The prediction of patient prognosis has always been essential to the practice of medicine. By the early 20th century, Halsted (1) and others believed that solid tumors spread contiguously over time through a series of stages, from the primary tumor site, through the lymphatics, to distant organs, with each stage conferring an increasingly poor prognosis. A corollary of this view, supported by later research, was that, at diagnosis (clinical tumor–node–metastasis [TNM] stage) or after surgery (pathologic TNM stage), tumor size or location (T), regional lymph node involvement (N), and distant metastases (M) were indices of disease spread and could be used to predict patient outcome. In 1953, the French surgeon Pierre Denoix proposed to the Union Internationale Centre le Cancer that these three factors be standardized and integrated into a prognostic system that could be used, with some accommodation for anatomic site, across all solid tumors (2). His proposal for a common language of solid tumor prognosis was adopted as the TNM staging system, which is currently used throughout the world. The TNM system has undergone six revisions and, in the United States, these changes have been guided by the American Joint Committee on Cancer (AJCC), which was established in 1959 and which has published a succession of revisions of its AJCC Cancer Staging Manual (3). The TNM staging system is a “bin model”; the TNM prognostic factors are used to create a mutually exclusive and exhaustive partitioning of patients, so that every patient is in one and only one bin, and the bins are grouped together into larger bins called stages (4). It uses the mean survival of the patients already in the bin to predict what will happen to a new patient placed in that bin. For example, if a new patient is placed in the {T1, N0, M0} bin, then that patient’s 5-year disease-specific survival is predicted to be the same as the mean survival of all the patients who were placed in that bin 5 years ago. The utility of the system arises from its ability to order patients by a decreasing probability of survival. It can be used for selecting patients for therapy and for providing patients with an estimate of their prognosis.

Until the sixth edition (3), the TNM stages for colon cancer had not changed substantially in 35 years. The stages in the first AJCC staging manual, published in 1976 (5), were virtually identical to those in the fifth edition (6), except for T2 having moved from stage II in the first edition to stage I by the fifth edition. In this issue of the Journal, O’Connell et al.(7) report that the sixth edition of the AJCC Cancer Staging Manual is substantially different from the fifth edition in terms of stages II and III. In the fifth edition, stage II was composed of the bins {T3, N0, M0} and {T4, N0, M0}. In the sixth edition, stage II is divided into two substages—IIa, which is the bin {T3, N0, M0}, and IIb, which is the bin {T4, N0, M0}. This change indicates that IIb patients will, on average, have a worse prognosis than stage IIa patients and a better prognosis than stage IIIa {T1–T2,