Rectal Carcinoma

Rectal Carcinoma Last Updated: July 6, 2005

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Synonyms and related keywords: adenocarcinoma of the rectum, carcinoma of the rectum

AUTHOR INFORMATION

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Author: Isaac Hassan, MBChB, DMRD, FRCR, Clinical Director of Radiology, Department of Radiology, Royal Bolton Hospital, UK Isaac Hassan, MBChB, DMRD, FRCR, is a member of the following medical societies: American Roentgen Ray Society, British Institute of Radiology, British Medical Association, and Royal College of Radiologists Editor(s): Ludwig G Strauss, MD, Associate Director, Professor, Department of Innovative Cancer Diagnostics and Therapy, Clinical Cooper, German Cancer Research Center; Bernard D Coombs, MBChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Udo P Schmiedl, MD, PhD, Fellowship Director, Professor, Department of Radiology, Division of Abdominal Imaging, University of Washington Medical Center; Robert M Krasny, MD, Visiting Assistant Professor of Radiology, University of California at Los Angeles Medical Center; Consulting Staff, Healthcare Management Partners; and Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Disclosure INTRODUCTION

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Background: 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 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. Almost all rectal cancers are primary adenocarcinomas. 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 and lymphatic and vascular structures to involve regional lymph nodes, adjacent structures, and distant sites, especially the liver. Several factors increase the risk for rectal cancer, including the following: • • • • • • • •

High-fat, low-fiber diet Patient older than 50 years Personal history of colorectal adenoma or carcinoma (3-fold risk) First-degree relative with colorectal cancer (3-fold 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 o Ulcerative colitis (risk is 30% after 25 y) o Crohn disease (4- to 10-fold risk)

Frequency: •

In the US: Colorectal cancers are the second most common cause of cancer death in developed countries and the most common GI cancer. In 2000, there were an estimated 130,200 new cases of colorectal cancer of which 36,400 involved the rectum and 18,500 the rectosigmoid junction. 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 incidence in the United States). Incidence rates, for colorectal cancer as a whole, declined significantly during 1992-1996 (-2.1% per year). Research suggests that these declines may be because of increased screening and polyp removal, preventing progression of polyps to invasive cancers. The death rate also has declined slightly.

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Internationally: The incidence 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 low rates in Africa, Asia, and South America. Rectal cancer shows less international variation than colon cancer. While a 60-fold difference is found in colon cancer incidence between countries with the highest and lowest rates, only an 18-fold difference is found in incidence for rectal

cancer. High colon-to-rectal cancer ratios (3-4:1) prevail in the westernized countries of 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 a tumor, node, metastases (TNM) international classification system and a staging CT system have been developed recently, the Dukes classification (or one of its modifications) remains in wide use (see Table 1). Prognosis also is affected by the histologic grade of the tumor. The complications of rectal cancer include obstruction (common); fistula formation to small bowel, bladder, or vagina (uncommon); and perforation (rare). Table 1. Modified Dukes Classification System and 5-year Survival Rate* Stage

Description

5-yr Survival Rate, %

A

Limited to the bowel wall

83

B

Extension to pericolic fat; no nodes

70

C

Regional lymph node metastases

30

D

Distant metastases (liver, lung, bone)

10

*Modified from Zinkin (Dis Colon Rectum, 1983) Race: •

In the United States, rectal cancer incidence rates are higher in white males than in black males, but the rates for white and black females are similar. Colon cancer incidence rates also are similar among white and black males and females.

•

Risk rates rise for populations migrating 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 approximate 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: An increased incidence exists in males in westernized countries. The maleto-female ratio may vary 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 from branches of the internal iliac arteries. The rectal lymphatics drain superiorly into the superior rectal, then the inferior mesenteric nodes, and laterally into the internal iliac nodes. The rectal wall comprises 5 layers, including the (1) mucosa (lined with columnar epithelium), (2) muscularis mucosa, (3) submucosa, (4) muscularis propria (an inner circular layer and an outer longitudinal layer, comprising 3 narrow bands), and (5) serosa. Clinical Details: Rectal cancers tend to be symptomatic earlier than colonic tumors. Overt rectal bleeding is more common in rectal than colonic tumors. A change in bowel habit or symptoms of large bowel obstruction, such as pain and abdominal distension, 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 distension 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. • • •

Possibly asymptomatic 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 prior to treatment to choose the most appropriate treatment. Although 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: •

•

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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; polyps may be excised by this method. Double-contrast barium enema: Detects most colorectal tumors (80-95%) but 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 the primary rectal tumor. These techniques cannot distinguish enlarged lymph nodes resulting from tumor from those resulting from inflammation. Normal-sized nodes containing tumor cannot be detected by either technique.

