The most common anorectal cancer is adenocarcinoma. Squamous cell (nonkeratinizing squamous cell or basaloid) carcinoma of the anorectum accounts for 3 to 5% of distal large-bowel cancers. Basal cell carcinoma, Bowen’s disease (intradermal carcinoma), extramammary Paget’s disease, cloacogenic carcinoma, and malignant melanoma are less common. Other tumors include lymphoma and various sarcomas. Metastasis occurs along the lymphatics of the rectum and into the inguinal lymph nodes.

Risk factors include infection with human papillomavirus (HPV), chronic fistulas, irradiated anal skin, leukoplakia, lymphogranuloma venereum, and condyloma acuminatum. Gay men practicing receptive anal intercourse are at increased risk. Patients with HPV infection may manifest dysplasia in slightly abnormal or normal-appearing anal epithelium (“anal intraepithelial neoplasia,” histologically graded I, II, or III). These changes are more common in HIV-infected patients, particularly gay men. Higher grades may progress to invasive carcinoma. It is unclear whether early recognition and eradication improve long-term outcome; hence, screening recommendations are unclear.

Wide local excision is often satisfactory treatment of perianal carcinomas. Combination chemotherapy and radiation therapy result in a high rate of cure when used for anal squamous and cloacogenic tumors. Abdominoperineal resection is indicated when radiation and chemotherapy do not result in complete regression of tumor and there are no metastases outside of the radiation field.

Frequency

United States
Colorectal cancers are the most common GI cancer and the second most common cause of cancer death in developed countries. In 2005, there were an estimated 145,290 new cases of colorectal cancer in the United States; 104,950 were in the colon and 40,340 rectal (only marginally less than lung cancer), and 56,300 related deaths were reported (47,700 colon cancer, 8,600 rectal), accounting for 11% of all cancer deaths. The highest GI cancer rates are in the Northeast and North Central states, and the lowest rates are in the southern and western states (except for the San Francisco Bay area and Hawaii, which have the highest incidences in the United States).

International
More than 940,000 new cases of colorectal cancer and nearly 500,000 related deaths are reported each year worldwide (World Health Organization, 2003). The incidence rate of rectal cancer is highest in the westernized countries of North America, northern Europe, Australia, and New Zealand. Intermediate rates are found in southern Europe, and there are low rates in Africa, Asia, and South America. Rectal cancer shows less international variation than colon cancer. Although there is a 60-fold difference in the incidence rates of colon cancer between countries with the highest incidence and those with the lowest incidence, there is only an 18-fold difference in the incidence rates for rectal cancer. High colon-to-rectal cancer ratios (3-4:1) prevail in North America, northern Europe, Australia, and New Zealand. Ratios equalling less than 1 are typical in Asia and Africa.

Mortality/Morbidity

Prognosis is related to the stage of the disease at diagnosis and to initial treatment. Although an international classification system known as TNM (Tumor, Node, Metastases) and a computed tomography (CT) system for staging have been developed recently, the Dukes classification (or one of its modifications) remains in wide use.

Prognosis is also affected by the histologic grade of the tumor. The complications of rectal cancer include obstruction (common); fistula formation to the small bowel, bladder, or vagina (uncommon); and perforation (rare).

Race

 
• In the United States, rectal cancer incidence rates are higher in white men than in black men, but the rates for white and black women are similar. Colon cancer incidence rates are similar among white and black men and women.  
• The rate of risk rises for populations that migrate from low-risk to high-risk areas, as demonstrated clearly in Japanese immigrants in Hawaii and the continental United States, where rates among immigrants have risen to approximately those of the native population. The 18-fold difference in rectal cancer rates between the country with the highest rate and the country with the lowest rate is significantly less than the 60-fold difference in colon cancer rates. This may reflect dietary differences in fat and fiber intake in different countries. These differences diminish when a western-type diet is adopted.

Sex
In westernized countries, men have a greater incidence of rectal cancer than women; the ratio varies from 8:7-9:5.

Age
Of patients with rectal carcinoma, 90% are older than 50 years. Only 5% of patients are younger than 40 years.

Anatomy
The rectum lies anterior to the sacrum and coccyx and is approximately 15 cm long. The rectosigmoid junction is located at the end of the sigmoid mesocolon. Its upper third is covered almost completely by peritoneum. Below this level, the peritoneum is reflected anteriorly onto the posterior surface of the uterus and vagina in females and onto the posterior surface of the bladder in males. The peritoneal recesses, the pouch of Douglas (rectouterine), and the rectovesical pouch lie between these organs.

The lower half of the rectum is entirely extraperitoneal. The rectum ends just below the level of the coccyx. It turns posteriorly, through the puborectal sling of the levator ani muscles, to become the anal canal. The rectum is supplied by the superior rectal branch of the inferior mesenteric artery and by branches of the internal iliac arteries. The rectal lymphatics drain superiorly into the superior rectal nodes, then through the inferior mesenteric nodes, and laterally into the internal iliac nodes.

