Colon Cancer Imaging

Updated: Apr 16, 2021
Author: Caroline R Taylor, MD; Chief Editor: Eugene C Lin, MD 

Practice Essentials

Colon cancer is the most common, and the most preventable, form of gastrointestinal cancer and the third most common cause of cancer-related death in the United States.[1]  Colorectal cancer is the third most common tumor in both men and women and constitutes 10% of all tumor types globally.[2, 3, 4]  While survival rates have improved markedly, and long-term survival can now be achieved in patients with advanced-stage cancers at diagnosis, the prevalence of the colorectal cancers still represents a considerable burden to the physician management. Five-year survival in patients with stage I (localized) colorectal cancer is approximately 90%, 70% for stage II (regional), but only 13% for patients with distal spread.[5]

The American Cancer Society estimated that 104,610 new cases of colon and rectal cancer were diagnosed in the United States in 2020. The 2020 estimate for mortality from colon and rectal cancer is 53,200.[1] According to the National Cancer Institute's Surveillance, Epidemiology, and End Results Program (SEER), rates of colorectal cancer have been declining approximately, on average, 2.4% each year; and death rates decreased 2.2% each year during 2007-2016.[6]

Most cancers are asymptomatic until the disease is advanced. Blood may be noted in the stool, but occult bleeding is more common; patients may often present with iron deficiency anemia. Increasing obstruction of the large bowel may cause symptoms such as constipation, change in bowel habits, and, in advanced cases, feculent vomiting. The tumor may hemorrhage acutely, perforate, or cause pain by invasion of adjacent organs.

(See the image below.)

CT colonography shows a large polypoid adenocarcin CT colonography shows a large polypoid adenocarcinoma in the cecum adjacent to the ileocecal valve.

Most colonic adenocarcinomas arise in preexisting adenomatous polyps, and progression from polypoid adenomas to carcinoma occurs over several years.

The examples below show a patient referred for screening CT colonoscopy who returned 5 years later with no interim follow-up.

CT colonography with 2- and 3-dimensional reformat CT colonography with 2- and 3-dimensional reformatted images shows a 14-mm polypoid lesion close to the ileocecal valve (yellow arrow).
CT scan sagittal reformatted image shows extensive CT scan sagittal reformatted image shows extensive posterior cecal wall thickening; pathologic diagnosis of adenocarcinoma of the colon.

Screening guidelines

The American Cancer Society 2018 guideline for colorectal cancer screening recommends that average-risk adults aged 45 years and older undergo regular screening with either a high-sensitivity stool-based test or a structural (visual) exam, based on personal preferences and test availability. As a part of the screening process, all positive results on non-colonoscopy screening tests should be followed up with timely colonoscopy.[2]

The European Society of Medical Oncology has published the following recommendations for colonoscopy[2] :

  • A complete colonoscopy is the recommended method for CRC screening in average-risk men and women based on higher sensitivity and specificity when compared with other tests. The optimal age range for testing is 50-74 years, with an optimal repetition interval for a negative test of 10 years.
  • Flexible sigmoidoscopy carried out every 5-10 years may be an alternative for those who refuse colonoscopy. The combination of this method with a yearly fecal occult blood test is recommended to reduce the risk of a right colon tumor, which may be difficult to visualize with colonoscopy alone.
  • Other invasive tests, including capsule colonoscopy, are not recommended for screening.

The US Multi-Society Task Force on Colorectal Cancer issued screening recommendations that divide screening tests into 3 tiers, based on their effectiveness.[7]

Tier 1 tests consist of the following:

  • Colonoscopy every 10 years
  • Annual FIT (fecal immunochemical test)

Tier 2 tests consist of the following:

  • CT colonography every 5 years
  • FIT–fecal DNA every 3 years
  • Flexible sigmoidoscopy every 5–10 years

Tier 3 testing is capsule colonoscopy every 5 years. Septin 9 testing, a novel DNA-based blood screen, has shown some efficacy in detecting advanced colorectal cancer, but due to poor sensitivity in early detection,[8]  it is not recommended.

Suggested timing of initial screening and intervals for subsequent testing for different risk populations are as follows[7] :

  • For patients at average risk, testing with a tier 1 test should begin at age 45 years for African Americans and at age 50 years for patients of all other races.
  • For patients with a family history of colorectal cancer or advanced adenoma that was diagnosed before age 60 years in 1 first-degree relative or at any age in 2 first-degree relatives, testing should begin with colonoscopy at an age 10 years younger than the youngest age at diagnosis of a first-degree relative, or age 40 years, whichever is sooner, to be repeated every 5 years.

  • In patients with 1 first-degree relative with colorectal cancer, advanced adenoma, or an advanced serrated lesion diagnosed at age 60 years or older, screening should begin with a tier 1 test at age 40 years and continue at the same intervals as in average-risk patients.

  • Colonoscopy screening should be discontinued in patients aged 75 years or older with prior negative screening tests or whose life expectancy is less than 10 years, or in those 85 years or older without prior screening.

Impact of the COVID-19 pandemic on colorectal cancer screening

According to Norman Sharpless, Director of the National Cancer Institute (2021), modeling the effect of COVID-19 on cancer screening and treatment for breast and colorectal cancer (which together account for about one sixth of all cancer deaths) over the next decade suggests almost 10,000 excess deaths in the United States from breast and colorectal cancer (ie, an approximately 1% increase in deaths from these 2 tumor types during a period when we would expect to see almost 1 million deaths).[9]

At the National Colorectal Cancer Roundtable, the consensus statement acknowledged: “Colonoscopies to detect colorectal cancer have been delayed or cancelled and patient fears about contracting COVID-19 have led to further reductions in screening. This drop has raised concern that COVID-19 related screening delays will lead to missed and advanced stage colorectal cancer diagnoses and to excess deaths from colorectal cancer.”[10]

The committee recommended “During a time when availability of elective screening colonoscopy may be limited by the COVID-19 pandemic, colorectal cancer screening can be safely offered through at-home stool-based tests (FIT which can be mail-based, and mt-sDNA requiring direct shipping and handling. A negative test (the most likely outcome) must be tracked and repeated at the appropriate time interval. A positive (abnormal) test must be referred promptly for colonoscopy because a delay of six months or longer after an abnormal FIT result is associated with higher rates of advanced adenomas and late-stage colorectal cancer.  Patients with an abnormal stool-based test should be carefully tracked and given priority as colonoscopy capacity remains below pre-COVID levels. Acting on an abnormal stool test requires close cooperation and communication between primary care and colonoscopy providers”[10]

Clinical staging of colorectal cancer

Stage is the strongest predictor of survival for patients with colorectal cancer.[11] The TNM (tumor, nodes, metastasis) staging system proposed by the American Joint Committee on Cancer/Union Internationale Contre le Cancer (AJCC/UICC) is the recommended staging system for colorectal cancer.[12]

The TNM system is derived from surgical resection specimens. The T refers to local extent of the untreated primary tumor. The N refers to tumor involvement of regional lymph nodes and the lymphatic system. The M refers to metastatic disease.

One of the primary advantages of the TNM system is that the system is data driven and susceptible to continuous improvement.[13] Another advantage of a single, united system is a common language regarding tumor burden that is understood by clinicians throughout the United States and internationally.

Tables 1-4 below are adapted from the AJCC Colon and Rectum Cancer Staging.

