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Gastrointestinal Imaging |
1 Departments of Radiology (H.M.F., P.D.C., J.T.F.)
2 Surgical Oncology (D.B.M.)
3 Gastroenterology (D.P.N.), Boston University School of Medicine, Boston Medical Center, 88 E Newton St, Boston, MA 02118.
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: Twenty-nine patients with occlusive colorectal carcinomas underwent preoperative virtual colonoscopy with use of a standard protocol. Patients with acute bowel obstruction were excluded. Results of virtual colonoscopy were compared with the findings of preoperative colonoscopy, preoperative barium enema examination, intraoperative colon palpation, histopathologic outcome, and postoperative colonoscopy and barium enema examination, where possible.
RESULTS: Virtual colonoscopy helped identify all 29 occlusive carcinomas and demonstrated two cancers and 24 polyps in the proximal colon. Both synchronous cancers were confirmed intraoperatively and resected. Postoperative conventional colonoscopy in 12 patients confirmed 16 polyps identified at virtual colonoscopy and demonstrated two subcentimeter polyps missed at virtual colonoscopy. Postoperative barium enema examination was performed in two patients and helped confirm two polyps identified at virtual colonoscopy. Virtual colonoscopy successfully demonstrated the proximal colon in 26 of 29 patients examined compared with preoperative barium enema examination, which failed to adequately demonstrate the proximal colon in any patient examined.
CONCLUSION: Virtual colonoscopy is a feasible and useful method for evaluating the entire colon before surgery in patients with occlusive carcinomas.
Index terms: Colon, CT, 75.12115 • Colon, neoplasms, 75.321 • Colonoscopy, 75.12117 • Computed tomography (CT), image processing, 75.12117 • Computed tomography (CT), three-dimensional, 75.12115, 75.12117
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Evaluation of the entire colon in patients with distal occlusive cancers, which are defined as tumors that cannot be traversed endoscopically, is difficult. Methods used to evaluate the proximal colon in these patients are a preoperative barium enema examination, intraoperative colon palpation, intraoperative colonoscopy, postoperative barium enema examination, and postoperative colonoscopy (14–17). Preoperative barium enema examination in these patients is technically difficult, is associated with an increased risk of barium inspissation, and may necessitate a delay before surgery to adequately cleanse the colon. While intraoperative colon palpation and intraoperative colonoscopy can be used to depict synchronous disease, most surgeons favor a thorough preoperative evaluation to avoid making on-table diagnoses. In practice, the majority of patients with occlusive colorectal cancers do not undergo an adequate total colon evaluation before surgery but are referred for follow-up postoperative colonoscopy (1). This approach may result in a delay to diagnosis of synchronous proximal lesions and requires a second surgical procedure if a carcinoma is identified.
The aim of this study was to evaluate the use of preoperative virtual colonoscopy to examine the proximal colon in patients with distal occlusive carcinomas.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Technique
All patients underwent virtual colonoscopy within 3 hours after conventional colonoscopy. Each patient, therefore, received a standard colonoscopic bowel preparation, consisting of either 4 L of polyethylene glycol electrolyte solution (GoLytely; Braintree Laboratories, Braintree, Mass) ingested the evening before the examination, or a 48-hour liquid diet combined with 8-oz doses of magnesium citrate and a commercially available bisacodyl and phospha soda preparation kit (Fleet Prep 3; Fleet Pharmaceuticals, Lynchburg, Va).
Virtual colonoscopic examinations were performed according to a standard protocol (18). Patients were placed in the right lateral decubitus position on the CT table, and a rectal enema tube was inserted. Patients were then turned supine and room air gently insufflated into the colon to maximal patient tolerance. One milligram of glucagon (Lilly, Indianapolis, Ind) was administered intravenously immediately before helical CT of the abdomen and pelvis to allow optimal colonic distention, minimize peristalsis, and alleviate spasm. A standard CT scout view image of the abdomen and pelvis was acquired to assess the degree of colonic distention, and more air was insufflated, if required. With the CT scout view image, each examination was tailored to encompass the entire colon from cecum to rectum.
