Overview of Colon Cancer

It is important to recognize the increased risk for cancer in patients with hereditary cancer syndromes, but by far the most common form of colon cancer is sporadic in nature, without an associated strong family history.

Although the cause and pathogenesis of adenocarcinoma are similar throughout the large bowel, colon cancer, significant differences in the use of diagnostic and therapeutic modalities separate colonic from rectal cancers. This distinction is largely due to the confinement of the rectum by the bony pelvis. The limited mobility of the rectum allows MRI to generate better images and

colon cancer

colon cancer

increases its sensitivity. In addition, the proximity of the rectum to the anus permits easy access of ultrasound probes for more accurate assessment of the extent of penetration of the bowel wall and the involvement of adjacent lymph nodes. The limited accessibility of the rectum, the proximity to the anal sphincter, and the close association with the autonomic nerves supplying the bladder and genitalia require special and unique consideration when planning treatment for cancer of the rectum. Therefore, colon and rectal adenocarcinomas are discussed separately.

The signs and symptoms of colon cancer are varied, nonspecific, and somewhat dependent on the location of the tumor in the colon as well as the extent of constriction of the lumen caused by the cancer. During the past several decades, the incidence of cancer in the right colon has increased in comparison to cancer arising in the left colon and rectum. This is an important consideration, in that at least half of all colon cancers are located proximal to the area that can be visualized by the flexible sigmoidoscope. colon cancers can bleed, causing red blood to appear in the stool (hematochezia). Bleeding from right-sided colon tumors can cause dark, tarry stools (melena). Often, the bleeding is asymptomatic and detected only by anemia discovered by a routine hemoglobin determination. Iron deficiency anemia in any male or nonmenstruating female should lead to a search for a source of bleeding from the gastrointestinal tract. Bleeding is often associated with colon cancer, but in about one third of patients with a proven colon cancer, the hemoglobin is normal and the stool tests negative for occult blood.

Cancers located in the left colon are often constrictive in nature. Patients with left-sided colon cancers may notice a change in bowel habit, most often reported as increasing constipation. Sigmoid cancers can mimic diverticulitis, presenting with pain, fever, and obstructive symptoms. At least 20% of patients with sigmoid cancer also have diverticular disease, making the correct diagnosis difficult at times. Sigmoid cancers can also cause colovesical or colovaginal fistulas. Such fistulas are more commonly caused by diverticulitis, but it is imperative that the correct diagnosis be established because treatment of colon cancer is substantially different than treatment of diverticulitis.

Cancers in the right colon more often present with melena, fatigue associated with anemia, or, if the tumor is advanced, abdominal pain. Although obstructive symptoms are more commonly associated with cancers of the left colon, any advanced colon cancer can cause a change in bowel habits and intestinal obstruction

Colonoscopy is the gold standard for establishing the diagnosis of colon cancer. It permits biopsy of the tumor to verify the diagnosis while allowing inspection of the entire colon to exclude metachronous polyps or cancers (the incidence of a synchronous cancer is about 3%). Colonoscopy is generally performed even after a cancer is detected by barium enema to obtain a biopsy and to detect (and remove) small polyps that may be missed by the contrast study .

January 2013
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