The non-small lung cancers are classified into several stages or phases following a fairly complex system known by the acronym TNM. The staging of the tumor is very important because it can, first, to distinguish patients cured of which are not and, secondly, to calculate the probability of being cured.

The T refers to tumor size. It is classified between T1 and T4, as the more bulky tumor affecting nearby structures or important as the main bronchi, arteries, or the heart itself.

The N indicates whether or not they affected the nearby lymph nodes. N0 means that they are not. The lymph node involvement is an important prognostic factor that graduates from N1 to N3. In particular it is vital to know whether or not invaded most central nodes of the thorax, a region known as the mediastinum. In general, involvement of the mediastinum means the tumor is inoperable.

M indicates if no metastasis (M0) or, conversely, if the cancer has already branched out to distant organs (M1).