Colon Cancer Imaging: Practice Essentials, Radiography, Computed Tomography (2024)

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. ^{[20, 21, 28, 29, 30, 31, 32, 33]}

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.

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. ^[34] Other complications that may present acutely include obstruction and fistula formation, as shown in the image below.

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. ^{[35, 36]}

Technique

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. ^[37] However, new developments in fecal tagging may be able to eliminate the bowel preparation process. ^[38] 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. ^[39]

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. ^[40] (See the images below.)

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. ^[41]

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. ^[42]

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 ^[43] :

• 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. ^[42]

Lesion categorization

Lesions are categorized using the CT Colonography Reporting and Data (C-RAD) system ^[44] 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.

Effectiveness

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. ^[44] Subsequent trials in the mid 2000s reported lower sensitivities, of 55% and 59%, ^{[45, 46]} 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. ^[47] Moreover, sensitivity for lesions measuring 5-9 mm was reported with a sensitivity of 85-89%.

Disadvantages

CT colonography has several disadvantages over traditional colonoscopy. The first is radiation exposure possibly leading to a long-term carcinogenic effect, although with newer image acquisition systems, radiation exposure has been significantly decreased. ^[48] 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. ^[44] Yet another disadvantage is interinterpreter, intersystem variability. The reported results may not be replicable without extensive training in the technique.

CT colonography carries a small risk of procedural complication, including bowel perforation. ^[49] 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, ^[28] lower than that of optical colonoscopy but not negligible.

The sensitivity of CT colonography for the detection of flat lesions is has not been established; such lesions may constitute 40% of all adenomas. ^[50] (See the images below.)

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. ^[10] 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. ^{[29, 43] .}

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%. ^[51] Determination of depth of tumor invasion through the colonic wall has not been accurate. ^[29]

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, ^[30] 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%. ^[52]

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. ^[53] One study evaluated CT colonograms from 246 patients to assess tumor staging. Overall accuracy for tumor stage was 79%. ^[54]

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.)

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.)

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%, ^[30] 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%. ^[52]

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).

Rectosigmoid tumors may metastasize to external iliac nodes.

Despite limitations in accuracy of staging, there is an association with survival ^[55] ; 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).

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.

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. ^[56]

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 ^[57] 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 ^[55] 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. ^[55]

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.)

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. ^[57]

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.)

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.)

Bony and cerebral metastases are uncommon.

Reporting of metastatic disease should concentrate on careful reporting of location 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 ^[58] is not generally applied in clinical practice, but iit s a requirement in clinical trials for standardization.

Postoperative complications

An example of a postoperative complication is shown below.

CT in surveillance after colon cancer therapy

Surveillance guidelines recommend annual CT surveillance of the chest, abdomen, and pelvis, ^[56] 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. ^[59] (See the images below.)

Contrast-enhanced CT colonography has been evaluated for efficacy of colorectal cancer in surveillance after curative resection, ^[60] 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 ^[31] 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, ^[61] 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.

In some settings, for example elevation of CEA following therapy for colon cancer, CT was found less accurate 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. ^[62]

False negatives/positives

Inadequate bowel preparation and/or distention, flat lesions, and small polyps are causes for missed lesions (false negatives) 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 classification of polyps as feces. ^[63]

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.