Table 1. Primary Tumor (T) (Open Table in a new window)

Primary Tumor (T)


Primary tumor cannot be assessed


No evidence of primary tumor


Carcinoma in situ: intraepithelial or intramucosal carcinoma (involvement of lamina propria with no extension through the muscularis mucosa)


Tumor invades submucosa (through the muscularis mucosa but not into the muscularis propria)


Tumor invades muscularis propria


Tumor invades through the muscularis propria into the pericolorectal tissues


Tumor invades through the visceral peritoneum (including gross perforation of the bowel through tumor and continuous invasion of tumor through areas of inflammation to the surface of the visceral peritoneum)


Tumor directly invades or is adherent to other organs or structures


Table 2. Regional Lymph Nodes (N) (Open Table in a new window)

Regional Lymph Nodes (N) a


Regional lymph nodes cannot be assessed


No regional lymph node metastasis


Metastasis in 1-3 regional lymph nodes (tumor in lymph nodes measuring ≥0.2 mm) or any number of tumor deposits are present and all identifiable nodes are negative


Metastasis in one regional lymph node


Metastasis in 2-3 regional lymph nodes


Tumor deposit(s) in the subserosa, mesentery, or nonperitonealized, pericolic, or perirectal/mesorectal tissues without regional nodal metastasis


Metastasis in 4 or more regional lymph nodes


Metastasis in 4-6 regional lymph nodes


Metastasis in 7 or more regional lymph nodes

a A satellite peritumoral nodule in the pericolorectal adipose tissue of a primary carcinoma without histologic evidence of residual lymph node in the nodule may represent discontinuous spread, venous invasion with extravascular spread (V1/2), or a totally replaced lymph node (N1/2). Replaced nodes should be counted separately as positive nodes in the N category, whereas discontinuous spread or venous invasion should be classified and counted in the Site-Specific Factor category Tumor Deposits (TD).

Definition of distant metastasis (M)

The terms pM0 and Mx are not valid categories in the TNM system. Assignment of the M category for clinical classification may be cM0, cM1, or pM1. Any of the categories (cM0, cM1, or pM1) may be used with pathologic stage grouping.

Table 3. Distant metastasis (M) (Open Table in a new window)

Category M Criteria


No distant metastasis by imaging or other studies, no evidence of tumor in distant sites or organs. (This category is not assigned by pathologists.)


Metastasis to one or more distant sites or organs or peritoneal metastasis is identified


Metastasis confined to 1 organ or site is identified without peritoneal metastasis


Metastasis to 2 or more sites or organs is identified without peritoneal metastasis


Metastasis to the peritoneal surface alone or with other site or organ metastases

pM1 Metastasis to 1 or more distant sites or organs or peritoneal metastasis is identified and microscopically confirmed
pM1a Metastasis to 1 site or organ is identified without peritoneal metastasis and microscopically confirmed
pM1b Metastasis to 2 or more sites or organs is identified without peritoneal metastasis and microscopically confirmed.
pM1c Metastasis to the peritoneal surface is identified alone or with other site or organ metastasis and microscopically confirmed


Table 4. Anatomic Stages and Prognostic Groups (Open Table in a new window)

Anatomic Stage/Prognostic Groups a






















































































Any T

Any N





Any T

Any N




a cTNM is the clinical classification and pTNM is the pathologic classification. The y prefix is used for those cancers classified after neoadjuvant pretreatment (eg, ypTNM). Patients who have a complete pathologic response are ypT0N0cM0 that may be similar to stage group 0 or I. The r prefix is to be used for cancers that have recurred after a disease-free interval (rTNM).

b Dukes B is a composite of better (T3 N0 M0) and worse (T4 N0 M0) prognostic groups, as is Dukes C (any TN1 M0 and any T N2 M0). MAC is the modified Astler-Coller classification.

Many patients with colon cancer who either have liver metastases at diagnosis or develop metastatic liver tumors following diagnosis may become eligible for hepatic resection following chemotherapy with immune modulators (conversion therapy).[14] For example, in one trial, 79% of patients who were initially unsuitable for resection responded to a regimen of cetuximab with FOLFOXIRI, and liver resection became possible in 60% of patients.[15] From the imaging point of view, this is stimulating efforts to increase the accuracy of staging, but current morphologic assessments are not necessarily well correlated with intermediate or final outcomes in patients with metastatic liver cancer.[16]


Optical colonoscopy remains the criterion standard against which the accuracy of other screening tests is measured.[17] The sensitivity for colonoscopy to detect cancerous and precancerous lesions has been estimated to be greater than 95%.[5]  A major advantage of colonoscopy is that polyps can be excised during the procedure. However, it has a relatively higher cost and requires conscious sedation. Not all patients may be able to tolerate sedation, or it may be too risky to discontinue anticoagulation, such as in persons with heart valves. It may not be as cost effective to use colonoscopy in a population with low prevalence of disease.

In a study by Doubeni et al of 1747 patients who died from colorectal cancer and 3460 colorectal cancer-free controls, screening colonoscopy was associated with a 67% reduction in the risk of death from any colorectal cancer, a 65% reduction in risk of death for right-colon cancers, and a 75% reduction for left-colon/rectal cancers.[18]

Alternatives such as screening CT colonoscopy may provide an advantage in being less invasive and perhaps more suitable to screen patients with a low prior probability of disease. Radiography, CT (including virtual CT colonoscopy), MRI, transrectal ultrasound, and positron emission tomography (PET) have roles in the diagnosis and management of colon carcinoma.[19]

CT Colonography

The efficacy of CT scanning to diagnose a primary tumor may be limited by tumor size or location. Despite no existing recommendations for the use of CT scanning as a screening modality, other than CT colonography, there may be cases in which a primary tumor may be evident on a scan obtained for another purpose and, if colon or rectal wall thickening, mesenteric infiltration, adenopathy, or liver lesions are not noted, could represent a reason for litigation. CT colonography in the United States is generally limited to patients who are not good candidates for colonoscopy either because of comorbidities or because of structural problems that prevent satisfactory optical examination.[20, 21, 22, 5, 23, 24]  The development of multislice CT and other technological advances have allowed CT colonography to become an alternative to colonoscopy or double-contrast barium enema for screening and detecting colorectal polyps.[21, 25]

CT colonography, although now included in the US Preventative Services Task Force guidelines as a direct visualization technique with colonoscopy and flexible sigmoidoscopy,[26]  is currently not approved for CMS reimbursement as a screening test, but has been approved for use in symptomatic individuals. If cost constraints were not a barrier, it has been shown that using a threshold for polyps at 6mm or above, the technique can detect twice the number of advanced and invasive colorectal carcinomas as compared to mt-sDNA testing from 2.8% to 5%.[27]

In the interpretation of CT colonography, diagnosis of lesions of less than 5 mm (and limitation in detection of flat lesions) is not accurate. Staging inaccuracies of imaging techniques are a limitation that is understood, and other validation by endoscopy, pathologic node and tumor analysis, tumor markers, and surgical techniques are used in the definitive clinical staging of colorectal carcinoma.

