You are in: eMedicine Specialties > Radiology > GASTROINTESTINAL Colon, AdenocarcinomaArticle Last Updated: Apr 4, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Isaac Hassan, MB, ChB, FRCR, DMRD, Former Senior Consultant Radiologist, Department of Radiology, St Bernard's Hospital, Gibraltar Isaac Hassan is a member of the following medical societies: American Roentgen Ray Society and Royal College of Radiologists Editors: Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Abraham H Dachman, MD, Professor, Department of Radiology, The University of Chicago School of Medicine; Director of CT, Department of Radiology, The University of Chicago Hospitals; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Author and Editor Disclosure Synonyms and related keywords: carcinoma of the colon, adenocarcinoma of the colon, colon cancer, colorectal cancer INTRODUCTIONSection 2 of 11
  BackgroundAlmost all colon cancers are primary adenocarcinomas, which are the third most common cancer in both men and women in North America and Western Europe. Colon cancers are the most common gastrointestinal (GI) carcinomas and have the best prognosis. The 5-year survival rates of approximately 50% may be improved by screening and removal of adenomatous polyps. PathophysiologyTumors of the colon arise as intramucosal epithelial lesions, usually in adenomatous polyps or glands. 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. FrequencyUnited StatesColorectal cancers are the second most common cause of cancer-related deaths in developed countries and the most common GI cancer. In 2005, in the United States, there were an estimated 145,290 new cases of colorectal cancer, of which 104,950 were colonic and 40,340 were rectal (only marginally less than lung cancer), with a reported 56,300 deaths (47,700 colonic; 8,600 rectal), which accounted for 11% of all cancer deaths. The highest rates of the disease are found in the northeastern and north central states, and the lowest rates are in the southern and western states (except the San Francisco Bay area and Hawaii). The incidence of colon cancer has risen since 1950, while the incidence of rectal cancer has remained stable. The increased incidence of colon cancer is believed to be a result of an increased intake of fat and beef and a decreased intake of fiber. InternationalThe incidence of colon 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 are found in Africa, Asia, and South America. A 60-fold difference exists between those areas with the highest incidence of colon cancer and those areas with the lowest incidence. More than 940,000 new cases of colorectal cancer and nearly 500,000 deaths associated with colorectal cancer are reported worldwide each year (World Health Organization, 2003). Mortality/MorbidityThe prognosis of patients with colon cancer relates to the stage of the disease at the time of diagnosis and to initial treatment. Although a tumor, node, metastasis(TNM)–based international classification and a computed tomography (CT) staging system have been developed recently, the Dukes classification (or one of its modifications) is widely used (Table 1). Prognosis is also affected by the histologic grade of the tumor. The complications of colon cancer include obstruction (common), perforation (uncommon), intussusception and ischemic colitis proximal to an obstructing tumor (rare), and fistula formation in the small bowel, bladder, or vagina (rare). Table 1. Dukes Classification and 5-Year Survival*
*Modified from Zinkin.1 Several factors increase the risk for colonic cancer.
