In 2013 there were approximately 143,000 Americans diagnosed with colon and rectal cancer. Approximately 2/3 will involve the colon and 1/3 the rectum. Epidemiological factors and pathogenesis are similar for rectal cancer as for colon cancer.
The rectum is generally divided into three portions: lower rectum, mid-rectum, and upper rectum. The distances from the anal verge are approximations and may vary with flexible endoscopic techniques.
Lower rectum: 4–8 cm from anal verge
Mid-rectum: 8–12 cm from anal verge
Upper-rectum: 12–16 cm from anal verge
Anal canal: 4 cm in length
The dentate line is the transition point between the squamous mucosa of the anus/perineum and the columnar mucosa of the rectum. Below the dentate line, the lymph drainage flows through the inguinal nodes and has implications for treatment.
In contrast to the sigmoid colon, peritoneum does not cover the circumference of the rectum. Rectal cancer has higher rates of local failure following surgery than colon cancer and requires aggressive local treatment.
Generally, rectal tumors should be no less than 6–7 cm from anal verge if a sphincter sparing operation is to be attempted in order to preserve muscle function while obtaining adequate margins.
The majority of patients diagnosed with rectal cancer present with symptoms, although many are nonspecific and this may lead to a delay in diagnosis. Common symptoms include bleeding (gross or occult), constitutional symptoms, abdominal pain, changes in stool caliber, and changes in bowel habits.
When cancer is in the differential diagnosis, the workup entails history and physical including digital rectal examination (DRE), complete blood count, liver and renal function tests, carcinoembryonic antigen (CEA), and endoscopy.
DRE DRE should be used to assess the location of the tumor in relation to the anal verge, the dentate line, and the anal sphincter. If possible, the tumor should be assessed with respect to anal sphincter involvement, circumferential extent, and possible fixation to normal structures. Baseline sphincter tone should be assessed.
Rigid proctosigmoidoscopy This is used both to assess the location of the tumor (especially when nonpalpable), and to take biopsies for tissue diagnosis.
Rectal cancer is staged using clinicopathological parameters and classified using the AJCC TNM system (see Table 49-1). Preoperative staging is used for prognostic purposes and to estimate the risk of recurrence after surgery to guide adjuvant therapy.