In a study by Ibanez et al of 217 patients (225 tumors) who had colon cancer and underwent preoperative CT colonography and elective colectomy, CT colonography identified all tumors and provided the precise location in 208 cases (92.4%). Findings changed the surgical plan in 31 patients (14.3%). In differentiating T3/T4 from T1/T2 tumors, accuracy was 87.1%, sensitivity 88.5%, and specificity 84.1%. For high-risk tumors (T3≥5 mm and T4), accuracy  was 82.7%, sensitivity 86%, and specificity 80%. For N-stage evaluation, accuracy was 69.3%, sensitivity 74%, and specificity 67.1%.[28]

Stool testing

Stool testing can indirectly identify lesions, such as the guaiac-based fecal occult blood test (gFOBT), the fecal immunochemical test (FIT), and fecal DNA.[29, 30, 31]

ESMO has made the following recommendations for noninvasive testing[2] :

  • Noncolonoscopic tests are recommended in average-risk men and women from the age of 50 years who are not already taking part in colonoscopic screening programs. The optimal frequency of testing is every year and no later than every 3 years. A colonoscopy must be carried out at the earliest convenience when the test results in any positive findings.
  • Among the available tests, FIT appears to be superior to high-resolution guaiac FOBT with respect to detection rate and positive predictive value for adenomas and cancer. Other novel methods, such as DNA-based or tests using other markers (eg, M2-PK), are undergoing formal comparisons of their performance, and integration with other assays needs to be monitored. 
  • DNA testing has better sensitivity than FIT testing: for advanced precancerous lesions was 42.4% with DNA testing and 23.8% with FIT (P< 0.001). The rate of detection of polyps with high-grade dysplasia was 69.2% with DNA testing and 46.2% with FIT (P=0.004); the rates of detection of serrated sessile polyps measuring 1 cm or more were 42.4% and 5.1%, respectively (P< 0.001). Specificities with DNA testing and FIT were 86.6% and 94.9%, respectively. [32]


Plain abdominal radiographs continue to be a useful first step in the evaluation of patients presenting with complications of colon cancer, such as large bowel obstruction or perforation. Free air below the diaphragm may be detected by plain erect chest radiograph. Rarely, mucin-producing colon cancers show calcification in the primary tumor and in hepatic and peritoneal secondary deposits visible on radiographs.

Double-contrast barium enema is one of the modalities approved for colon cancer screening, but the frequency of its use has markedly decreased over the past decade. As Ferrucci et al note,[33] “DCBE [double-contrast barium enema] is a low-yield procedure for detecting polyps, with a high false-positive rate, and is not likely to be performed by experienced practitioners in the future.”

Although barium enema is no longer among common screening tools, the following examples offer an overview of its utility in diagnosis of polyps and carcinomas, which can be synchronous, and gross tumor morphology, as shown by double-contrast barium enema.

Approximately 5% of patients with colon cancer have more than 1 cancer at diagnosis (see the image below).

Synchronous annular carcinomas in the ascending co Synchronous annular carcinomas in the ascending colon and splenic flexure.

Approximately 35% of patients with colon cancer have an associated adenomatous polyp (see the image below).

Annular carcinoma of the transverse colon is assoc Annular carcinoma of the transverse colon is associated with a 2-cm polyp in the sigmoid colon.

The appearance of colonic tumors on double-contrast barium enema reflects diverse morphologic types: polypoid, annular, or flat.

Polypoid lesions vary from small, smooth tumors to larger lobulated masses with an irregular surface and an associated contour deformity along 1 margin of the bowel wall (see the image below). The incidence of carcinoma in an adenomatous polyp is related to its size and surface features; larger, more irregular ulcerated lesions are more likely to contain carcinoma.

Polypoid carcinoma. A large, irregular lobulated m Polypoid carcinoma. A large, irregular lobulated mass is present in the rectosigmoid junction.

Annular lesions result from irregular, circumferential masses that severely constrict the bowel lumen. The margins of the carcinoma show overhanging edges, the tumor shelf or shoulder (termed "apple-core" lesion). The mucosal folds in the narrowed segment are destroyed; ulceration may be present (see the image below).

Annular carcinoma of the sigmoid colon. The lumen Annular carcinoma of the sigmoid colon. The lumen of the sigmoid is narrowed severely by the circumferential mass with mucosal destruction and the overhanging edges or shouldering at the tumor margins.

Flat lesions, which are rare, are visualized as a unilateral, broad-based, contour defect. Ulceration may be present (see the image below). Flat lesions may infiltrate the bowel wall and, if extensive, cause areas of nondistensibility.

Flat carcinoma in the transverse colon. A broad-ba Flat carcinoma in the transverse colon. A broad-based contour defect with central ulceration.

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.

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 (see the image below).

Double-contrast barium enema. An 18-mm sessile pol Double-contrast barium enema. An 18-mm sessile polyp in the sigmoid colon showing crescent sign.

Pedunculated polyps have stalks that may be identified easily on profile (see the image below). When the stalk is seen through the polyp itself, this results in a target 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.

Double-contrast barium enema. Stalked 15-mm polyp Double-contrast barium enema. Stalked 15-mm polyp in sigmoid colon.



Computed Tomography

CT scanning (including multidetector computed tomography [MDCT] and CT colonography) is used as an adjunct in screening for colon carcinoma, in staging colon cancer before surgery, for assessing and staging recurrent disease, and for detecting the presence of distant metastases. Colon tumors may be diagnosed on a CT scan as an incidental finding (see the images below) or in patients presenting with acute symptoms related to complications of a colonic tumor, such as perforation.[23, 24, 34, 35, 36, 37, 38, 39]

Incidental finding in an 87-year-old man undergoin Incidental finding in an 87-year-old man undergoing CT angiography for aneurysm stent planning, arterial-phase imaging. A cecal mass is present with distinct vessels.
A 62-year-old man with hematuria undergoing excret A 62-year-old man with hematuria undergoing excretory urography; an incidental finding of a colocolic intussusception was noted, which was due to an adenocarcinoma of the colon as a lead point.
Coronal-plane delayed-phase post-contrast image ob Coronal-plane delayed-phase post-contrast image obtained during CT urography in the same patient with hematuria, showing a large-bowel colocolic intussusception.

Adenocarcinoma of the colon presenting as an acute abdominal condition

Elderly patients are more likely to present with symptoms associated with complications from late-stage colonic tumors. Two examples are demonstrated in the images below of patients previously undiagnosed as having colon carcinoma, presenting with signs and symptoms of acute abdominal perforation.

A 61-year-old man with a history of umbilical hern A 61-year-old man with a history of umbilical hernia presented with acute periumbilical pain due to incarcerated, perforated adenocarcinoma of the transverse colon.
Acute presentation of an 87-year-old man with righ Acute presentation of an 87-year-old man with right lower abdominal pain and tenderness for 3 days, due to perforated sigmoid carcinoma.

Spontaneous perforation may be associated with slightly higher postoperative mortality, but long-term survival in such patients is a function of the stage at presentation, which tends to be more advanced.[40] Other complications that may present acutely include obstruction and fistula formation, as shown in the image below.

An 82-year-old patient presented with vomiting and An 82-year-old patient presented with vomiting and abdominal pain, which was secondary to an obstructing lesion in the splenic flexure. Transverse and ascending colon are markedly dilated.

CT in screening for colorectal carcinoma

CT colonography

CT colonography, better known as virtual colonoscopy, has been introduced as an alternative method for screening for colorectal carcinoma and is gaining in acceptance as an alternative or complementary diagnostic test to conventional colonoscopy. Its efficacy was established in the multicenter trial, the American College of Radiology Imaging Network (ACRIN) study.[41, 42]


CT colonography is a minimally invasive examination of the colon and rectum designed to evaluate for polyps and neoplasms. It is similar to diagnostic colonoscopy in that it requires 24 hours of bowel preparation with a combination of cathartics, contact laxatives, dietary restriction, and hydration.[43] However, new developments in fecal tagging may be able to eliminate the bowel preparation process.[44] Once the patient has prepared for the examination, a rectal tube is inserted and room air or carbon dioxide is insufflated into the colon under controlled pressure to provide full colon distention.[45]

CT images are typically obtained through the colon in at least 2 positions (usually supine and prone). Additional imaging is obtained on an as-needed basis after repositioning and reimaging to ensure that all colonic segments are imaged adequately. Once images are obtained, they are sent to an integrated computer system for image review and further manipulation. Recent advances in technology and software have allowed the conversion of 2-dimensional (2D) axial, coronal, and sagittal slices into 3-dimensional (3D) endoluminal rendering or “fly-through” techniques.[46]  (See the images below.)

