The rectum is the lower part of the colon that connects the large bowel to the anus. The rectum’s primary function is to store formed stool in preparation for evacuation. Like the colon, the three layers of the rectal wall are as follows:
- Mucosa: This layer of the rectal wall lines the inner surface. The mucosa is composed of glands that secrete mucus to help the passage of stool.
- Muscularis propria: This middle layer of the rectal wall is composed of muscles that help the rectum keep its shape and contract in a coordinated fashion to expel stool.
- Mesorectum: This fatty tissue surrounds the rectum.
In addition to these three layers, another important component of the rectum is the surrounding lymph nodes - also called regional lymph nodes. Lymph nodes are part of the immune system and assist in conducting surveillance for harmful materials - including viruses and bacteria - that may be threatening the body. Lymph nodes surround every organ in the body, including the rectum.
Around 41,000 people are diagnosed with colorectal cancer - or bowel cancer - in the UK each year, with around one third of these being rectal cancer. According to Cancer Research UK, excluding non-melanoma skin cancer, this makes colorectal cancer the 4th most common cancer overall. Around two-thirds of these colorectal cancers are colon cancers and the rest are rectal cancers.
The most common type of rectal cancer is adenocarcinoma, which is a cancer arising from the mucosa. Cancer cells can also spread from the rectum to the lymph nodes on their way to other parts of the body.
Like colon cancer, the prognosis and treatment of rectal cancer depends on how deeply the cancer has invaded the rectal wall and surrounding lymph nodes. However, although the rectum is part of the colon, the location of the rectum in the pelvis poses additional challenges in treatment when compared with colon cancer.
This article only discusses issues related to rectal adenocarcinoma.
Rectal cancer causes
Rectal cancer usually develops over several years, first growing as a pre-cancerous growth called a polyp. Some polyps have the ability to turn into cancer and begin to grow and penetrate the wall of the rectum.
The actual cause of rectal cancer is unclear. However, the following are risk factors for developing rectal cancer:
- Increasing age
- Family history of colon or rectal cancer
- Diet high in red and processed meat
- Personal or family history of polyps or colorectal cancer
- Alcohol consumption
Rectal cancer symptoms
Rectal cancer can cause many symptoms that require a person to seek medical care. However, rectal cancer may also be present without any symptoms. Symptoms to be aware of include the following:
- Bleeding. Seeing blood mixed with stool (poo) is a sign to seek immediate medical advice. Although many people bleed due to haemorrhoids, a doctor should still be notified in the event of rectal bleeding. Prolonged rectal bleeding - perhaps in small quantities that is not seen in the stool - may lead to anaemia, causing fatigue, shortness of breath, light-headedness, or a fast heartbeat.
- Obstruction. A rectal mass may grow so large that it prevents the normal passage of stool. This blockage may lead to the feeling of severe constipation or pain when having a bowel movement. In addition, abdominal pain or cramping may occur due to the blockage.
- The stool size may appear narrow so that it can be passed around the rectal mass. Therefore, pencil-thin stool may be another sign of an obstruction from rectal cancer.
- A person with rectal cancer may have a sensation that the stool cannot be completely evacuated after a bowel movement.
- Weight loss: Cancer may cause weight loss. Unexplained weight loss - in the absence of dieting or a new exercise programme - requires a medical evaluation.