A. Lung cancer begins when cells in one or both lungs start to grow out of control. At first, pre-cancerous cells can’t be seen on an x-ray and they don’t cause symptoms. But over time, these cells may progress to true cancer. They may continue to grow 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. There are different types of lung cancer. The most common type is non-small cell lung cancer, which accounts for about 85% to 90% of lung cancers. The other main type is small cell lung cancer, which accounts for about 10% to 15%. Other types, such as lung carcinoid tumors account for less than 5% of lung cancers. The different types are treated very differently.