All CT examinations were performed by using a helical CT scanner (PQ 5000; Picker International, Cleveland, Ohio). Images were acquired by using 5-mm collimation with a table speed of 6.25 mm/sec and a pitch of 1.25, 110 mA, 110 kVp, and a 512 x 512 matrix. A single breath-hold acquisition was used when possible to encompass the entire colon. Images were reconstructed at 2-mm intervals, with a 3-mm section overlap and a reconstruction index of 2. After the supine image was obtained, helical CT was repeated with the patient prone.
The CT data were downloaded to an independent workstation (Voxel Q; Picker International) equipped with software for perspective volume rendering (epi-Scope 3.4, Voyager 3.4; Picker International). Using this software, a single radiologist (H.M.F.) blinded to the results of the conventional colonoscopy generated a retrograde intraluminal "fly-through" navigation through the volume of CT data from rectum to cecum. The navigation was then repeated in an antegrade direction from cecum to rectum. Both antegrade and retrograde virtual colonoscopic studies were stored in a cine loop format and viewed directly from the workstation monitor.
Interpretation
All 29 virtual colonoscopic studies and the corresponding axial CT images were evaluated by two experienced gastrointestinal radiologists (J.T.F., P.D.C.) who jointly reviewed the CT images and arrived at a consensus decision. The radiologists were aware that an occlusive carcinoma had been identified at conventional colonoscopy, but they were blinded to specific details, including the location and size of the tumor. The virtual colonoscopic studies were reviewed on a 17-inch monitor (Voxel Q; Picker International) at a variable frame rate of five to 30 frames per second. Virtual colonoscopic studies generated from the CT data acquisitions obtained with the patient supine were used for initial interpretation. Both antegrade (cecum to rectum) and retrograde (rectum to cecum) navigations were evaluated to enable visualization of both sides of the haustral folds. Prone studies were reconstructed when virtual colonoscopic studies obtained from supine CT data were limited by poor distention or retained intraluminal fluid and stool, or when difficulties arose in differentiating polyps or cancers from retained stool. In addition, the supine and prone axial CT images were printed to hard copy and viewed at lung window settings (window level, -1,000 HU; window width, 500 HU). Final interpretations were made on the basis of combined evaluation of the virtual colonoscopic studies and axial CT images.
The number, size, location, and morphology of all polyps and cancers identified at virtual colonoscopy were recorded. Lesion location was determined by reference to adjacent bone and soft-tissue landmarks, as determined from both the hard-copy axial images and the multiplanar reformatted images, which are routinely used to assist navigation at the workstation monitor. The degree of colonic distention and adequacy of colonic preparation were also recorded. For quantification of the extent of proximal colon successfully visualized at virtual colonoscopy, the large intestine was divided into five anatomic segments: rectum, sigmoid colon, descending colon, transverse colon, and ascending colon and cecum. The results of virtual colonoscopy were correlated with the findings of preoperative conventional colonoscopy and preoperative barium enema examination, when available, and with the surgical findings and histopathologic outcome for each patient. In addition, the results of preoperative virtual colonoscopy were correlated with those of the postoperative conventional colonoscopy and postoperative barium enema examination when possible.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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The surgical findings confirmed that virtual colonoscopy enabled correct prediction of the location of all 29 occlusive cancers: four in the rectum, 12 in the sigmoid colon, nine in the descending colon, and four in the transverse colon. Correct lesion localization was achieved with reference to both the axial CT images and the multiplanar reconstructions that are automatically generated at the workstation monitor to assist virtual colonoscopic navigations. Conversely, the location of five of the 29 index cancers was incorrectly reported at conventional colonoscopy as being more proximal than their actual location determined at the time of surgery.
On average, the total CT room time for virtual colonoscopy was 20 minutes (range, 15–30 minutes), physician input time for image manipulation was 35 minutes (range, 25–50 minutes), and the time required for virtual colonoscopic interpretation was 12 minutes (range, 8–15 minutes).
