Cancers are staged depending on how far they have spread. Staging a cancer correctly is essential to select the most appropriate treatment option. A number of diagnostic tests, such as CT scans, MRIs, bronchoscopy, blood tests, bone scans, and biopsies are used to find out how far the cancer has spread.

Non-small cell lung cancers, which account for about 80 percent of lung cancers, are staged using the Roman numerals 0 through IV. If a cancer is too small to be detected, it is called an occult or hidden cancer and not assigned any numeral. Stage 0 cancer, or carcinoma in situ, is limited to the lung and only involves a few layers of cells. Stage I cancer is still limited to the lung, with an area of normal tissue surrounding it. Stage I cancers are further divided into Stage IA and Stage IB, depending on the size of the tumor. In stage II cancer, the cancer may have spread to nearby lymph nodes, the chest wall, the diaphragm, or to the tissues lining the lung (pleura) and the heart (pericardium). In stage III cancer, lymph nodes in the central chest or on the other side of the body from the original tumor are involved. Stage III cancer is further divided into stage IIIA and stage IIIB. In stage IV cancer, the cancer has spread to other parts of the body, such as the liver, bones or brain, or to a lobe of the other lung. Cancers that are stages 0 through IIIA may be treated with surgery. Treatment of stage III and stage IV cancers requires chemotherapy and radiotherapy.

Small cell lung cancer has two stages. Limited-stage cancer is limited to only one lung, the tissues between the lungs, and nearby lymph nodes. In extensive-stage small cell lung cancer, the cancer has spread to other parts of the body. The brain is commonly involved in extensive-stage cancer.