RaceInternational incidences reflect dietary differences in fat and fiber intake rather than racial differences. When a developing country adopts a Western diet, colon cancer rates rise. Similarly, immigrants from a low-incidence country soon experience the approximate incidence rate of their adopted country. SexMales and females are equally affected. AgeOf patients with colon cancer, 90% are older than 50 years. The highest incidence rates are in individuals aged 70-85 years. Only 10% of patients are younger than 50 years. AnatomyThe colon is 150 cm long and is subdivided into the cecum and the ascending, transverse, descending, and sigmoid colons. The ileocecal valve forms the junction between the small and large bowel and demarcates the cecum from the ascending colon. The transverse and sigmoid colons have a mesentery and are entirely intraperitoneal. The ascending and descending colons are partially extraperitoneal. The superior mesenteric artery supplies the colon between the ileocecal valve and the splenic flexure. The inferior mesenteric artery supplies the colon distal to the splenic flexure. The colon wall comprises 4 layers, including the mucosa, submucosa, muscularis propria (inner circular layer and outer longitudinal layer, comprising 3 narrow bands), and serosa. Clinical DetailsColon cancers progress slowly and may be asymptomatic for as many as 5 years; however, patients usually have occult blood loss from their tumors. Symptoms depend on the location of the primary tumor. Cancers of the cecum and ascending colon usually grow larger than left-sided tumors before symptoms occur. Fatigue, shortness of breath, and angina resulting from microcytic hypochromic anemia are common presenting features. Vague abdominal discomfort or a palpable mass may occur later, but obstruction is uncommon (unless the ileocecal junction is involved) because of the larger diameters of the cecum and ascending colon. Cancers of the descending and sigmoid colons may present with large bowel obstruction. 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). The primary tumor may be palpable in the abdomen. Overt rectal bleeding is more common in tumors of the sigmoid colon, whereas occult bleeding is typical with proximal tumors. A change in bowel habits may be the only presenting feature. Weight loss, jaundice, and ascites are associated with advanced metastatic disease. Signs and Symptoms:
Preferred Examination
Limitations of TechniquesSigmoidoscopy The 60-cm flexible sigmoidoscope has greater range than the rigid sigmoidoscope, which, at best, only reaches the distal sigmoid (20 cm). Double-contrast barium enema A double-contrast barium enema study detects most colon tumors (80-95%); however, flexible sigmoidoscopy should precede the barium enema as it is more accurate in detecting small rectal lesions. The double contrast barium enema has a low perforation rate (1 in 25,000). Colonoscopy Colonoscopy detects more adenomatous polyps than a barium enema, and polyps can be excised during the procedure. Colonoscopy is approximately 3 times more expensive, has a much higher perforation rate (1 in 1700) than barium enema, and fails to reach the cecum in 5-30% of patients. DIFFERENTIALSSection 3 of 11
Colitis, Ischemic Colon, Diverticulitis Colon, Polyps Crohn Disease Tuberculosis, Gastrointestinal Ulcerative Colitis Other Problems to Be ConsideredColon lymphoma
RADIOGRAPHSection 4 of 11
FindingsAdvanced carcinoma Most colon cancers are relatively advanced, measuring 3-4 cm in diameter at diagnosis. The appearance of the tumors on double-contrast barium enema reflects the 3 morphologic types: polypoid, annular, or flat.
Early carcinoma Small carcinomas usually present as a polypoid mass with a smooth outline; they may be indistinguishable from a benign polyp. Rarely, they may present as a small flat lesion (Image 3). Radiologic appearances Radiologically, a polypoid mass is visualized 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 (Image 4 ). Lobulation is common in polypoid lesions larger than 2 cm in diameter. Pedunculated polyps have stalks that may be identified easily on profile (Image 5). When the stalk is seen 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. Risk is less than 1% in polyps with less than a 1 cm diameter. This risk of malignancy increases to 5% in adenomas of 1-2 cm in diameter. Patients with polyps larger than 2 cm have a risk of 11-50%. Thus, all polypoid lesions from 0.5-3 cm require endoscopic removal and histologic examination. Local complications Findings that result from complications of the primary tumor include the following:
Synchronous lesions Approximately 5% of patients with colon cancer have more than 1 cancer at diagnosis (Image 9). Approximately 35% of patients with colon cancer have an adenomatous polyp (Image 10). Second tumors are more likely to be overlooked. Plain abdominal radiography Plain abdominal radiographs are useful in patients presenting with large bowel obstruction or perforation. Free gas below the diaphragm is detected best by plain erect chest radiograph. Rarely, mucin-producing colon cancers show calcification in the primary tumor and in hepatic and peritoneal secondary deposits. Degree of ConfidenceDouble-contrast barium enema detects approximately 90% of colonic tumors. The overall detection rate for single-contrast barium enema is approximately 80% but is much lower for small polypoid tumors. Colonoscopy and biopsy are recommended in patients whose findings are equivocal. False Positives/NegativesFalse-positive findings
