Once food is swallowed, it travels down the esophagus to the stomach, where digestion begins, and then travels to the small intestine, where significant nutrients are absorbed. The food waste travels to the colon, where water is absorbed, while the rectum acts as a holding bay for the stools until they are eliminated via the anus.
The colon consists of 4 parts: the ascending, transverse and descending colon, and the sigmoid colon (see image 1). The rectum is the last segment of the large intestine, which leads to the anus and measures 15 cm in length.
In order to describe the position of a tumor in the rectum, we usually divide it into three parts: the upper, middle and lower third. The upper rectum is located immediately beneath the sigmoid colon and the lower rectum is in the area where the large intestine enters the anus.
The rectal walls consist of tissue layers. Cancer starts developing in the interior layer (mucosa), but as it spreads it infiltrates the other layers and can come out into the fat that surrounds the rectum. In more advanced stages it can infiltrate neighboring organs and lymph nodes and/or metastasize to remote areas of the body.
Most cancers develop slowly over a number of years and start as small benign lesions known as polyps. A small percentage of colorectal cancers (mainly hereditary forms) may create a large number of polyps, but these forms are rare. In the early stages, the disease remains within the organ, but in more advanced stages it tends to metastasize, primarily to the liver and the lungs.