The colon and colorectal tumors
What are the colon and rectum?
The colon, which extends to the small intestine, constitutes the terminal part of the digestive tract; it is also called the large intestine. This 1.5-meter-long organ begins with the cecum, where the appendix is located. It is divided into four main zones defined by their orientation (ascending, transverse, descending, sigmoid). The colon extends through the rectum, then through the anus.
The wall of the colon is made up of four layers of cells performing different functions. From the inside to the outside of the colon, we find:
- the mucosa, which mainly plays a role in absorbing the nutrients or water present in the matter being digested;
- the submucosa, which has many blood vessels and lymphatic vessels;
- the muscular, which ensures the contractions of the intestine necessary for the movement of its contents;
- the serosa, which is the outer lining of the colon. It is absent in the rectum.
The colon is linked to the deep abdominal wall by a membrane called the mesocolon; the rectum is surrounded by a tissue called the mesorectum.
On its inner face, the intestinal wall forms microscopic folds: this alternation of villi and crypts, within which are found different glands (mucus glands, Lieberkühn glands, etc.), gives it a brush-like appearance.
Once concentrated, the feces are transported to the rectal ampulla, which acts as a reservoir. Stool retention is ensured by a muscle: the anal sphincter.
Polyps and colorectal tumors
Because of their similarities, cancers of the colon and rectum are often grouped under the term colorectal cancer.
Colorectal cancer most often develops from a benign non-cancerous tumor called an adenomatous polyp or adenoma. These anomalies are frequent and most of the time not serious; however, they can slowly progress to a cancerous form called adenocarcinoma. The larger the polyp, the greater the risk of progression to cancer. On average, the occurrence of an adenoma precedes the appearance of cancer by nine years. It is therefore important to detect them and remove them as early as possible.
The colorectal tumor
About 40% of cancers affect the rectum and 60% the colon. The vast majority (70%) of colon tumors develop on the last part of the colon: the sigmoid.
Adenocarcinoma is the most common form of cancer affecting the colon and rectum (95%). Depending on the cell of origin of the tumor, we speak of lieberkühnian adenocarcinomas (because they resemble the Lieberkühn glands of the colonic mucosa) or mucinous adenocarcinomas. The former is the most frequent (85%), the latter account for approximately 10% of cases. The remaining few percent correspond to rarer forms from other cell types.
According to its evolution, there are different stages of adenocarcinoma:
- we speak of cancer in situ when the tumor, little developed, remains localized at the level of the internal mucosa of the intestinal wall;
- we speak of invasive cancer when the tumor has progressed towards the submucosal layer;
- in the absence of treatment, the tumor can continue to progress and spread towards the lymph nodes which drain the region (nodal extension) and, ultimately, distant organs (liver, lung, etc.): this is then called cancer metastasized.
In 5% of cases, cancers that affect the colon develop from other types of cells than those at the origin of the adenocarcinoma. We speak of carcinoid tumors when they develop from digestive nerve cells; melanoma when cancer arises from a melanocyte cell in the anal canal; lymphoma when cancerous cells form in a lymph node. These rare cancers require specific treatment which will not be covered in this brochure dedicated to adenocarcinomas.