Applied Anatomy of Human Colon
This pouch-like structure, present at the beginning of the colon is called as caecum. This part, present on the right side of the abdomen, extending upwards from the caecum is called an ascending colon. The longest part of the colon, that extends from the right to the left side of the abdomen, is called as transverse colon. And this part, which extends downwards from transverse colon, on the left side of the abdomen, is called as descending colon. And this last part, which connects colon to the rectum and anal canal is called as sigmoid colon. These are the loops of the small intestine that occupy the central part of the abdomen. This is the cross-section from the wall of the colon, with the upper part being inside and lower part being outside of the wall. The innermost layer is epithelium, followed by this layer called as lamina propria. Outer to which lies muscularis mucosa. Then comes the submucosa. Outer to which lies the muscularis propria which is followed by a layer of pericolorectal tissue. On the outermost aspect, lies this layer which is called as serosa.
TNM Staging of Colon Cancer
Tis – Pre-cancerous or cancer cells present only in the superficial layer (epithelium) of colorectal mucosa
T1 – Cancer extends to the lamina propria
T2 – Cancer extends to the muscular layer (muscularis propria)
T3 – Cancer extends through the muscularis propria into the pericolorectal tissues
T4a – Cancer has invaded up to the outermost serosa layer (or visceral peritoneum)
T4b – Cancer has invaded into the adjacent structures/organs like small intestine, kidneys, pancreas, or stomach.
Now we will discuss the N staging or the nodal staging for colon cancer. In this figure, you can see the draining lymph nodes for the colon. It can be called as N0, N1 or N2, depending upon the number of lymph nodes involved.
N0 – Cancer has not spread to regional lymph nodes
N1 – Cancer has spread to 1 to 3 nearby lymph nodes
N2a – Cancer has spread to 4 to 6 nearby lymph nodes.
N2b – Cancer has spread to >/=7 nearby lymph nodes.
Lastly, we will discuss the M-staging or the metastatic staging of the colon cancer. Liver is the most common site for the distant spread of the tumor from colon.
Due to the direction of vascular drainage, after liver, lungs are the second most common organs involved by metastasis.
This figure shows the spread of colon cancer to both the lungs, in form of multiple nodular deposits. And sometimes, both lungs and liver maybe involved in the metastatic spread of the disease, as we can see in this figure. The tumor may also spread to the peritoneum, in form of peritoneal deposits. It may rarely spread to one or both the ovaries. Very rarely, the tumor may also spread to the brain or bones.
M1a – Cancer has spread to one distant organ without peritoneal spread.
M1b – Cancer that has spread to >/=2 distant organs without peritoneal spread.
M1c – Cancer that has spread to peritoneum with or without other sites.
4 Stages of Colon Cancer
Based on the TNM classification described above, colon cancer may be divided into 4 stages as descibed below.
|0||Tis N0 M0|
|I||T1-2 N0 M0|
|IIA||T3 N0 M0|
|IIB||T4a N0 M0|
|IIC||T4b N0 M0|
|IIIA||T1-2 N1/1c M0|
|T1 N2a M0|
|IIIB||T3-4a N1/1c M0|
|T2-3 N2a M0|
|T1-2 N2b M0|
|IIIC||T4a N2a M0|
|T3-4a N2b M0|
|T4b N1-2 M0|
|IVA||Any T Any N M1a|
|IVB||Any T Any N M1b|
|IVC||Any T Any N M1c|
Stage 1 Colon Cancer
Stage 1 colon cancer includes cases with T1 and T2 disease, that is, disease extending upto muscularis propria.
Stage 2 Colon Cancer
In T3, cancer extends through the muscularis propria into the pericolorectal tissues.
In T4a, cancer has invaded up to the outermost serosa layer of colonic wall.
In T4a, colon cancer has invaded into the adjacent structures/organs like small intestine, kidneys, pancreas, or stomach.
A tumor present in almost any part of the colon may infiltrate into the small intestine.
Tumor in the ascending colon may infiltrate into the right kidney. And that in the descending colon may infiltrate into the left kidney. A transverse colon tumor may extend to involve pancreas. And may even extend to involve the stomach.
Stage 3 Colon Cancer
All node positive cases come under stage 3 disease. It may be N0, N1 or N2 depending upon the number of lymph nodes involved. It has been discussed above in N staging.
Stage 4 Colon Cancer
It is also called as metastatic colon cancer. When the disease spread to non-regional lymph nodes or distant sites, it is called as stage 4. It has been discussed in detail above in M staging.
Survival/Life Expectancy of Colon Cancer based on Staging
- Cancer is limited to the bladder.
- 5 year survival 90%.
- Cancer has spread to nearby structures or lymph nodes
- 5 year survival 71%.
- Cancer has spread to distant body parts like lungs, liver or bones.
- 5 year survival 14%.