Rectal cancer is a disease that results from the formation of malignant cancer cells in the tissues of the rectum.
The rectum is part of the body’s digestive system. The digestive system absorbs a variety of nutrients (vitamins, minerals, carbohydrates, fats, proteins, and water) from food and helps get waste out of the body. The digestive system consists of the esophagus, stomach, small intestine and large intestine. The colon (large intestine) is the first part of the large intestine and is about 5 feet long. Meanwhile, the rectum and anal canal form the last part of the large intestine, which is 6 to 8 inches long. The anal canal terminates in the anus (the opening of the large intestine to the outside of the body).
Anatomy of the lower digestive system; shows the colon and other organs.
For more information on rectal cancer, see the following PDQ overview:
Rare childhood cancer treatments (see colorectal cancer section)
Colorectal Cancer Prevention
Colorectal Cancer Screening
Treatment of mesenchymal tumors of the gastrointestinal tract (adults)
Colorectal cancer genetics
Health history affects the risk of developing rectal cancer.
Any factor that increases your chance of getting the disease is called a risk factor. Having a risk factor does not mean you will get cancer; not having a risk factor does not mean you will not get cancer. If you think you are at risk for colorectal cancer, talk to your doctor.
Risk factors for developing colorectal cancer include the following:
A family history of a first-degree relative (parent, sibling, or child) who has colon or rectal cancer.
Have a personal history of colon, rectal, or ovarian cancer.
Personal history of high-risk adenomas (colorectal polyps 1 cm or larger, or abnormal cells under the microscope).
Hereditary variants in certain genes that increase the risk of familial adenomatous polyposis (FAP) or Lynch syndrome (hereditary nonpolyposis colorectal cancer).
History of chronic ulcerative colitis or Crohn’s disease for more than 8 years.
Drinking three or more alcoholic beverages per day.
Smoking
Black
obese
Age is a major risk factor for most cancers. As you get older, your chances of getting cancer increase.
Signs of rectal cancer include changes in bowel habits or blood in the stool.
These and other signs and symptoms can be caused by rectal cancer or other diseases. Consult your doctor if any of the following symptoms occur.
Blood in the stool (bright red or very dark).
Changes in bowel habits.
Diarrhea
Constipation
Feeling that the bowel is not completely empty.
Thinner stools or different shape than before.
A variety of abdominal discomfort (frequent bloating, distention, fullness, or cramping).
Changes in appetite.
Unexplained weight loss.
Feeling very tired.
Used to detect and diagnose rectal cancer by examining the rectum and colon.
There are several tests used to diagnose rectal cancer:
Physical exam and history: A physical exam to detect general health, including detecting signs of disease, such as lumps or other conditions that seem abnormal. The patient’s health history, past medical history, and treatment history are also considered in the examination for a rectal cancer diagnosis.
Diagnosis of the rectum (DRE): A method for diagnosing rectal cancer. The doctor or nurse uses a lubricated, gloved index finger to reach into the lower rectum and palpate a mass in the rectum or any other part of the rectum that looks abnormal. For female patients, the vagina should also be examined at the same time.
Colonoscopy: A procedure in which a colonoscope is used to look at polyps (small raised pieces of tissue), abnormal areas, or tumors inside the rectum and colon. A colonoscope is a thin, tube-like instrument with a light and a lens to allow for visualization. A colonoscope may also come with an instrument to remove polyps or to take tissue samples that are placed under a microscope to check for signs of cancer. 
Colonoscopy. A thin, glowing tube is passed through the anus and rectum and into the colon, looking for abnormal areas.
Biopsy: Cells or tissues are removed so that they can be placed under a microscope to look at them and detect signs of cancer. To see if a patient may have a genetic mutation that causes hereditary nonpolyposis colon cancer (HNPCC), the tumor tissue that is removed during the biopsy may be examined. This also helps in the development of a treatment plan. The following tests may be used.
Reverse transcription-polymerase chain reaction (RT-PCR) test:A laboratory test that measures the amount of genetic material called mRNA produced by a specific gene. An enzyme called reverse transcriptase is used to convert a specific piece of RNA into a matching piece of DNA, which can be amplified (mass replicated) by another enzyme called DNA polymerase. RT-PCR can be used to detect the activation of certain genes that indicate the presence of cancer cells. This test can be used to look for specific changes in genes or chromosomes that can help in the diagnosis of cancer.
Immunohistochemistry test:A laboratory test that uses antibodies to check for certain antigens (markers) in a patient’s tissue sample. Antibodies are usually used in combination with enzymes or fluorescent dyes. After the antibody binds to a specific antigen in the tissue sample, the enzyme or dye is activated and the antigen can then be seen under a microscope. This type of test is used to aid in the diagnosis of cancer and to help distinguish between different types of cancer.
Carcinoembryonic antigen (CEA) assay: A test that detects levels of CEA in the blood. CEA is released into the blood stream from cancer cells and normal cells. When carcinoembryonic antigen is found in the blood above normal, this may be a sign of rectal cancer or other disease.
Some factors can affect prognosis (chance of recovery) and treatment choices.
Prognosis (chance of recovery) and choice of treatment depends on:
Stage of the cancer (whether the cancer has affected only the lining of the rectum, the entire rectum, or has spread to lymph nodes, adjacent organs, or other parts of the body).
Whether the tumor has spread to the bowel wall or has spread through the bowel wall.
Where in the rectum is the cancer located.
Whether there is bowel obstruction or bowel perforation.
Whether all of the tumor can be removed surgically.
The patient’s general health.
Whether the cancer was just diagnosed or has recurred (reappeared).