Axial, coronal, and sagittal CT images highlightin Axial, coronal, and sagittal CT images highlighting a lesion following insufflation of carbon dioxide to distend the colon. The images are sent to 3-dimensional rendering software, which is able to reformat the images into a 3-dimensional projection.
Two- and 3-dimensional reformatted images. The 2-d Two- and 3-dimensional reformatted images. The 2-dimensional image was obtained after administration of oral contrast, and a stalked polyp was demonstrated. "Electronic cleansing" has been applied post process to the 3-dimensional image, and the lesion is shown in profile.

Indications/guidelines for ordering

Indications for CT colonography are similar to those for colonoscopy. Individuals undergoing the examination fall into 1 of 3 categories designated as screening, surveillance, or diagnostic.[47]

Screening seeks to detect polyps or colon cancer without signs or symptoms of the disease. For disease screening, the US Multi-Society Task Force on Colorectal Cancer, American Cancer Society, and the American College of Radiology recommend a screening CT colonography every 5 years.[48]

Diagnostic CT colonography encompasses symptomatic individuals or individuals in whom a screening examination detected an abnormality and further imaging is required. A common indication is for “completion colonography” following an incomplete optical colonoscopy.

In contrast to screening and diagnostic CT colonography, surveillance CT colonography includes monitoring people with known colorectal cancer. Repeat examinations are tailored to a patient’s individual case and treatment plan.

CT colonography can also be offered to patients who are at increased risk for complications from conventional colonoscopy, such as increased anesthesia risk. Contraindications for CT colonography include the following[49] :

  • Symptomatic acute colitis

  • Acute diarrhea

  • Recent acute diverticulitis

  • Recent colorectal surgery

  • Symptomatic colon containing abdominal wall hernia

  • Recent deep endoscopic biopsy

  • Suspected colonic perforation

  • Symptomatic/high-grade small bowel obstruction

In addition, there are few indications for which CT colonography has not been approved, including routine follow-up of inflammatory bowel disease, hereditary polyposis/ nonpolyposis syndromes, evaluation of anal canal disease, or pregnancy.[48]

Lesion categorization

Lesions are categorized using the CT Colonography Reporting and Data (C-RAD) system[50] as C0 if the study was inadequate and as C1 if the study was normal. Polyps of 6-9 mm and fewer than 3 in number are classified as C2 (indeterminate), and continued surveillance or colonoscopy is recommended. C3 lesions include those larger than 10 mm in diameter or if more than 3 lesions of 6-9 mm are present, for which colonoscopy is recommended. C4 is used to describe a colonic mass with associated luminal narrowing or extracolonic extension, for which urgent referral for consideration of surgery is recommended. The system also recommends categorization of significant extracolonic findings.


The accuracy of CT colonography compared with traditional colonoscopy has been greatly debated over the past few years. The first large CT colonography–based screening trial in the United States in 2005 estimated a sensitivity greater than 90% for the detection of polyps larger than 1 cm.[50] Subsequent trials in the mid 2000s reported lower sensitivities, of 55% and 59%,[51, 52] but a large-scale study sponsored by ACRIN in 2008 reported a specificity greater than 90% and a sensitivity of 90% for polyps larger than 9 mm.[53] Moreover, sensitivity for lesions measuring 5-9 mm was reported with a sensitivity of 85-89%.


CT colonography has several disadvantages over traditional colonoscopy. The first is radiation exposure possibly leading to long-term carcinogenesis, especially if used for repeated screenings. Although in recent years and with newer image acquisition systems, radiation exposure has been significantly decreased.[54] Another disadvantage is that if a significant lesion is found on CT colonography, the patient would likely have to be brought back for a second study with a second bowel preparation (ie, colonoscopy) for confirmation, surveillance, or removal of the lesion if graded C2 or higher.[50] Yet another disadvantage is interinterpreter, intersystem variability. The reported results may not be replicable without extensive training in the technique.

While CT colonography carries a small risk of procedural complication, including bowel perforation[55] , risk of perforation is lower than for optical colonoscopy. Recent retrospective analyses of almost 29,000 mostly symptomatic patients screened in the United Kingdom and Israel reported a perforation rate of 0.06% and 0.08%, respectively,[34]  on average a fraction of the risk of optical colonoscopy (reportedly between 0.3-0.7%).[56, 57]

The sensitivity of CT colonography for the detection of flat lesions has not been well established independently; such lesions may constitute 40% of all adenomas.[58]  (See the images below.) However, the addition of a contrast agent has shown utility in highlighting mucin-secreting serrated sessile tumors, a common feature in many lesions.[59]

Color 3-dimensional rendering of CT colonography s Color 3-dimensional rendering of CT colonography study, showing 17-mm adenomatous polyp in the sigmoid colon.
A plaque-shaped 3.8 x 5.9-cm lesion in the cecum ( A plaque-shaped 3.8 x 5.9-cm lesion in the cecum (biopsy-proven adenocarcinoma) just proximal to the ileocecal valve at 233 cm from the anal verge. Maximum thickness was up to 1 cm.

CT in staging adenocarcinoma of the colon.

CT scanning plays an integral role in the staging of colon cancer. CT scanning can help identify the tumor site, the size of the tumor, and locoregional and metastatic spread, which can help guide surgical planning.[13] The degree of wall invasion, tumor extension into the mesentery, and the presence of lymph node and distant metastases are major factors that influence patient prognosis.[35, 49]  .

The current role of CT in patients with known colon cancer is controversial; overall accuracy rates for preoperative staging of colon cancer based on early evaluations of CT, prior to the introduction of multidetector technology, ranged from 48% to 77%.[60]  While recent studies have shown the feasibility of CT staging, some features such as determination of depth of tumor invasion through the colonic wall has not been reliably predicted by readers.[61]

The hope is that the introduction of MDCT scanning and improved processing software can improve accuracy. One study found that using MDCT, the accuracy rates for identifying T1 and T2 tumors were as high as 90.4% and 73.9%, respectively,[36] although the accuracy of correctly identifying tumor involvement of lymph nodes was lower for N0 and N1 patients, at 61.6%. This limitation is related to size criteria for evaluation of lymph nodes, with an inability to identify malignant infiltration of nonenlarged lymph nodes.

An earlier small study using axial MDCT in conjunction with multiplanar reformatted images found similar results with tumor staging accuracy, at 83%, and slightly improved nodal staging, at 80%.[62]

CT colonography also offers some promise as a staging modality. CT colonography is increasingly being performed after incomplete colonoscopy to assess for synchronous lesions and metastasis.[63] One study evaluated CT colonograms from 246 patients to assess tumor staging. Overall accuracy for tumor stage was 79%.[64]

As technology advances, it is likely that MDCT scanning and CT colonography will play increasingly important roles, along with MRI, in the initial staging of colorectal cancer.

