Lung cancers can start in the cells lining the bronchi (the main breathing tubes) and other parts of the lung such as the bronchioles (smaller breathing tubes) or alveoli (tiny air sacs at the end of the bronchioles).
Lung cancers are thought to start as areas of pre-cancerous changes in the lung. The first changes happen in the genes of the cells themselves and may cause them to grow faster. The cells may look abnormal if seen under a microscope, but at this point they do not form a mass or tumor. They cannot be seen on an x-ray and they do not cause symptoms.
Over time, these pre-cancerous changes in the cells may progress to true cancer. As a cancer develops, the cancer cells may make chemicals that cause new blood vessels to form nearby. These new blood vessels nourish the cancer cells, which can keep growing and form a tumor large enough to be seen on imaging tests such as x-rays.
At some point, cells from the cancer may break away from the original tumor and spread (metastasize) to other parts of the body. Lung cancer is often a life-threatening disease because it tends to spread in this way even before it can be detected on an imaging test like a chest x-ray.