DIFFERENTIALS

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Carcinoid, Gastrointestinal Colon, Polyps Crohn Disease Endometrioma/Endometriosis Ulcerative Colitis Other Problems to be Considered: Extrinsic compression by adjacent neoplasm or benign mass including endometrioma Lymphoma involving the rectum X-RAY

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Findings: Double-contrast barium enema • •

• • • •

Most rectal cancers are 3-4 cm in diameter at diagnosis. Polypoid lesions vary from small smooth tumors to larger lobulated masses with an irregular surface and associated contour deformity along one margin of the bowel wall (see Image 1). Annular lesions result from irregular circumferential masses that severely constrict the bowel lumen. Margins of the carcinoma show overhanging edges, which are the tumor shelf or shoulder (see Image 2). Mucosal folds in the narrowed segment are destroyed, and ulceration may be present. Flat lesions are rare and consist of a unilateral broad-based contour defect. Ulceration may be present. Flat lesions may infiltrate the bowel wall, and, if extensive, cause areas of nondistensibility.

Early carcinoma/polyps • •

Small carcinoma usually present as a polypoid mass with a smooth outline and may be indistinguishable from a benign polyp. Rarely, they may present as a small flat lesion.

Radiologic appearances •

A polypoid mass is visualized radiologically either as a filling defect in the barium column (single contrast study) or more commonly as a barium-

• • •

coated soft tissue mass protruding into the air-filled lumen (double contrast study). A sessile polyp may be visualized as a crescent (or ring) shadow on the bowel wall. Lobulation is common in polypoid lesions larger than 2 cm in diameter. Pedunculated polyps have stalks that may be identified easily on profile. When the stalk is observed through the polyp itself, this results in a target (or Mexican hat) appearance. Malignant change may occur in the head of a stalked polyp. A long (2 cm or more) thin (5 mm or less) stalk may hinder the spread of carcinoma from the head of the polyp into the wall.

Risk of malignancy •

The risk of malignancy in a polyp increases with its size. It is less than 1% in polyps less than 1 cm in diameter. This increases to 5% in 1-2 cm adenomas. Polyps larger than 2 cm have a risk of 11-50%. Thus, all 0.5-3 cm polypoid lesions require endoscopic removal and histologic examination.

Local complications of the primary tumor • • • •

A large bowel obstruction usually results from an annular carcinoma in the upper rectum or rectosigmoid junction. A localized perforation resulting from tumor necrosis may result in a pararectal abscess that simulates an inflammatory process. Perforation also may occur proximal to an obstructing tumor, usually in the cecum. Local invasion of adjacent organs (bladder, uterus, vagina) and fistula formation are late manifestations.

Synchronous lesions • • •

Approximately 5% of colorectal cancers demonstrate multiple lesions at diagnosis. An adenomatous polyp is present elsewhere in the colon or rectum in 35% of patients diagnosed with a primary colorectal carcinoma. Second tumors are more likely to be overlooked (“satisfaction of search error”).

Plain abdominal radiographs • •

These are useful in patients presenting with large bowel obstruction or perforation. Free gas under the diaphragm is detected best by a plain erect chest radiograph.

•

Rarely, mucin-producing colonic cancers demonstrate calcification in the primary tumor and in hepatic and peritoneal secondary deposits.

Degree of Confidence: Double-contrast barium enema detects approximately 90% of rectal tumors. The overall detection rate for single-contrast barium enema is approximately 80% but is much lower for small polypoid tumors. False Positives/Negatives: False-positive examinations may result, since residual stool may be adherent to the bowel wall and mimic a tumor. A submucosal mass, such as a lipoma or benign mucosal adenoma or hyperplastic polyp, may be indistinguishable from a small polypoid cancer. False-negative examinations may result from inadequate bowel preparation in which multiple filling defects resulting from residual stool may obscure carcinoma. In this case, repeat examination or sigmoidoscopy is required. Small lesions may be missed in a dense pool of barium. Errors of perception account for more than 50% of missed cancers. These can be reduced by asking a different observer to perform a second reading. Multiple cancers can produce false negatives, since second lesions are more likely to be overlooked (“satisfaction of search error”). Strictures resulting from inflammatory bowel disease, diverticulitis and radiation colitis may mimic malignant strictures. Extrinsic compression of the rectum by an adjacent mass may mimic a primary rectal tumor.

