Excerpt from Rectal Carcinoma

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Background

Almost all rectal cancers are primary adenocarcinomas. Adenocarcinoma of the rectum is a major cause of mortality and morbidity in North America and Western Europe. Rectal cancers are, after colon cancers, the second most common gastrointestinal (GI) carcinoma, and have the best prognosis. The 5-year survival rate is approximately 50%. Screening for and removing adenomatous polyps may improve survival rates.

For excellent patient education resources, visit eMedicine's Esophagus, Stomach, and Intestine Center and Cancer and Tumors Center. Also, see eMedicine's patient education articles Colon Cancer, Colonoscopy, Sigmoidoscopy, and Rectal Cancer.

Pathophysiology

Adenocarcinoma of the rectum arises as an intramucosal epithelial lesion, usually in an adenomatous polyp or gland. As cancers grow, they invade the muscularis mucosa, lymphatic structures, and vascular structures and involve regional lymph nodes, adjacent structures, and distant sites, especially the liver.

Many factors increase the risk for rectal cancer, including the following:

• High-fat, low-fiber diet

• Age greater than 50 years

• Personal history of colorectal adenoma or carcinoma (3-fold greater risk)

• First-degree relative with colorectal cancer (3-fold greater risk)

• Familial polyposis coli, Gardner syndrome, and Turcot syndrome (in which all patients without a colectomy develop colorectal carcinoma)

• Juvenile polyposis syndrome, Peutz-Jeghers syndrome, and Muir syndrome (risk increased slightly)

• Hereditary nonpolyposis colorectal cancer (as many as 50% of patients are affected)

• Inflammatory bowel disease
  • Ulcerative colitis (risk is 30% after 25 y)

  • Crohn disease (4- to 10-fold risk)

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 (see Table 1).

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).

Table 1. Modified Dukes Classification System and 5-year Survival Rate*

*Modified from Zinkin.¹

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.

Clinical Details

Rectal cancers tend to be symptomatic earlier than colon cancers. Overt rectal bleeding is more common in rectal than colon tumors, and a change in bowel habit or symptoms of large bowel obstruction, such as pain and abdominal distention, may be the presenting features in patients with a rectosigmoid or upper rectal tumor. The primary tumor may be palpable by digital examination of the rectum. Weight loss, jaundice, and ascites are associated with advanced metastatic disease. Perforation is rare but may occur as a result of distention proximal to the tumor (usually in the cecum) or locally at the site of the tumor. Pneumaturia and feculent vaginal discharge may occur as a result of fistula formation into the bladder or vagina.

Rectal tumors may be asymptomatic, but the possible symptoms of rectal tumors include the following:

• Palpable mass on digital rectal examination

• Overt rectal bleeding

• Microcytic anemia with fatigue, shortness of breath, and angina

• Vague abdominal discomfort

• Change in bowel habit

• Large bowel obstruction

• Pneumaturia

• Feculent vaginal discharge

• Perforation (rare)

• Weight loss

• Jaundice

• Ascites

Preferred Examination

Evaluation begins with a history and physical examination, including a digital rectal examination.

• Inspect the stool and test for occult blood.

• Order blood tests (ie, complete blood count, liver function tests, and carcinoembryonic antigen levels).

• Perform either sigmoidoscopy (rigid or flexible) or a double-contrast barium enema.

• Perform CT studies to stage the tumor before treatment and to choose the most appropriate treatment. Although magnetic resonance imaging (MRI) is slightly more accurate than CT in staging primary rectal tumors, CT is much more widely available. Most institutions and departments have more extensive experience using CT than MRI and continue to use CT for staging rectal tumors. This may change in the future.

Limitations of Techniques

• Sigmoidoscopy: The 60-cm flexible sigmoidoscope has an increased range over the rigid sigmoidoscope, which at best reaches only to the rectosigmoid junction (20 cm). The sigmoidoscope also is more accurate in the rectum. Sigmoidoscopy detects smaller adenomatous polyps than barium enema; also, polyps may be excised by this method.

• Double-contrast barium enema: Detects most colorectal tumors (80-95%), but it should be preceded by flexible sigmoidoscopy. It has a low perforation rate (1/25,000).

• CT and MRI cannot be used to assess the exact degree of mural invasion of a primary rectal tumor. These techniques cannot distinguish enlarged lymph nodes resulting from tumor from those resulting from inflammation. Normal-sized nodes that contain tumors cannot be detected by CT, MRI, sigmoidoscopy, or barium enema.

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