False-negative findings
CT SCANSection 5 of 11
FindingsIndications for CT scanCT scan is used for staging colon cancer before surgery, for assessing and staging recurrent disease, and for detecting the presence of distant metastases. Preoperative CT scan is indicated if there is clinical suggestion of distant metastases or local invasion of the adjacent organs or abdominal wall. In older patients who may be unable to undergo colonoscopy or barium enema, modified CT scan may be performed for primary detection of colorectal tumors. Colonic tumors may be diagnosed on CT scan as an incidental finding. Tumor staging: CT scan findings of primary colon cancer CT scan staging (Table 2)or TNM staging (Table 3) systems may be used to assess colonic neoplasms. Table 2. CT Scan Staging System for Colonic Cancer*
* Modified from Thoeni.2 Table 3. TNM/Modified Dukes Classification System*
*American Joint Committee on Cancer.3 Findings A localized tumor may be seen on CT scan as an intraluminal or intramural mass of soft tissue density adjacent to the gas-filled or contrast-filled bowel lumen; this is the appearance of a stage A tumor (Tables 2 and 3). There is no mural thickening or pericolic fat invasion in stage A tumors. To opacify the entire bowel, oral water-soluble contrast (1% Gastrografin) is administered at 12 hours and at 2 hours before examination. More advanced tumors are associated with thickening of the bowel wall (>6 mm) and infiltration of the pericolic fat. Thin strands of tissue may extend from the tumor into the pericolic fat (Image 11). Annular carcinomas are detected by a thickening of the bowel wall and narrowing of the lumen. This thickening is concentric if the scanning plane is at right angles to the long axis of the bowel (Image 12). Extracolonic tumor spread is indicated by a loss of tissue fat planes between the colon and surrounding structures (Image 13). Invaded muscle may be enlarged (Image 14). The comparative barium enema findings are shown in Image 15. Colonic tumors may invade the anterior abdominal wall, liver, pancreas, spleen, or stomach. Complications of the primary tumor Obstruction, perforation, and fistula formation can be demonstrated by CT scan. An intussuscepting colonic tumor may have a typical targetlike appearance with alternating rings of soft tissue and fat on CT scan, if mesenteric fat is present between the intussusceptum and the intussuscipiens. A local perforation of a carcinoma may be associated with an extraluminal fluid collection. N staging Nodes greater than 10 mm in diameter are considered abnormal. CT scan is unable to distinguish between enlarged benign nodes and enlarged malignant nodes. Furthermore, malignant foci may be present in nodes less than 1 cm in diameter. Overall, 60% of affected nodes are detected by CT scan. Enlarged nodes may be detected in the mesentery and retroperitoneum (Image 16). Occasionally, enlarged nodes are observed around the porta hepatis (Image 17). Rectosigmoid tumors may metastasize to external iliac nodes. M Staging Hepatic metastases are the most common site of distant spread. Following injection of intravenous contrast medium (Image 18), CT scan detects hepatic metastases as well-defined areas of low density (compared with normal liver parenchyma) in the portal venous phase. In the earlier arterial phase, hepatic metastases may show rim enhancement or become hyperdense or isodense (in relation to normal liver). Hepatic metastases may be suitable for surgical resection if they are small (usually <3 cm), number fewer than 3, and are suitably located (Image 19), but others are suitable only for intra-arterial chemotherapy or radiofrequency (RF) ablation (see Intervention). Other common sites include the lungs, adrenal glands, peritoneum, and omentum. Although pulmonary metastases may be detected by chest radiograph (Image 20), CT scan has a higher sensitivity for small pulmonary metastases (<10 mm). Adrenal metastases may occur in as many as 14% of patients with colon cancer. They manifest with enlargement (>2 cm), asymmetry, and heterogeneity. Bony and cerebral metastases are uncommon (Image 21). Tumors less than 2 cm in diameter cannot be detected reliably by the standard CT scan technique. In 1996, Vining introduced CT scan colonography (virtual colonoscopy) as a screening tool for the detection of colorectal polyps and small cancers.4 This technique involves a 3-dimensional computer reconstruction from a volumetric data set that uses a workstation as well as distention of a clean colon with air. Images are read as soft copy from the workstation using a combination of paging through the 2-D axial images, aided by multiplanar and 3-D endoluminal images. The recent arrival of multisectional helical scanners has reduced the time required to obtain the images (usually 30 seconds for each series; scans involve the patient in the prone and supine positions, using a reduced tube current to minimize the radiation dose). The length of time required for image analysis (currently ranging from 5-30 min) also has decreased with the introduction of sophisticated software programs that enable a mathematically straightened colon to be viewed. 