The most common type of lung cancer is non–small cell lung cancer. Surgery is the usual treatment if the cancer has not spread outside the lung. Sometimes the cancer cells invade nearby tissue, for example, lymph nodes in the area between the lungs (the mediastinum), and spread through the blood and lymph system to other parts of the body (that is, they metastasize). Doctors usually use imaging tests, such as bone scan, computed tomography (CT), or positron emission tomography (PET), to see whether lung cancer has spread to the mediastinum or to other parts of the body (extrathoracic spread). The CT scans give a series of detailed x-ray pictures of body structures. Both the bone scan and PET involve injecting a radioactive substance into a vein. They give pictures of areas of the body that “light up” because the radioactive substance goes to areas where new bone is being made (which show up on the bone scan) and the radioactive substance is rapidly trapped by cancer cells (which shows up on PET). The purpose of these imaging tests is to identify patients who have cancer that has spread and who thus are not candidates for curative surgical resection. The usual strategy to detect spread, called conventional staging, consists of bone scan and CT of the abdomen.