Colon cancer refers to cancer that originates in the colon. Colon cancer is often associated with colorectal cancer, as the cancer often is located in both the colon and the rectum. Approximately 95 percent of colon cancer begins with an adenoma, a polyp that grows in the lining of the colon. Typically, the cancer is slow growing and spreads from the polyp to the center of the colon or rectum. Colon cancer is staged in four stages, like most other types of cancer, however there is a Stage II A and B, and a Stage III A, B and C.
Staging of Colon Cancer
Like most cancers, colon cancer is staged using a standard system created by the American Joint Committee on Cancer. The system is called the AJCC staging system, or the occasionally the TNM, based on the three factors used to stage the cancer. The "T" in TNM is determined by the size and growth of the primary tumor (how many cancer cells there are) and how far the tumor has grown into the intestine wall or surrounding areas. The "N" addresses whether the cancer has spread into the nearby lymph nodes. The "M" refers to whether the cancer is metastatic (has spread to other parts of the body).
Because the colon has several layers, the "T" factor is determined by the depth of colon layers the tumor has impacted. The layers of the colon include the mucosa (the inner lining), the muscularis mucosa (the thin muscle layer next to the mucosa), the submucosa (the tissue and fibers above the muscle layer), the muscularis propi (the muscle layer which contracts in order to move the intestines contents throughout the intestines) and finally the subserosa and serosa (the outer tissue layers that surround the colon).
There are two different combinations of TNM numbers associated with stage III A. The first is T1 with an N1 and an M0. The second is a T2 number with an N1 and an M0. A T1 number means the cancer has extended into the submucosa. A T2 number means the cancer has spread to the the muscularis propia. An N1 number means the cancer is present in between 1 and 3 lymph nodes near the colon. An M0 number means the cancer has not metastasized or spread to parts of the body other than the colon.
There are also two different combinations of TNM numbers associated with Stage IIIB Colon Cancer. Both have an N1 and an M0, like Stage IIIA. However, Stage IIIB indicates that the T number is either T3 (the cancer is in the outer layer of the colon) or T4 (the cancer extends beyond the colon wall into nearby tissue or organs).
Stage IIIC colon cancer has an M of 0. The "T" number does not matter with Stage IIIC, as IIIC is characterized by an N number of N2. This "N" number means that the cancer is present in four or more lymph nodes near the colon.
According to the National Cancer Institute, based on data from its SEER database, which looked at 120,000 cases of colon cancer from 1991 and 2002, the prognosis for colon cancer is dramatically different for Stage IIIA, Stage IIIB and Stage IIIC. While 83 percent of people survive for five years after diagnosis with Stage IIIA colon cancer, 64 percent survive with Stage IIIB and 44 percent with Stage IIIC.