The primary tumor may be inapparent on CT staging, particularly if the lesion is relatively small, or flat, and the bowel is filled with feces. Lesions appear as asymmetric, wall-thickening focal intraluminal masses, or they may be annular. Occasionally, a colon carcinoma presents as a stricture with large bowel obstruction. Annular carcinomas are detected by a thickening of the bowel wall and narrowing of the lumen. Use of multiplanar images to view the colon in the orthogonal plane facilitates assessment of tumor thickness and lesion length. Adenomatous polyps and adenocarcinomas are enhanced by iodinated contrast. As tumors progress, vascularity is recruited into the tumor and adjacent mesentery. (See the images below.)

Preoperative CT: Cecal carcinoma with circumferent Preoperative CT: Cecal carcinoma with circumferential involvement of the cecal wall.
Endoscopic view of cecal lesion showing narrowed l Endoscopic view of cecal lesion showing narrowed lumen circumferential lesion with ulceration.

Extracolonic tumor spread is indicated by a loss of tissue fat planes between the colon and surrounding structures. Advanced colonic tumors may directly invade the anterior abdominal wall, retroperitoneum, liver, pancreas, spleen, or stomach. (See the images below.)

Locally advanced cecal carcinoma with concentric w Locally advanced cecal carcinoma with concentric wall thickening of cecum and invasion of pericecal mesentery.
The same patient (locally advanced cecal carcinoma The same patient (locally advanced cecal carcinoma) has enlarged nodes within mesentery, likely indicating regional metastasis.

Nodal staging

The accuracy of correctly identifying tumor involvement of lymph nodes is lower, with accuracy rates for N0 and N1 patients at 61.6%,[36] although a small study using axial MDCT in conjunction with multiplanar reformatted images found similar results, with tumor staging accuracy of 83% and slightly improved nodal staging, with an accuracy rate of 80%.[62]

Nodes greater than 10 mm in diameter are considered abnormal. No CT tissue characteristics have been identified that enable discrimination between enlarged benign nodes and enlarged malignant nodes. Furthermore, malignant foci may be present in nodes less than 1 cm in diameter.

Enlarged nodes may be detected in the mesentery and retroperitoneum (see the first image below). Enlarged nodes may also be observed around the porta hepatis (see the other images below).

Retroperitoneal lymphadenopathy from sigmoid carci Retroperitoneal lymphadenopathy from sigmoid carcinoma.
Primary lesion in sigmoid colon. Primary lesion in sigmoid colon.
Enlarged portal nodes in same case (primary lesion Enlarged portal nodes in same case (primary lesion in sigmoid colon).

Rectosigmoid tumors may metastasize to external iliac nodes.

Despite limitations in accuracy of staging, there is an association with survival[65] ; in a study of more than 500 patients who underwent curative resection for colonic cancer, radiological (CT) nodal staging (not using subclassifications) was associated with a survival rate at 5 years of 83% (N0), 76% (N1; 1-3 regional lymph nodes), and 54% (N2; 4 or more regional lymph nodes).

CT colonography could play a useful role in pre-operative planning: A prospective study was performed on 217 patients who underwent preoperative CT colonography prior to their elective colectomy. The accuracy in differentiating T3/T4 vs T1/T2 tumours was 87.1%, with a sensitivity and specificity of 88.5% and 84.1%, respectively (kappa = 0.71). For high-risk tumours (T3 ≥ 5 mm and T4), CTC showed an accuracy, sensitivity and specificity of 82.7%, 86% and 80%, respectively (kappa = 0.65). The accuracy of N-stage evaluation was 69.3%, the sensitivity 74% and the specificity 67.1% (kappa = 0.37). The authors concluded that surgical planning was improved, as that was changed in 31 (14.3%) patients.[66]

Metastasis staging

Hepatic metastases are the most common site of distant spread and develop in 50% of cases. Following injection of intravenous contrast medium (see the images below), hepatic metastases are demonstrated as well-defined areas of low density (compared with normal liver parenchyma) best seen (ie with maximum difference in tissue attenuation between unaffected liver and lesion) 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.

Contrast-enhanced CT showing liver metastases. Sev Contrast-enhanced CT showing liver metastases. Several low-density metastases from the colon primary tumor involve both lobes of the liver.
Liver metastases containing cloudlike calcificatio Liver metastases containing cloudlike calcifications.

Even extensive hepatic metastases may be suitable for surgical resection, provided the primary tumor has been resected for cure (R0) and that complete resection is achievable based on anatomic criteria and maintenance of hepatic function. Increasingly sophisticated techniques may include staged resection, a combination with preoperative portal vein embolization, or radiofrequency ablation prior to, or in conjunction with, resection.[67]

Many clinical trials are in process to evaluate the timing and optimal regimens of preoperative and adjuvant chemotherapy, and a review of surgical consensus conferences by Masi et al[68] summarizes that the only rules when performing surgical resection of colorectal liver metastases, in addition to a complete (R0) resection, are the preservation of 2 contiguous liver segments (with adequate vascular inflow and outflow) and adequate future liver remnant (>20% of the total volume in a healthy liver). Limited portal adenopathy does not exclude surgical resection.

Kanas et al[65] performed a review and meta-analysis of studies and determined that median 5-year survival rate after resection of colorectal liver metastasis was highly variable (ranging from 16-74%) and depended on a number of factors. Median survival was better in patients with a carcinoembryonic antigen (CEA) level of less than 200, with a negative tumor margin, and fewer than 3 liver metastases. Patients with higher tumor grade had the worst median survival, and those with negative nodes did better. Survival chances were also improved by higher annual clinic volume. More recent studies did not show improved survival compared with the earliest studies. The overall median survival following colorectal liver metastasis liver resection was 3.6 years.[65]

Given the continuing controversy regarding the benefit of hepatic resection, CT evaluation for intrahepatic metastatic disease should focus on accurate reporting of intrahepatic location and size of metastases, bearing in mind the need for 2 contiguous segments with adequate vascular supply and drainage for resection. Radiologists may be requested to make assessments of liver volume in patients being considered for hepatic resections. (See the image below.)

CT scan in a patient following a partial hepatecto CT scan in a patient following a partial hepatectomy for metastasis in the right lobe, with radiofrequency ablation for further recurrence, now disease free for 2 years. Note hypertrophy of the left lobe and caudate lobe.

The concept of unresectability is changing, and patients with disease previously thought unresectable may still be candidates for surgery with curative intent, but there is as yet no consensus on the selection of this subset.[68]

There is potential for application of radiomics in evaluating tumor characteristics that are related to patient outcomes: a recent study used textural analysis of contrast-enhanced CT images to predict survival in colorectal cancer in a group of patients receiving targeted therapy with bevacizumab and cetuximab).[69]  

Common sites of metastatic involvement include the lungs, adrenal glands, peritoneum, and omentum. In females, the ovary may be involved.

Although pulmonary metastases may be detected by chest radiography, CT scanning has a higher sensitivity for small pulmonary metastases (< 10 mm). (See the image below.)

CT scan in patient with rectal carcinoma and liver CT scan in patient with rectal carcinoma and liver metastases, showing pulmonary metastasis in right lower lobe.

Adrenal metastases may occur in as many as 14% of patients with colon cancer. They manifest with enlargement (>2 cm), asymmetry, and heterogeneity. Adrenal metastases are fluorodeoxyglucose (FDG) avid and may be further characterized by MRI evaluation. (See the images below.)

A 62-year-old man with rectal carcinoma; staging C A 62-year-old man with rectal carcinoma; staging CT demonstrates heterogeneous left adrenal mass on portovenous-phase images.
Same patient (62-year-old man with rectal carcinom Same patient (62-year-old man with rectal carcinoma): rectal carcinoma. Note rectal wall thickening with enhancement and left adrenal mass on coronal-phase images.
Fluorodeoxyglucose (FDG)-avid adrenal metastasis i Fluorodeoxyglucose (FDG)-avid adrenal metastasis in same patient, FDG-positron emission tomography examination, increasing suspicion for metastatic disease.

