Colon cancer is usually classified using the American Joint Committee on Cancer’s (AJCC) TNM system. The classification of the main stages of colon cancer is by assigning a number ranging from 0 to 4 in Roman numerals with 0 being the early stage and 4 or IV being the most advanced stage of the disease. This system studies and scores three important factors together and depending on them, a number is assigned to denote the stage. The classification helps to get a clear picture of the overall stage of the disease and determine the line of treatment that would be most beneficial. The TNM system evaluates colon cancer based on three factors which are represented by the alphabets T, N, and M.
T denotes the size of the tumor and how far into the layers of the colon it has spread. The colon’s inner lining is called the mucosa and this has a thin muscular layer called muscularis mucosa. Most colon cancer starts in the mucosa of the colon. The next layers are the submucosa and the thick muscular layer of the muscularis propria. The outermost layers are the subserosa and serosa which are made up of connective tissue. This outermost layer is only present in the colon and not the rectum. A tumor that cannot be scored because of lack of sufficient information is scored as TX while T0 is the score given when there is no evidence of a primary tumor. When there is a lack of information about the regional lymph nodes, the score given is NX. The N component of the classification system is to denote if cancer has spread to the nearby lymph nodes. The M component denotes whether cancer has metastasized or spread to other organs or distant lymph nodes.
There is no stage of colon cancer that is classified as the main stage, each stage has to be evaluated differently and has appropriate steps and procedures that need to be followed to manage it.
Stage 0 and stage I of colon cancer
The earliest one, i.e. stage 0 denotes cancer that is in the innermost layer of the colon or rectum. It is called carcinoma in situ or intramucosal carcinoma (Tis). The Stage I is cancer has not spread to the nearby lymph nodes (N0) or other parts of the body (M0) but has grown through the muscularis mucosa and into the submucosa (T1) or further into the muscularis propria (T2).
Stage II of colon cancer
The second stage or stage II colon cancer has not spread to the nearby lymph nodes (N0) or other parts of the body (M0). It has three sub-classifications based on the T component. When cancer has reached the outer layer of the colon but has not gone through (T3) it is stage IIA. Stage IIB is assigned when cancer has gone through the outer wall of the colon (T4a). Stage IIC is when cancer has grown through the colon wall and into or attached to the nearby organs or tissue (T4b).
By stage III colon cancer, which in common parlance may be referred to as one of the main stages, cancer has spread past the colon’s lining and has affected the lymph nodes. This stage is also further divided into three categories. In stage IV colon cancer, it has spread to distant organs in the body and the lymph nodes.
