What for is cancer staging useful

Staging is used by the doctors to evaluate how much the cancer has invaded the organism and to decide further what methods of therapy to apply.

The colon cancer staging began with the Dukes classification system which contained three stages: A, B, and C. After a while Astler-Coller brought modifications to this classification, adding one more stage: the D stage. In 1978 Gunderson & Sosin brought other modifications to the classification and a few years ago, the American Joint Committee on Cancer has come with a new classification called TNM containing four stages.

Nowadays the doctors use more often the TNM classification and the Dukes classification in staging the colon cancer.

Duke stage A refers to the fact that the cancer affects only the mucosa of the bowel and does not get further to other structures of the colon.
Duke stage B1 shows that the cancer has reached the muscularis propria of the colon but did not get through it.
In Duke stage B2 the tumor has got through the muscularis propria of the colon.
In Duke stage C1 the cancer has reached the muscularis propria and has also affected the local lymph nodes.
In Duke stage C2 the tumor has got through the muscularis propria of the colon and has also reached the local lymph nodes.
Duke stage D shows that the cancer has spread towards other tissues and organs.

The TNM staging refers to tumor, nodes and metastasis.

T1: the cancer has affected the mucosa and the submucosa too.
T2: the cancer has 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 colons layers and it is spreading o the nearby organs.

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.

M0: it means that no metastases are present.
M1: it means that distant metastasis can be seen.

Another classification which is more accurate contains both the Dukes 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 II: T3 N0 M0; T4 N0 M0 means that the nearby structures of the colon and rectum have been invaded but the lymph nodes are clear.
Stage III: any T, N1-2, M0 refers to the fact that the lymph nodes are affected by the disease but the distant organs are still safe.
Stage IV: any T, any N, and M1 means that metastases have appeared, and so the cancer has spread to distant organs like lungs and liver.