Bony and cerebral metastases are uncommon.

Metastatic disease should be carefully reported, particularly the locations and size of lesions, which will be the continued focus of attention as the patient undergoes further treatment. At this point, use of Response Evaluation Criteria in Solid Tumors (RECIST) Group criteria[70] is not generally applied in clinical practice, but it's a requirement in clinical trials for standardization.

Postoperative complications

An example of a postoperative complication is shown below.

Postoperative complication: Anastomotic breakdown Postoperative complication: Anastomotic breakdown after right hemicolectomy in patient with Serratia sepsis. CT showing extravasation into peritoneal collection, superior mesenteric vein (SMV) thrombus.

CT in surveillance after colon cancer therapy

Surveillance guidelines recommend annual CT surveillance of the chest, abdomen, and pelvis,[67] in addition to frequent clinical evaluation, CEA testing, and follow-up colonoscopy. The American Society of Clinical Oncology (ASCO) guidelines recommend CT surveillance based on meta-analyses, showing CT scanning or liver imaging is associated with a survival benefit. The mortality rate for patients who undergo liver imaging was found to be 25% lower than that for patients who do not undergo such imaging. Rectal cancer patients with poor prognostic factors may also benefit from pelvic CT scanning, especially if they have not been treated with radiation.

Chest CT scanning is beneficial in the ASCO panel’s conclusions in that the highest proportion of resectable lung metastases, particularly in patients with rectal cancer, is found by using chest CT and is often not associated with elevation of CEA markers.[71]  (See the images below.)

Recurrence of tumor in the peritoneal cavity, with Recurrence of tumor in the peritoneal cavity, with invasion of spleen and kidney. Liver metastases also present.
Recurrence at the ileocolic anastomosis in a 58-ye Recurrence at the ileocolic anastomosis in a 58-year-old man with invasion of the duodenum and antrum who presented with perforation, developed liver metastasis, and was placed on chemotherapy.
Within a month, the same patient (58-year-old man) Within a month, the same patient (58-year-old man) developed acute cholelithiasis as the tumor continued to invade the duodenum and caused compression and obstruction of the common bile duct.

Contrast-enhanced CT colonography has been evaluated for efficacy of colorectal cancer in surveillance after curative resection,[72] and the authors concluded that the technique matched colonography CT screening trials in accuracy, could detect extracolonic metastatic disease, and may eliminate the need for more invasive surveillance colonoscopy during routine postoperative surveillance. However, sensitivity for adenomas was more limited at 80% versus 100% for carcinoma, and the authors conceded that alternating techniques may be a more prudent approach.

Degree of confidence

Early evaluations of the accuracy of CT scanning were published in the late 1980s through early 2000. The introduction of MDCT scanning, which enables rapid acquisition of images in multiple phases and multiplanar reformatting of images based on thin section slices as low as 0.5 mm, has provided radiologists with images of high quality and improved spatial resolution. The accuracy of tumor staging has improved, but prediction of tumor involvement within nonenlarged lymph nodes is still limited. Based on a meta-analysis of 19 series, Dighe et al[37] concluded that MDCT has the potential to make a considerable impact in improving the accuracy of staging of colon cancer, with a specificity of 93% and a sensitivity of 86% for detecting tumor invasion and a specificity of 87% and a sensitivity of 70% for detecting lymph node metastasis.

Huh et al,[73] who reviewed more than 500 cases (excluding stage IV) scanned from 1997-2007, noted radiologic staging was a significant independent predictor of long-term survival. Five-year survival progressively fell in patients staged by CT as T1 (96%), T2 (89%), T3 (75%), and T4 (79%), as well as in those TNM-staged by CT, from 90% (stage I) to 81% at stage II, and 70% in stage III disease. CT scanning cannot replace pathologic staging, but it provides useful insight into disease manifestations and locoregional and distant spread, which can help in operative planning and management.

Meta-analysis of articles published between 2009-2019 in colorectal cancer patients with known or suspected liver metastases identified 12 prospective studies suitable for study[74]  and the pooled sensitivity of MDCT as lower than MRI on a per lesion basis (84% vs 89%), 90% vs 97% for lesions >10mm, and 69% vs 89% for lesions < 10mm. CT was superior to PET/CT (sensitivity 62%) and similar to contrast-enhanced ultrasound (86%). 

In other settings, for example elevation of CEA following therapy for colon cancer, CT was found less accurate in detecting sites of recurrence than positron emission tomography (PET) scanning; onestudy showed a sensitivity of MDCT of 70.3%, as compared to PET scanning, with a sensitivity of 97%. Limitations of CT were in detection of recurrence in the presacral space, in metastatic subcentimeter lymph nodes, in peritoneal deposits, at the periphery of ablated liver metastases, and in the uterine cervix.[75]

CT retains a significant role in the staging of patients with colo-rectal cancer in view of its accessibility, ability to scan both thorax and abdomen, enabling evaluation of the local, regional and metastatic spread with acceptable performance characteristics. Where presence or absence of liver metastases is key to treatment planning, MRI is a more sensitive technique.

False negatives/positives

Common causes for missed lesions (false negatives) include inadequate bowel preparation and/or distention, flat lesions, and small polyps at multidetector row CT colonography. In one study, 3 of 3 flat lesions larger than 1 cm were not distinguished. Other reasons for error included misclassification of polyps as feces.[76]

CT scanning signs for colon cancer are not specific and may be caused by any disease associated with focal thickening of the colon wall, including 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.

In cachectic patients, the absence of fat planes is a result of nutritional status and may limit evaluation for tumor extension.

Chronic radiation changes in the pelvis may mimic recurrent colon tumors and require PET scanning correlation and/or biopsy for differentiation.

Tumors in the transverse colon and colon flexures may be visualized incompletely. A primary gastric carcinoma with extension into the colon may be indistinguishable from a colon tumor invading the stomach.

Hypodense hepatic lesions may be caused by simple cysts rather than metastases. Hemangiomas also may cause confusion.


Magnetic Resonance Imaging

MRI has roles in staging of rectal tumors and in assessment of liver metastases in colorectal carcinoma. Before the introduction of high field–strength magnets and endorectal or phase array coils, CT was considered superior to MRI for local staging of rectal cancer. Although no advantage has been yet found in the use of 3-Tesla field-strength imaging in comparison to 1.5-Tesla in tumor staging of rectal cancer,[77] MRI has now been shown to be able to reliably exclude T3 tumors with a negative predictive value of 97%.[78]

One notable pitfall is the difficulty with differentiating a desmoplastic reaction in the perirectal fat (T2) and true tumor infiltration (T3), although in practice it may not change therapy.

Preoperative MRI is also used to identify tumor distance from the mesorectal fascia, which is a predictor of tumor positivity at the circumferential resection margin (CRM) after a total mesorectal excision.[79] A positive resection margin is defined as tumor extension to within 1 mm of the mesorectal fascia. (See the image below.)

T2-weighted axial image of a 71-year-old patient w T2-weighted axial image of a 71-year-old patient with rectal carcinoma shows a low lesion extending anteriorly from the rectum with invasion of the anterior muscular propria, abutting the posterior aspect of the urethra without evidence of invasion, but with invasion into the mesorectal fascia anteriorly, extending approximately 15 mm from the rectal wall.

