The colon and rectum are the two main components of the large intestine. The intestinal wall has several layers, including the mucosa (inner lining), submucosa, several layers of muscle and the serosa (outer layer). A network of blood vessels and lymph nodes also feed the tissue in and around the colon. The colorectal cancer stage is determined by the extent to which the disease has spread through those layers of the colon or rectum wall and/or into lymph nodes or other organs.
Each stage reflects how the disease has advanced into or through the colon or rectum or has spread to nearby or distant organs. Doctors also use the TNM system to more precisely determine the extent of certain cancers in each stage. This is how the system is defined:
T (tumor): This describes the size of the original tumor.
N (node): This indicates whether the cancer is present in the lymph nodes.
M (metastasis): This refers to whether cancer has spread to other parts of the body.
A number (0-4) or the letter X is assigned to each factor. Using this colorectal cancer staging system, a higher number indicates increasing severity. For instance, a T1 score indicates a smaller tumor than a T2 score. The letter X means the information could not be assessed.
Once the T, N and M scores have been assigned, one of these overall stages is determined:
Stage 0: Abnormal cells or growths, such as polyps, are found on the mucosa (the inside lining of the colon or rectum). This is known as carcinoma in situ because the cells are confined to their place of origin and there is no evidence they have spread to other layers of the colon or rectum or to lymph nodes or have metastasized to other organs. Cells found in stage 0 colorectal cancer may be cancerous or precancerous.
Stage I (stage 1 colorectal cancer): The cancer has grown into the intestinal wall, through the mucosa (the inner lining) and into the submucosa and may have entered the muscle. There is no evidence the cancer has spread to lymph nodes or distant organs.
Stage II (stage 2 colorectal cancer): Stage II colorectal cancer is divided into three categories:
Stage III (stage 3 colorectal cancer): Stage III colorectal cancer is divided into three categories:
Stage IV (stage 4 colorectal cancer): Stage IV is the most advanced stage of colorectal cancer. If you have been diagnosed with stage IV colorectal cancer, it means that the cancer has metastasized to distant sites, such as the liver or lungs. The cancer may or may not have grown through the wall of the colon or rectum, and lymph nodes may or may not have been affected.
Stage IV colorectal cancer is further divided into two categories, depending on whether or not the metastasis has affected more than one organ. The original tumor can be of any size and lymph nodes may or may not be involved, but if the cancer has spread to one different organ it is considered stage IVA, while more than one organ would be defined as stage IVB.
Stage IV colorectal cancer can be defined by any T or N category, with the only difference stemming from whether the M1 or M2 assignment is more appropriate.
In both forms of stage IV colorectal cancer, the tumor can be of any size (T), and lymph nodes may or may not be involved (N). M1a indicates that the cancer has spread to just one organ, while M1b would mean that more than one organ has been affected.
Next topic: How is colorectal cancer daignosed?