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Rectal Carcinoma Imaging

Practice Essentials

Imaging studies are a major component in the evaluation of patients for the screening, staging, treatment and surveillance of rectal cancer. 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%.  Almost all rectal cancers are primary adenocarcinomas (see the images below). Adenocarcinoma of the rectum is a major cause of mortality and morbidity in North America and Western Europe. Prognosis is related to the stage of the disease at diagnosis and to initial treatment. Screening for and removing adenomatous polyps may improve survival rates.

Polypoid carcinoma of the upper rectum.

Polypoid carcinoma of the upper rectum.

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CT scan for low rectal carcinoma preoperative stag

CT scan for low rectal carcinoma preoperative staging. Note circumferential thickening of the rectal wall.

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The National Comprehensive Cancer Network (NCCN) guidelines
utilizes the The American Joint Committee on Cancer (AJCC) tumor/node/metastasis (TNM) classification and staging system (see Colon Cancer Staging for more information).
However, the Dukes classification (or one of its modifications, such as the  modified Astler–Coller [MAC]) remains in wide use (see Table 1 for a comparision of anatomic stage/prognostic groups).

Table 1. Anatomic stage/prognostic groups (Open Table in a new window)

Stage T N M Dukes MAC
0 Tis N0 M0
I T1 N0 M0 A A
  T2 N0 M0 A B1
IIA T3 N0 M0 B B2
IIB T4a N0 M0 B B2
IIC T4b N0 M0 B B3
IIIA T1-T2 N1/N1c M0 C C1
  T1 N2a M0 C C1
IIIB T3-T4a N1/N1c M0 C C2
  T2-T3 N2a M0 C C1/C2
  T1-T2 N2b M0 C C1
IIIC T4a N2a M0 C C2
  T3-T4a N2b M0 C C2
  T4b N1-N2 M0 C C3
IVA Any T Any N M1a D D
IVB Any T Any N M1b D D
IVC Any T Any N M1c D D

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). See the table below for the Modified Dukes Classification for 5-year survival rates.

Table 2. Modified Dukes Classification System and 5-year Survival Rate* (Open Table in a new window)



5-yr Survival Rate, %


Limited to the bowel wall



Extension to pericolic fat; no nodes



Regional lymph node metastases



Distant metastases (liver, lung, bone)


*Modified from Zinkin.

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.

Endoscopic ultrasonography (EUS) is an acceptable alternative when MRI is contraindicated.

Positron emission tomography (PET)/CT colonography is valuable in the evaluation of extra-colonic and hepatic disease. PET/CT colonography is useful in patients with obstructing colorectal cancers that cannot be traversed colonoscopically. PET/CT colonography is able to localize synchronous colon cancers proximal to the obstruction precisely. However, there is no definite evidence to support the routine clinical use of PET/CT colonography.

Limitations of techniques

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).

T staging of rectal cancer by MRI is an established modality because MRI can diagnose rectal wall laminar structure. However, on MRI it is difficult to differentiate fibrosis from tumor infiltration, which compromises the ability to distinguish early stage T3 tumors from stage T2 tumors.
 CT and MRI cannot be used to assess the exact degree of mural invasion of a primary rectal tumor. N staging in patients with rectal cancer is still challenging using any imaging modality. These techniques cannot differentiate between enlarged lymph nodes resulting from tumor and those resulting from inflammation. Normal-sized nodes that contain tumors cannot be detected by CT, MRI, sigmoidoscopy, or barium enema.

EUS cannot fully image high or bulky rectal tumors or regions beyond the immediate area of the primary tumor.

For patient information resources, see the Esophagus, Stomach, and Intestine Center and Cancer and Tumors Center, as well as Colon Cancer, Colonoscopy, Sigmoidoscopy, and Rectal Cancer.

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