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 virtual colonoscopy using multisectional helical scanners is greater than that of the double-contrast barium enema. For polyps larger than 10 mm, it has a sensitivity of 91% but a specificity of 76%. Sensitivity falls to 81% for 5- to 10-mm polyps. The examination has the advantage of displaying incidental extracolonic findings as well. Data from several ongoing retrospective and prospective multicenter trials are expected in the near future. Colorectal cancer screening CT colonography (virtual colonoscopy) has become an acceptable noninvasive option for colorectal cancer screening that can reliably depict clinically important colorectal lesions. However, substantial controversy remains regarding its exact role. Recent studies have shown that the sensitivity of CT colonography may not be as high when performed and interpreted by radiologists who do not have the required expertise and training. Significant lesions may be missed, and mucosal folds and residual fecal matter may be misinterpreted as polyps, leading to unnecessary colonoscopy. CT scan findings in recurrent colorectal cancer A baseline CT scan study is obtained 3 months following resection of a colonic tumor and reanastomosis. 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 metastasis rate of approximately 35% after curative resection. Most of these distant metastases occur within 2 years after surgery. Although colonoscopy and barium enema reveal better mucosal detail of a local anastomotic recurrence, CT scan is able to detect recurrence away from the anastomosis as well as lymphadenopathy and distant metastases. A recurrent tumor mass is typically large and often extrinsic to the bowel wall (see Images 14 and 22). CT scan criteria of a recurrent tumor include invasion of adjacent structures, enlargement, and associated lymphadenopathy. An inflammatory mass following surgery or radiation therapy may mimic a recurrent tumor and may require biopsy for differentiation. Postoperative soft tissue masses are usually from granulation tissue but may be the result of a hematoma or abscess. Of these, 60% decrease but 40% may remain unchanged for up to 2 years. Both recurrent tumor and inflammatory masses can cause hydronephrosis by ureteric obstruction (see Images 23 and 24). Degree of ConfidenceDegree of Confidence: Colonic lesions smaller than 2 cm usually are not detected. The accuracy and quality of CT scan studies can be increased using air contrast (rectal air insufflation), smooth muscle relaxants, and laxatives. CT scan:
For polyps larger than 10 mm, CT scan colonography (virtual colonoscopy) has a sensitivity of 91% but a specificity of 76%. Sensitivity falls to 81% for 5- to 10-mm polyps. False Positives/NegativesColon cancer may be indistinguishable from a large benign tumor as well as from metastasis to the colon (usually from an ovarian primary). CT scan signs for colon cancer are not specific and may be caused by any disease associated with focal thickening of the colonic wall. These diseases include diverticulitis, Crohn disease, ischemic colitis, and tuberculous colitis. In cachectic patients, the absence of fat planes is a result of nutritional status and not tumor invasion. A paracolic collection may be seen in diverticulitis, as well as in local perforation of a carcinoma. Chronic radiation changes in the pelvis may mimic recurrent colonic tumors and require biopsy for differentiation. Tumors in the transverse colon and colonic flexures may be visualized incompletely. A primary gastric carcinoma with extension into the colon may be indistinguishable from a colonic tumor involving the stomach. Enlarged lymph nodes may result from inflammation rather than tumor. Lymph nodes of normal size may contain tumor. Hypodense hepatic lesions may be caused by simple cysts rather than metastases (Image 18). Hemangiomas also may cause confusion. MRISection 6 of 11
