung. Adenocarcinomas arise from glandular cells that form mucous in the air sacs. They tend to grow slowly, making earlier detection more likely.
    • Squamous cell—Arise from the flat cells that line the inside the respiratory pathways and lungs. These cancers tend to be found near major structures in the airways like the bronchi.
    • Large cell—This type can develop in any part of the lung from several types of large cells. It generally grows and spreads faster than the previous types mentioned above.
  • Small cell—This type generally grows more quickly and is mainly associated with smoking. It is more likely to spread to other parts of the body. Small cell cancers are classified as oat cell or combined based on the appearance of cells under the microscope.
  • Carcinoid tumors—Rare subtype of slow-growing tumor from epithelial endocrine cells.

Many types of metastatic cancers in other parts of the body spread to the lungs. This fact sheet focuses on non-small cell and small cell cancers that start in the airways and/or lungs.