Lung cancer is a type of cancer that begins in the lungs. The lungs are two spongy organs in your chest that take in oxygen when you inhale and release carbon dioxide when you exhale. Lung cancer is the leading cause of cancer deaths worldwide, among both men and women. People who smoke have the greatest risk of lung cancer, though lung cancer can also occur in people who have never smoked. The risk of lung cancer increases with the length of time and the number of cigarettes you've smoked.

There are two major types of lung cancer based on the appearance of lung cancer cells under the microscope: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). Both types grow and spread in different ways and are treated differently, which is why it’s important to know which type you have.