The rectal wall is composed of 5 layers: the mucosa (lined with columnar epithelium), the muscularis mucosa, the submucosa, the muscularis propria (an inner circular layer and an outer longitudinal layer, comprising 3 narrow bands), and the serosa.

Rectal Carcinoma

The rectum is part of the digestive system. Once food nutrients have been absorbed by the small intestines, the waste is moved by muscular contractions into the large intestine (bowel). Water is removed and the waste is temporarily stored in the rectum, which makes up the last 20cm or so of the bowel. From the rectum, wastes pass out of the body through the anus. The rectum’s lining (epithelium) secretes mucus that helps to lubricate the faeces through the anus.

Cancer of the rectum begins as cellular changes in the topmost layer of the epithelium. Rectal cancer tends to affect people over the age of 50 years, with men more at risk than women. Some people have an increased risk due to genetic factors and may develop the disease sometime after the age of 40 years.

If treated in its earliest stages, rectal cancer is highly curable. If untreated, stray cancer cells can migrate around the body via the lymphatic system and develop secondary cancers.

Symptoms
The symptoms of rectal cancer include:

 
• A change in previous bowel habits, such as constipation or diarrhoea  
• Urgency to pass bowel motions  
• A sensation that the bowel isn’t empty after going to the toilet  
• Bleeding from the anus  
• Abdominal pain.

Risk factors
Factors that may increase a person’s risk of rectal cancer include:

 
• Advancing age  
• Family history of colon cancer  
• Polyps in the rectum  
• Pre-existing inflammatory bowel disease, such as ulcerative colitis  
• High fat, low fibre, low calcium diet  
• Obesity.

Diagnosis
Rectal cancer is diagnosed using a range of tests including:

 
• Rectal examination – the doctor inserts a gloved finger into the rectum and feels for lumps.  
• Faecal occult blood test – faecal matter is examined in the laboratory.  
• Proctoscope or sigmoidoscope – a slender instrument is inserted into the anus to allow the doctor to look at the rectum.  
• Colonoscopy – a longer instrument is inserted (under sedation) to check the rectum and colon.  
• Biopsy – a small tag of tissue is removed and examined in a laboratory for the presence of cancerous cells.  
• Barium enema – special fluid is squirted into the rectum and x-rays are taken.  
• Endorectal ultrasound – an ultrasound device is inserted and pictures taken of the rectum.  
• Magnetic resonance imaging (MRI) – a high-definition scan is used to determine the size and spread of rectal cancer.

Stages of the disease
Rectal cancer is categorised into different stages including:

 
• Stage 0 – the cancer is limited to the lining (epithelium) of the rectum.  
• Stage 1 – the cancer has spread to the deeper layers of the rectal lining.  
• Stage 2 – the cancer has spread to tissues beyond the rectum, but not to the lymph nodes in the local area.  
• Stage 3 – the cancer has spread to the lymph nodes in the local area.  
• Stage 4 – the cancer has developed secondary cancers at other sites around the body.

Treatment
Treatment for rectal cancer depends on its stage, but may include:

 
• Surgery – the cancer and associated rectal tissue as well as nearby lymph nodes are removed either through the anus or via an abdominal incision. The rectum is then sewn back together. If a large amount of tissue is removed, it may not be possible to close the rectum and a colostomy will be needed, either temporarily or permanently.  
• Radiation therapy – high doses of precisely targeted radiation are used to kill cancer cells.  
• Chemotherapy – cancer-killing drugs are administered by intravenous injection or by mouth.

There are several specialised centres in most Australian States and Territories that treat rectal cancer. Results from treatment are generally better when treatment is provided by these specialised centres.

Colostomy
Sometimes, so much tissue is removed that the rectum can’t be sewn back together or reattached to the anus. In this case, the bowel is diverted from the anus to a small hole in the abdomen (stoma) and a colostomy bag fitted. A colostomy bag is made from plastic and is stuck to the stoma with special glue. Wastes are passed into the bag, which is then thrown away and replaced with a fresh one.

A temporary colostomy bag may sometimes be needed while a successfully remodelled rectum heals properly. Less than a third of patients that have an operation for rectal cancer will require colostomy.

Things to remember

 
• The rectum makes up the last 20cm or so of the large intestine.  
• The function of the rectum is to temporarily store faeces.  
• Rectal cancer usually affects people over the age of 50 years, with men more susceptible than women.  
• Treatment includes surgery, chemotherapy and radiation therapy.

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Key words: anal cancer; chemoradiation; molecular biology; human papilloma virus; AIDS
Gervaz P, Allal AS, Villiger P, Bühler L, Morel P.