CAT SCAN

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Findings: Indications for performing CT in rectal carcinoma •

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CT is used for staging the rectal carcinoma prior to treatment, for staging of recurrent disease, and for detecting the presence of distant metastases after surgery. In older patients who may be unable to undergo colonoscopy or barium enema, modified CT is performed for primary detection of colorectal tumors. Rectal tumors may be diagnosed on CT as an incidental finding.

Tumor staging •

CT staging (see Table 2) or TNM staging (see Table 3) systems may be used to assess colonic neoplasms.

Table 2. CT Staging System For Rectal Cancer* Stage

Description

T1

Intraluminal polypoid mass; no thickening of bowel wall

T2

Thickened rectal wall >6 mm; no perirectal extension

T3a

Thickened rectal wall plus invasion of adjacent muscle or organs

T3b

Thickened rectal wall plus invasion of pelvic side wall or abdominal wall

T4

Distant metastases, usually liver or adrenal

*Modified from Thoeni (Radiology, 1981) Table 3. TNM/Modified Dukes Classification System*

TNM Stage

Modified Dukes Stage

T1 N0 M0

A

Limited to submucosa

T2 N0 M0

B1

Limited to muscularis propria

T3 N0 M0

B2

Transmural extension

T2 N1 M0

C1

T2, enlarged mesenteric nodes

T3 N1 M0

C2

T3, enlarged mesenteric nodes

T4

C2

Invasion of adjacent organs

Any T, M1

D

Distant metastases present

Description

*Modified from the American Joint Committee on Cancer (1997) Findings on CT •

•

•

• •

The rectal tumor often is observed as a focal mass of soft tissue density adjacent to the gas-filled or Gastrografin-filled bowel lumen. Oral watersoluble contrast (1% Gastrografin) is administered 12 hours and 2 hours prior to examination to opacify the entire bowel. Malignant strictures are detected by a thickening of the bowel wall (see Image 3). This thickening is concentric if the scanning plane is at right angles to the long axis of the rectum (see Image 4). Extrarectal tumor spread is suggested by a loss of tissue fat planes between the rectum and surrounding tissues as well as perirectal fat stranding and nodularity. Invaded muscle may be enlarged. Small strands of tissue may extend from the rectal wall into the perirectal fat.

Staging •

N Staging o Nodes greater than 10 mm in diameter are considered abnormal. CT is unable to distinguish enlarged nodes from benign causes from

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enlarged malignant nodes. Furthermore, malignant foci may be present in nodes less than 1 cm in diameter. o Overall, 60% of affected nodes are detected by CT. o Enlarged nodes may be detected in the mesentery and retroperitoneum. Rectal tumors may metastasize to internal iliac nodes. M Staging o Hepatic metastases are the most common site of distant spread. CT detects hepatic metastases as well-defined areas of low density (compared to normal liver parenchyma) in the portal venous phase following injection of intravenous contrast medium (see Image 5). In the earlier arterial phase, hepatic metastases may demonstrate rim enhancement or become hyperdense or isodense (in relation to normal liver). o Hepatic metastases may be suitable for surgical resection if they are small (usually <3 cm), number less than 3, and are suitably located, but others are only suitable for intra-arterial chemotherapy or radiofrequency (RF) ablation (see Intervention). o Pulmonary metastases are more frequent from low rectal carcinomas than upper rectal or colon carcinomas. This is because low rectal tumors drain into the systemic venous system (via the internal iliac veins) rather than into the portal venous system (via the superior and inferior mesenteric veins) like colon and upper rectal cancers. Thus, low rectal tumors may have pulmonary metastases and no evidence of hepatic metastases. Although pulmonary metastases may be detected by chest radiograph, CT has a higher sensitivity for small pulmonary metastases (<10 mm). o Other common sites include the adrenals, the peritoneum, and omentum. Adrenal metastases may occur in as many as 14% of patients with colonic carcinoma. They are manifested by enlargement (>2 cm), asymmetry, and heterogeneity.

Bony and cerebral metastases are uncommon. •

CT findings help determine surgical options. Precise information concerning the site and local extent of the tumor is required before the appropriate surgical choice can be made. Well-defined tumors (T1 or T2) may be amenable to simple resection or low anterior resection. More advanced tumors (T3) may require abdominoperineal resection or anterior resection, depending on their location. Perioperative adjuvant radiotherapy or chemotherapy may be used.