Advances in MR colonography have been spurred on by improvements in equipment and techniques. The method for MR colonography is similar to CT colonography in that the patient is imaged prone and supine. Fecal tagging techniques have also been developed to make the preexamination preparation more tolerable. Contrast between the colonic lumen and potential polyps is created through either a bright lumen (gadolinium enema) or dark lumen (water/air/carbon dioxide) technique.[80] MR colonography can accurately identify polyps 10 mm or greater while not exposing the patient to ionizing radiation.[81] While the lifetime risk of radiation-induced cancer in the average patient aged 50 years and older is low, it is significant when one considers using CT colonography as a widespread screening technique. Additionally, MR colonography may play a role in screening for younger patients with genetically increased risks for colon cancer.[82]

MRI provides greater contrast between soft tissues than CT scan. Although both T1- and T2-weighted sequences are recommended, T2-weighted sequences appear the most useful, combined with high-resolution matrices, thin-section imaging, and small field of view. Fat suppression (used to improve detection of perirectal fat invasion) and diffusion-weighted imaging may also be helpful.[83, 84] Gadolinium administration for evaluation of primary rectal tumor extension has not been found to be helpful.[85]

Tumor enhancement can be achieved by paramagnetic agents such as gadolinium. Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD) in patients with renal impairment.[86] Patients with an estimated glomerular filtration rate (GFR) of less than 30 mL/min/1.73 m2 should not receive gadolinium-based contrast agents unless the benefits significantly outweigh the risks. 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 MR angiography scans. 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.

The recently developed technique of MRI colonography (during which no intravenous contrast is used) can detect colon polyps greater than 1 cm in diameter and is likely to compete with CT colonography in screening programs.

Nodal staging

MRI has limited accuracy in determining nodal involvement, when solely relying on anatomical criteria. The addition of MR diffusion-weighted imaging has increased the sensitivity and specificity up to 97% and 81%, respectively.[83]

Metastasis staging

MRI may be used in symptom-based evaluations of other organs systems, if, for example, there is concern for brain metastasis or to characterize an adrenal mass in the setting of a patient with newly diagnosed colon carcinoma. (See the image below.)

MRI, T1-weighted axial image, post gadolinium. A 6 MRI, T1-weighted axial image, post gadolinium. A 62-year-old with newly diagnosed rectal carcinoma. A heterogeneously enhancing 5.1 x 4-cm right occipital mass, with mass effect and midline shift. A left parietal lesion of less than 1 cm was also noted (not shown). The patient underwent gamma knife surgery in addition to chemotherapy for a rectal tumor with adrenal metastasis.

MRI has been advocated as the most sensitive liver imaging in patients with colorectal cancer with liver metastases, who need evaluation for suitability for partial hepatectomy.[87] In a meta-analysis of prospective studies of 3391 previously untreated patients (39 articles; 1990-2010), it was determined that on a per-patient basis, the sensitivities of CT, MRI, and fluorodeoxyglucose (FDG) positron emission tomography (PET) were 83.6%, 88.2%, and 94.1%, respectively, with comparable specificity. For lesions smaller than 10 mm, the sensitivity estimates for MRI were significantly higher than those for CT. The sensitivity of MRI increased significantly after January 2004. The use of liver-specific contrast material and multisection CT scanners did not provide improved results in this study. Data on FDG PET/CT were too limited for comparisons with other modalities. A recent 2020 meta-analysis of 12 prospective studies performed 2009-2019 has shown that MRI maintains its position as the most sensitive diagnostic imaging test in detection of colo-rectal liver metastases, at 89% vs 86% for contrast enhanced US, 84% for Multidetector CT, and 62% for FDG PET/CT.[74]  (See the image below.)

MRI of liver in patient with liver lesion initiall MRI of liver in patient with liver lesion initially detected on screening for hepatitis C, found to have colon adenocarcinoma with liver metastasis. MRI (clockwise) shows high-intensity signal with high lesion visibility on diffusion-weighted imaging. The lesion has low signal intensity on lava sequence precontrast, enhances peripherally on T2 lava sequence postcontrast, and appears hyperintense on T2-weighted fast-spin echo imaging.

Typically, metastases from adenocarcinoma of the colon appear hypointense on T1-weighted images (not shown) and hyperintense on T2- and diffusion-weighted sequences, with peripheral enhancement following administration of gadolinium. Diffusion-weighted imaging (DWI) particularly when combined with hepatobiliary phase liver specific contrast ie gadoxetate (Eovist R) imaging (Gd-EOB-DPTA enhanced MRI), has further improved sensitivity.

Use of agents that undergo biliary excretion in addition to renal excretion, such as gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) and gadobenate dimeglumine (Gd-BOPTA), is recommended.[88] With these agents, delayed T1-weighted images demonstrate normal parenchyma as hyperintense, and metastatic lesions appear hypointense.

Degree of confidence

MRI has lower sensitivity and higher specificity than CT scanning in tumor staging. The techniques have similar overall accuracy in tumor staging, as well as similar overall accuracy (approximately 60%) in the detection of enlarged lymph nodes (nodal staging). In detecting local recurrence, MRI has a higher sensitivity (91%) than CT scan (82%) and a higher specificity (100%) than CT scan (69%).

MRI, while useful for specific problem solving (eg, staging of rectal carcinoma, evaluation of liver metastases), is not a cost-effective screening modality for the entire abdomen and pelvis.

However, MRI is recommended as the preferred first-line modality for evaluating colorectal liver metastases in patients who have not previously undergone therapy, as it has the highest sensitivity for lesions smaller than 1 cm.[87, 74]

A meta-analysis of 12 prospective studies performed in 2009-2019 has shown that MRI maintains its position as the most sensitive diagnostic imaging test in detection of colorectal liver metastases, at 89% vs 86% for contrast-enhanced US, 84% for multidetector CT, and 62% for FDG PET/CT.[74]  

False positives/negatives

Limitations 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 colon 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. In addition, chronic radiation changes in the pelvis may mimic recurrent colon tumors and require biopsy for differentiation. Enlarged lymph nodes may result from inflammation rather than tumor, and lymph nodes of normal size may contain a tumor.



Transrectal ultrasound (TRUS) may be used to distinguish between layers of the rectal wall and thus detect depth of tumor penetration and perirectal spread. The technique requires no ionizing radiation and is relatively inexpensive to perform.[89] Tumor staging accuracy for TRUS (84.6%) is superior to that of CT (70.5%), but both underperform similarly in nodal staging (64.1% vs 61.5%, respectively).[90] T2 lesions with peritumoral inflammation, which obscures differentiation of the rectal wall layers, remain a typical overstaging pitfall for TRUS.[91]

Although not primarily used as a primary staging modality, hepatic metastases from a colon primary tumor may be detected as hyperechoic masses (increased echogenicity in relation to normal liver) on transabdominal real-time ultrasound, but their appearances can be variable.[92]  (See the image below.)

Ultrasound scan through the right lobe of the live Ultrasound scan through the right lobe of the liver showing large hyperechoic metastasis from colon cancer.

Ultrasonic contrast agents, although approved for use in Europe and Asia, are of limited availability in the United States for noncardiac indications. These agents generate microbubbles of high acoustic amplitude when activated, which recirculate, and may be viewed over intervals of several minutes by real-time or Doppler ultrasonography. Lesion detectability is enhanced and Doppler is more sensitive in detecting tumor vascularity. (See the images below.)

