In order to decide what treatment is best for the patient the doctor will stage the cancer after it has been diagnosed to see how much it has affected the inner layers of the colon and if it has spread to other organs. Depending in what stage the colon cancer is the doctor will decide what therapy to apply. For those who are in an early stage the surgery will be the only treatment needed but for those who have an advanced cancer stage surgery and chemotherapy is needed.

Doctors can use different classifications of staging like the Duke's staging, the Astler-Coller staging and the American Joint Committee on Cancer system known as the TNM system. These systems are using in their staging the layers of the colon, the lymph nodes and the local organs and the distant organs. Generally the doctors find out the exact cancer stage only after surgery is done and only the pathologist can give the verdict.

The layers of the colon from the inside to the outside are: the mucosa, the submucosa, the muscularis propria, the subserosa and the serosa.
The TNM staging is one of the most important ways of staging the cancer. The T category determines how much the cancer has spread in the affected organ and if other organs have been affected by cancer too.
Tx refers to the fact that the information is insufficient and so the cancer's evolution can not be determined.
Tis: it means that the cancer is in an early stage and it has affected only the mucosa; it is also named as carcinoma in situ.
T1: the cancer has affected the mucosa and the submucosa too.
T2: the cancer has passed through the submucosa and extended to the muscularis propria.
T3: the cancer has passed through the muscularis propria and has reached the subserosa.
T4: the cancer has affected all the colon's layers and it is spreading o the nearby organs.

The N category shows if the lymph nodes are affected and how many of them are in this situation.

Nx refers to the fact that the information is not sufficient for determining the lymph node involvement.
N0: it means that no lymph nodes are affected yet.
N1: the cancer has affected 1 to 3 local lymph nodes.
N2: the cancer is found in more than 4 local lymph nodes.

The M category shows whether the cancer has spread to other organs like lungs, liver or distant lymph nodes.
Mx refers to the fact that the information is incomplete and so there can not be determined whether the cancer has given metastasis or not.
M0: it means that no metastases are present.
M1: it means that metastasis can be seen.

Another classification which is more accurate contains both the Duke's and the TNM classification.

Stage I: T1 N0 M0; T2 N0 M0 means that the disease has extended in the inner layers of the colon but it has not spread to other structures yet.
Stage IIA: T3, N0, M0 means that the cancer has affected all of the colon's layers but it has not affected the lymph nodes or other structures.
Stage IIB: T4, N0, M0 means that the cancer has affected all the layers of the colon and has spread in the nearby structures. The lymph nodes are still clear.
Stage IIIA: T1-2, N1, M0 refers to the fact that the disease has affected the mucosa, the submucosa and might have affected even the muscularis propria. 1 to 3 lymph nodes are affected too.
Stage IIIB: T3-4, N1, M0 means that the cancer has got to the nearby structures or organs and got 1 to 3 lymph nodes.
Stage IIIC: Any T, N2, M0 means that the cancer might have affected more than 4 lymph nodes but still not spread to distant organs.
Stage IV: Any T, Any N, M1refers to the fact that metastases have appeared, and so the cancer has spread to distant organs like lungs, ovary, and liver.