Complications of the primary tumor •

CT can demonstrate obstruction, perforation, and fistula formation. A local perforation of a carcinoma may be associated with an extraluminal fluid

collection. Early cancers and polyps • •

•

•

•

Tumors less than 2 cm in diameter cannot be detected reliably by standard CT techniques. CT colonography or virtual colonoscopy was introduced by Vining in 1996 as a screening tool for the detection of colorectal polyps and small cancers. It involves a 3-dimensional computer reconstruction from a volumetric data set using a workstation as well as distending a clean colon with air. Images are read as soft-copy from the workstation using a combination of pagingthrough the 2D axial images, aided by multiplanar and 3D endoluminal images. The recent arrival of multisectional helical scanners has reduced the time required to obtain the images (usually 30 seconds for each series, scanning the patient prone and supine using a reduced tube current to minimize the radiation dose). The length of time required for image analysis (currently ranging from 5-30 minutes) also has decreased with the introduction of sophisticated software programs that enable a "mathematically-straightened" colon to be viewed while co-referencing the 3D images with the cross sectional images. Advances in computer-aided diagnosis and novel methods of display are expected to improve the performance of this test and reduce the reading time. The sensitivity of this recently introduced technique is greater than that of double-contrast barium enema. For polyps larger than 10 mm, it has a sensitivity of 91% but a specificity of 76%. This sensitivity falls to 81% for 510 mm polyps.

CT findings in recurrent rectal cancer •

•

•

A baseline CT study is obtained 3 months following resection of a rectal tumor. Recurrent tumor is staged by similar criteria as described above for primary cancers. There is a local recurrence rate of 20-40% and a distant metastases rate of approximately 35% after curative resection. Most of these occur within 2 years of surgery. CT can be used to detect local recurrence as well as lymphadenopathy and distant metastases. CT criteria of a recurrent tumor include invasion of adjacent structures, increasing size, and associated lymphadenopathy (see Image 6). An inflammatory mass following surgery or radiation therapy may mimic a recurrent tumor and may require biopsy for differentiation (see Image 7). Postoperative soft tissue masses usually are the result of granulation tissue but may be from a hematoma or abscess. Of these, 60% decrease, but 40% may remain unchanged for up to 2 years.

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Both recurrent tumor and inflammatory masses can cause hydronephrosis by ureteric obstruction (see Image 8).

Degree of Confidence: CT is more accurate in assessing T4 cancers; however, the spatial resolution of CT is too low to distinguish T2 from T3 lesions. CT has 50% sensitivity for local invasion but does not distinguish between direct tumor infiltration and an inflammatory reaction induced by the tumor. CT detects up to 60% of mesenteric nodes but is unable to detect tumor in normalsized nodes (<1 cm in diameter); in most lymph nodes, metastases are less than 1 cm in diameter. Nodes may be enlarged for other reasons, such as infection. Rectal lesions smaller than 2 cm may not be detected. The accuracy and quality of CT can be increased using intravenous (IV) contrast medium, rectal contrast (air or Gastrografin), smooth muscle relaxants, and laxatives. The sensitivity of virtual colonoscopy or CT colonography is greater than that of double-contrast barium enema. For polyps larger than 10 mm, the technique has a sensitivity of 91% (81% for 5- to 10-mm polyps) but a specificity of 76%. Its future role in colorectal polyp screening is assured. False Positives/Negatives: •

• • •

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CT signs for rectal cancer are not specific and may be caused by any disease associated with focal thickening of the rectal wall, including Crohn disease. A polypoid mass may result from an adenoma, carcinoid tumor, or lymphoma rather than rectal carcinoma. In cachectic patients, absence of fat planes is a result of nutritional status and not tumor invasion. Enlarged lymph nodes may result from inflammation rather than tumor. Lymph nodes of normal size may contain tumor. Hypodense hepatic lesions may be simple cysts rather than hepatic metastases. Hepatic metastases do not enhance following injection of IVcontrast medium and appear as hypodense lesions (see Image 5, Image 8). Recurrent tumor (see Image 6) may be difficult to differentiate from postoperative fibrosis on imaging grounds alone (see Image 7) and may require biopsy. MRI

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Findings: •

Rectal tumors have low signal intensity (similar to adjacent skeletal muscle) on T1-weighted sequences, which facilitates their differentiation from highsignal perirectal fat (see Image 9).

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T2-weighted images are used to detect pelvic sidewall invasion. Tumor enhancement can be achieved by paramagnetic agents such as gadolinium.