FindingsMRI provides greater contrast between soft tissues than CT scan. Colonic tumors have low signal intensity (similar to adjacent skeletal muscle) on T1-weighted sequences, which facilitates their differentiation from high-signal perirectal fat. T2-weighted images are used to detect pelvic sidewall invasion. Tumor enhancement can be achieved by paramagnetic agents such as gadolinium. Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin;yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape. Degree of ConfidenceMRI has lower sensitivity and higher specificity than CT scanning in T staging. The techniques have a similar overall accuracy in T staging, as well as a similar overall accuracy (approximately 60%) in the detection of enlarged lymph nodes (N staging) and liver metastasis (M staging). In detecting local recurrence, MRI has a higher sensitivity (91%) than CT scan (82%) and a higher specificity (100%) than CT scan (69%). Nevertheless, most centers tend to use CT scanning rather than MRI for staging and follow-up imaging of colonic neoplasms because of their greater experience with and the wider availability of CT scans. In addition, spiral CT scan (and the newer multislice CT scan) can assess the whole abdomen and pelvis in a much shorter time than MRI. False Positives/NegativesLimitations of MRI are similar to those of CT scanning. Colon cancer may be indistinguishable from a large benign tumor and from metastasis to the colon (usually from an ovarian primary). MRI signs for colon cancer are not specific and may be caused by any disease associated with focal thickening of the colonic wall. These diseases include diverticulitis, Crohn disease, ischemic colitis, and tuberculous colitis. A paracolic collection may be seen in diverticulitis, as well as in local perforation of a carcinoma. Chronic radiation changes in the pelvis may mimic recurrent colonic tumors and require biopsy for differentiation. Enlarged lymph nodes may result from inflammation rather than tumor. Lymph nodes of normal size may contain tumor. ULTRASOUNDSection 7 of 11
FindingsThe primary role of ultrasound (US) in patients with colon cancer is the detection of hepatic metastases. US has a detection rate of 70-90% for hepatic metastases, which reflects the operator dependence of this modality, the range of equipment available, and the size of the individual metastasis. Hepatic metastases from a colonic primary tumor are usually hyperechoic (increased echogenicity in relation to normal liver; Image 25) but also may be hypoechoic (decreased echogenicity). On US, a colonic tumor typically appears as an echo-poor mass with a hyperechoic center, which is known as the target sign (Image 26). Other findings include localized irregular colonic wall thickening, an irregular contour, lack of normal peristalsis, and an absence of the normal layered appearance of the colonic wall. US may detect a colonic tumor as a chance finding or may be used specifically in instances when a palpable abdominal mass is observed that is consistent with a colonic tumor (Image 26). Intussuscepting colonic tumors have a characteristic targetlike appearance from concentric rings of soft tissue and mesenteric-fat density (Image 27). Degree of ConfidenceUS usually cannot detect colonic tumors smaller than 2 cm. US is difficult to use in rectosigmoid lesions. False Positives/NegativesUS has a high false-negative rate for the detection of colonic tumors, with a sensitivity of 31-80%, depending on tumor size, and cannot be used as a screening tool. Conversely, US has a low false-positive rate, with a specificity greater than 90%. US may reduce the need for more intrusive procedures in older patients and in those with advanced disease. Its primary role in colon cancer management is to detect hepatic metastases, where it has a detection rate of 70-90%. NUCLEAR MEDICINESection 8 of 11
FindingsNuclear medicine has a small peripheral role in colon cancer. Consider using radioimmunoscintigraphy with monoclonal antibody that recognizes CEA or tumor-associated glycoprotein-72 to detect disease recurrence in the pelvis or extrahepatic abdomen. Consider using positron emission tomography (PET) with fluorodeoxyglucose (FDG) to detect recurrent disease. Degree of ConfidenceA recent study by Meta et al evaluated the impact of FDG-PET on the management of patients with colorectal carcinoma.5 They noted a change in the clinical stage and major management decisions in approximately 40% of patients. Of the changes in clinical stages in 25 patients, the disease was upstaged in 20 patients (80%) and downstaged 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/NegativesFalse-positive results may occur with FDG from nonspecific inflammatory reactions following radiotherapy or in patients with abscesses. INTERVENTIONSection 9 of 11
Stent placement is a relatively simple procedure that rapidly improves the general condition of patients with colonic obstruction. Consider placing metallic stents across obstructing carcinomas of the left colon as a temporary measure to reduce the need for emergency surgery. In patients unfit for surgery or with unresectable tumors, stents are used as a palliative procedure. Intra-arterial chemotherapy may be 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 tumors. Lesional heating techniques such as radiofrequency (RF) ablation and interstitial laser photocoagulation cause preferential tumor necrosis. RF electrodes or laser fibers are inserted into the hepatic metastasis under CT scan or US control. Promising results (40% 5-year survival) have been achieved from RF thermal ablation in selected patients with hepatic metastases from colorectal cancer Medical/Legal Pitfalls
MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
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