Contrast enhanced ultrasound has been shown to detect additional liver metastasis in 8 of 296 patients (2.7%) with newly diagnosed colorectal cancer, and to improve diagnostic specificity in 98% of the 62 patients with indeterminate focal liver lesions based on CT alone.[93]  

Liver metastasis from colon adenocarcinoma. A 42-y Liver metastasis from colon adenocarcinoma. A 42-year-old patient 2 years following diagnosis of colon cancer with new lesion in liver, hypoechoic and therefore atypical for metastatic disease.
Contrast-enhanced scan of liver: metastatic adenoc Contrast-enhanced scan of liver: metastatic adenocarcinoma. On ultrasound contrast scan at a high maximum intensity (1.2), and delay of 7 seconds, neovascularity was demonstrated.

Intraoperative ultrasound (IOUS) is routinely used during partial hepatectomy to confirm and localize lesions in guiding surgical planning, and it is common to find additional lesions not demonstrated by other imaging techniques. Contrast enhancement is now also being used.[94]

In a study of 60 patients already selected for liver resection, who had undergone preoperative staging with CT or MRI, IOUS performed after intravenous injection of SonoVue detected liver metastases with higher sensitivity than CT/MRI or IOUS alone (96.3% vs 76.7% and 81.5%, respectively; P< .05). Surgical management was also altered in 30% of cases because of additional (19.3%) or fewer (3.5%) metastases or characterization of lesions as benign (5.3%) or, rarely, vascular proximity (1 case), and CE-IOUS altered combined IOUS/CT/MR staging in 35%.

A primary colon tumor typically appears as an echo-poor mass with a hyperechoic center, which is known as the target sign (see the image below). Other findings include localized irregular colon wall thickening, an irregular contour, lack of normal peristalsis, and an absence of the normal layered appearance of the colon wall.

Ultrasound scan of a large cecal carcinoma showing Ultrasound scan of a large cecal carcinoma showing concentric thickening of the hypoechoic bowel wall by the tumor.

Intussuscepting colon tumors have a characteristic target-like appearance from concentric rings of soft tissue and mesenteric-fat density (see the image below).

Ultrasound scan demonstrating intussuscepting ceca Ultrasound scan demonstrating intussuscepting cecal carcinoma.

Nuclear Imaging

PET scanning

The most commonly used radiotracer is fluorodeoxyglucose (FDG), a glucose analog that is taken up and trapped in cells. To differentiate tumor from background, FDG positron emission tomography (PET) relies on the increased glucose metabolism of tumor cells and calculates a semiquantitative measure of glucose uptake, also known as standard uptake value (SUV). With the advent of simultaneous CT scanning, PET scanning also improves both spatial resolution and anatomic localization of findings.

FDG-PET scanning has become more popular in the imaging of colorectal cancer and is used in pretreatment staging, posttreatment follow-up, and surveillance. It achieves sensitivity and specificity similar to that of contrast-enhanced CT scanning. For certain indications, such as monitoring response to therapy,[95] assessing for recurrence in patients with elevated carcinoembryonic antigen (CEA),[72] assessing for hepatic metastases after partial hepatectomy,[96] and differentiating local recurrence from scarring,[97] small studies have shown incremental benefit over conventional diagnostic imaging. For now, the lack of larger studies and the higher costs of FDG-PET scanning prohibit more widespread adoption.[98, 99, 100]

(See the images below.)

Patient undergoing staging for adenocarcinoma of t Patient undergoing staging for adenocarcinoma of the descending colon. Axial scan demonstrates circumferential wall thickening and invasion into pericecal mesentery.
Arterial phase image in same patient (undergoing s Arterial phase image in same patient (undergoing staging for adenocarcinoma) with primary colon cancer of descending colon. A lesion in segment 7 shows peripheral enhancement and relative central hypoattenuation.
On portal venous-phase CT imaging, the peripheral On portal venous-phase CT imaging, the peripheral enhancement is no longer as prominent and central hypoattenuation persists.
Positron emission tomography scan confirmed activi Positron emission tomography scan confirmed activity in the hepatic lesion suspicious for metastasis, in addition to the primary tumor.
Dark maximum-intensity projection images demonstra Dark maximum-intensity projection images demonstrate fluorodeoxyglucose (FDG) activity in primary tumor and liver in addition to physiologic activity.
Positron emission tomography demonstrating uptake Positron emission tomography demonstrating uptake in left adrenal gland consistent with metastatic disease in a patient undergoing staging for newly diagnosed right colonic adenocarcinoma.

Degree of confidence

A study by Meta et al evaluated the impact of FDG-PET on the management of patients with colorectal carcinoma and noted a change in the clinical stage and major management decisions in approximately 40% of patients.[101] Of the changes in clinical stages in 25 patients, the disease was up-staged 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-positive results may occur with FDG from nonspecific inflammatory reactions following radiotherapy or in patients with abscesses. Infectious/inflammatory conditions also demonstrate FDG uptake, which limits the ability to differentiate from tumor. For example, it is not able to differentiate diverticulitis from tumor, a pitfall PET scanning shares with conventional CT scanning. FDG-PET also has decreased sensitivity for mucinous (58%) versus nonmucinous (92%) adenocarcinomas.[102]

It has been recently been determined that rectal cancers with KRAS or NRAS mutations have higher glucose metabolism than wild –type cancers, but 18F-PET/CT does not yet have a meaningful clinical role (18F-FDG PET/CT imaging in rectal cancer: relationship with the RAS mutational status.[103]  However, PET/CT has a better performance profile than CT in the evaluation of postoperative patients with increasing CEA levels, with a detection rate of 71% vs 55% compared to CT alone, which in the study of 73 patients, changed management in 55% of patients, and showed recurrence in more cases than CT (36 vs 23) of patients with elevated CEA >3ng/ml.[104]




Angiography has a role in the administration of chemotherapy via the hepatic artery, an invasive approach based on the rationale that higher doses of chemotherapy may be delivered to the tumors with reduced systemic toxicity and reduced toxicity to the unaffected liver, since normal supply is predominantly via the portal venous system. As yet, consensus has not been reached on the ideal combination of agents.[68]

One study evaluated a protocol of intra-arterial therapy with mitomycin C, alone or in combination with irinotecan or gemcitabine, in 463 patients who had not responded to systemic chemotherapy.[105] The infusions were followed by embolization with lipiodol and starch microspheres in order to occlude small tumor vessels and obstruct the vascular bed of the liver tumors. Although 37% had evidence of progressive disease, partial response was obtained in 15% and 48% remained stable. There was no difference between the protocols, and median survival from date of diagnosis of liver metastases was 38 months and from the start of chemoembolization treatment was 14 months. Survival for those with progressive disease was 13 months, as compared to 18.2 months in those with partial response, a statistically significant difference.

Results for patients with unresectable, chemorefractory liver metastases using radioembolization with yttrium-90 (90Y) microspheres are also encouraging. Response rates of 30-60% are being achieved, and in one study, median survival of 457 days for patients with colorectal tumors was achieved.[106]

Minor complications of 90Y therapy include fatigue, pain, nausea, bilirubinemia, and reduction in lymphocyte count. Major complications occurred in less than 5%, including gastrointestinal ulceration, radiation-induced cholecystitis, biloma, and hepatic abscess, all requiring further intervention.

The role of hepatic intra-arterial infusion also includes use of F-fluoridine, using an implanted pump. Combining infusion with chemotherapy may increase the percentage of patients who are eligible to up to 50%, and 5-year survival of 56% has been achieved, compared to zero for those unable to undergo resection. There are potentially serious complications, however, such as pump failure and biliary sclerosis associated with the therapy, although reportedly of low frequency.[107, 108]