Degree of Confidence: MRI provides greater contrast in soft tissues than CT. MRI is more accurate than CT at preoperative staging of rectal and rectosigmoid tumors and in the detection of direct tumor spread into the perirectal fat and adjacent pelvic organs. MRI and CT have similar overall accuracy in the detection of enlarged lymph nodes (N staging) and liver metastases. MRI has a higher sensitivity (91%) than CT (82%) in detecting local recurrence and a higher specificity (100%) than CT (69%). Nevertheless, most centers continue to use CT rather than MRI for staging and follow-up imaging of rectal neoplasms. This is because of the wider availability of CT and their much longer experience with CT. This is likely to change in the future. The new technique of MR colonography can detect colonic polyps and may compete with CT colonography in screening programs. ULTRASOUND

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Findings: The primary role of ultrasound (US) is in detecting liver metastases. US sensitivity is as high as 85%. Hepatic metastases resulting from rectal carcinoma usually are hyperechoic (see Image 10) but may be hypoechoic (see Image 11). Unlike CT and MR, transrectal ultrasound (TRUS) can depict individual rectal wall layers. The extent of spread through the rectal wall may be assessed by means of a rotating high-frequency probe placed in the rectum (see Image 12). The rectal wall is visualized as 5 concentric bands as follows: • • • • •

Mucosa (echogenic) Muscularis mucosa (hypoechoic) Submucosa (echogenic) Muscularis propria (hypoechoic) Serosa (echogenic)

The rectal tumor is demonstrated as a hypoechoic mass with varying mural invasion (see Image 13). Invasion of the bladder and prostate and adjacent lymph nodes may be demonstrated. Lymph nodes involved by tumor become spherical and hypodense rather than oval and hyperdense, as is seen in normal lymph nodes. Degree of Confidence: TRUS is limited to lesions located less than 14 cm from

the anus and may not be used for the upper rectum. It may overestimate tumor size and extent as a result of tumoral inflammatory response. Spread beyond the rectal wall to the pelvic cavity cannot be detected. TRUS only detects adjacent lymph nodes. The sensitivity of TRUS for detection and local staging of rectal tumors (within 14 cm of the anus) is 90-100% (CT is 50-80%), and its specificity is 75% (CT is 3380%). TRUS cannot assess the extent of any distant spread beyond its narrow range. NUCLEAR MEDICINE

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Findings: Nuclear medicine studies have an increasing role in colorectal cancer. Radioimmunoglobulin scintigraphy uses monoclonal antibody that recognizes carcinoembryonic antigen or tumor-associated glycoprotein 72 and may be used in the detection of disease recurrence in the pelvis or extrahepatic abdomen. This technique is being replaced by positron emission tomography (PET). PET may detect recurrent or metastatic disease using fluorine-18fluorodeoxyglucose (F-18-FDG). Degree of Confidence: A recent study (Meta, 2001) evaluated the impact of FDG PET on the management of patients with colorectal carcinoma. They noted a change in the clinical stage and major management decisions in approximately 40% of patients. Of the changes in clinical stage in 25 patients, the disease was upstaged in 20 patients (80%) and down-staged in 5 patients (20%). As a result of PET findings, physicians avoided major surgery in 41% of patients for whom surgery was the intended treatment. False Positives/Negatives: False-positive results may occur with FDG in patients with abscesses from nonspecific inflammatory reactions following radiotherapy or tracer uptake in bowel, bladder, or ureters. INTERVENTION

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Intervention: Metallic stents may be placed across obstructing carcinomas of the rectum as a temporary measure to reduce the need for emergency surgery. In

patients unable to undergo surgery or who have unresectable tumors, stents are used as a palliative procedure. Stent placement is a relatively simple procedure that rapidly improves the general condition of patients with large bowel obstruction. In some institutions, intra-arterial chemotherapy via the internal iliac arteries is performed in patients with unresectable tumors. Similarly, intra-arterial chemotherapy via the hepatic artery may be used in the management of liver metastases from colorectal cancer. Guided liver-directed therapy such as RF ablation and interstitial laser photocoagulation cause preferential tumor necrosis. RF electrodes or laser fibers are inserted into the hepatic metastasis under CT or US control followed by tumor ablation procedures. Promising results, (eg, a 40% 5-year survival), have been achieved from RF thermal ablation in selected patients with hepatic metastases from colorectal cancer. Medical/Legal Pitfalls: •

Failure to recognize the signs and symptoms of rectal cancer

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Failure to appropriately screen patients at various levels of risk

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Failure to detect a carcinoma or polyp (>10 mm) by double-contrast barium enema or sigmoidoscopy

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Failure to stage the carcinoma correctly using CT or MRI