Following a positive identification of cancer, doctors will try to establish the stage of the cancer. Cancers are ranked into stages depending on the specific characteristics that they possess; stages correspond with severity. Determining the stage of a given cancer helps doctors to make treatment recommendations, to form a likely outcome scenario for what will happen to the patient (prognosis), and to communicate effectively with other doctors.
There are multiple staging scales in use. One of the most common ranks cancers into five progressively more severe stages: 0, I, II, III, and IV. Stage 0 cancer is cancer that is just beginning, involving just a few cells. Stages I, II, III, and IV represent progressively more advanced cancers, characterized by larger tumor sizes, more tumors, the aggressiveness with which the cancer grows and spreads, and the extent to which the cancer has spread to infect adjacent tissues and body organs. Another popular staging system is known as the TNM system, a three dimensional rating of cancer extensiveness. Using the TNM system, doctors rate the cancers they find on each of three scales, where T stands for tumor size, N stands for lymph node involvement, and M stands for metastasis (the degree to which cancer has spread beyond its original locations). Larger scores on each of the three scales indicate more advanced cancer. For example, a large tumor that has not spread to other body parts might be rated T3, N0, M0, while a smaller but more aggressive cancer might be rated T2, N2, M1 suggesting a medium sized tumor that has spread to local lymph nodes and has just gotten started in a new organ location.
Still another staging system, called summary staging, is in use by the National Cancer Institute for its SEER program. Summary stages include: "In situ" or early cancer (stage 0 cancer), "localized" cancer which has not yet begun to spread, "regional" cancer which has spread to local lymph nodes but not yet to distant organs, "distant" cancer which has spread to distant organs, and finally, "unknown" cancer to describe anything not fitting elsewhere.