When suffering from cancer, the location, type, and spread of cancer determine which stage it is at. In stage III, the cancer does not spread to distant lymph nodes (M0) but it does spread to the nearby lymph nodes. This stage has classifications based on the N and T components.

Stage IIIA

Stage IIIA has two variations. The first is a score of T1/T2 N1/N1c M0. This indicates that cancer has grown through the mucosa and into the submucosa (T1) or even further into the muscularis propria (T2). The N component indicates that cancer has affected either the fat near the lymph nodes or only one to three of the nearby lymph nodes. The second variation has a score of T1 N2a and M0. This indicates that cancer has grown through the mucosa and into the submucosa (T1) and spread to four to six nearby lymph nodes (N2a).

Stage IIIB

Stage IIIB also has three variations T3/T4a N1/N1c M0, T2/T3 N2a M0, and T1/T2 N2b M0. The M0 indicates that in this stage cancer has not spread to the distant lymph nodes. T3 indicates that cancer has grown into the outermost layers of the colon or rectum while T4a is cancer through the visceral peritoneum. If cancer has spread to one or more lymph nodes, it is denoted by N1a or N1b and spread to the fat near the lymph nodes; however, it is indicated by N1c. The second subtype of T2/T3 N2a M0 is cancer that has grown into the muscularis propria (T2) or the outermost layers of the colon or rectum (T3). The N2a denotes that it has spread to four to six nearby lymph nodes. The third subtype of T1/T2 N2b M0 denotes cancer that has grown through the mucosa and into the submucosa (T1) or the muscularis propria (T2). N2b indicates that it has spread to seven or more nearby lymph nodes.

Stage IIIC

Stage IIIC cancer subtypes are T4a N2a M0, T3/T4a N2b M0, and T4b N1/N2 M0. T4a N2a M0 indicates that cancer has grown through the wall of the colon or rectum and the visceral peritoneum but not the nearby organs (T4a). N2a indicates that it has spread to four to six nearby lymph nodes. The scores of T3/T4a N2b M0 show that cancer has grown into the outermost layers of the colon or rectum (T3) or the visceral peritoneum (T4a) but has not reached the nearby organs. N2b indicates that it has spread to seven or more nearby lymph nodes. The label of T4b N1/N2 M0 indicates that it has grown through the wall of the colon or rectum and is attached or grown into other nearby tissues or organs (T4b). N1 or N2 indicates that it has spread to at least one nearby lymph node or areas of fat near the lymph nodes.

Stage IV

Cancers are classified as stage IV cancers when they have metastasized to other parts of the body. So, the T and N scores are not considered. IVa stage cancers are those that have a score of M1a. This indicates that colorectal cancer has spread to a distant set of lymph nodes or one distant organ but not to farther than the peritoneum, the lining of the abdominal cavity. IVb cancers have an M score of M1b which indicates that cancer has spread to more than one distant organ or distant lymph nodes but not to the distant parts of the peritoneum. A stage IVc has a score of M1c, which means cancer has spread to distant parts of the peritoneum and may or may not have spread to distant organs or lymph nodes.