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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The preoperative diagnosis of synchronous carcinomas is difficult, and controversy continues as to how best to evaluate the entire colon before resection in patients with colorectal carcinoma (8). Although conventional colonoscopy is regarded as the best method for evaluating the colon before surgery, prospective studies have shown that the entire colon is visualized at colonoscopy in only 42%–60% of patients with colon cancer (7,8,10–12). Some authors hypothesize that preoperative colonoscopy constitutes an unnecessary risk for the patient, as it may promote seeding of neoplastic cells throughout the colon and manipulation of the tumor may accelerate hematogenous or lymphatic spread (19). On the other hand, studies (20,21) have shown that preoperative barium enema examination may miss up to 36% of synchronous colon malignancies and up to 67% of coexisting polyps detectable by using colonoscopy.
At least one report (19) in the surgical literature advocates thorough intraoperative palpation of the colon combined with early postoperative endoscopy rather than preoperative colonoscopy or barium enema examination. Up to 30% of synchronous cancers, however, may be missed with intraoperative palpation (10,22). Moreover, it can be difficult to palpate a synchronous tumor in a dilated colon and in regions such as the splenic flexure, where organs must be carefully handled.
For several reasons, preoperative evaluation of the entire colon is particularly important for patients with distal occlusive colorectal carcinomas. First, compared with all colorectal cancers, occlusive cancers are found predominantly in the distal colon, leaving a substantial length of proximal colon inaccessible to colonoscopy (14). Second, synchronous neoplasms may be more common in patients with occlusive cancers than among those with nonocclusive cancers (14). In one series, the colon proximal to occlusive cancers harbored multiple adenomatous polyps in 29 (58%) patients and synchronous invasive cancers in three (6%) patients (14). The increased risk of synchronous neoplasms in patients with occlusive carcinomas has been attributed to increased patient age and more advanced tumor stage compared with the risk in patients with nonocclusive carcinomas. Third, synchronous neoplasms in patients with occlusive carcinomas may be difficult to detect during laparotomy if the proximal colon becomes distended by gas or feces. For example, in the series of patients with occlusive cancers examined by Bat et al (14), none of the synchronous neoplasms was detected with intraoperative palpation, even though 46% of the neoplasms were larger than 1 cm in diameter. It is clear, therefore, that patients with occlusive carcinomas are in double jeopardy with respect to synchronous neoplasms, these being more prevalent and less accessible than those in patients with nonocclusive tumors.
Virtual colonoscopy was described by Vining et al (23) in 1994. It involves cleansing the patient's bowel by using a standard barium enema or colonoscopic bowel preparation, insufflation of room air into the cleansed colon with a rectal enema catheter, and thin-section helical CT of the abdomen and pelvis followed by off-line computerized manipulation of the CT data to generate an "endoscopic" view of the colonic mucosa. Many authors (18,24–32) have since reported its usefulness in colorectal cancer and polyp detection. Hara et al (24,25) used a variation of the technique (CT colography) to evaluate 30 endoscopically proved polyps in 10 patients. They detected 100% of all polyps larger than 1 cm, 71% of polyps 0.5–0.9 cm, and 28% of polyps smaller than 0.5 cm, results that surpass the performance of barium enema examination in most modern series. A recent study (26) comparing virtual colonoscopy and single-contrast barium enema examination reported depiction rates of 91% and 64% for polyps greater than or equal to 10 mm, 95% and 52% for polyps 6–9 mm, and 13% and 0% for polyps smaller than 5 mm, respectively.
Royster et al (18) compared the diagnostic accuracy of axial two-dimensional CT images of the air-distended colon, virtual colonoscopic images, and conventional colonoscopic images in 20 patients with colorectal cancer. With the two-dimensional axial CT technique, all 20 carcinomas (size range, 2.5–6.0 cm) and 12 of 13 polyps (all smaller than 1 cm) were successfully detected. Comparable results were obtained with interpretation of axial CT images and virtual colonoscopic images for cancers, but there was a substantially higher rate of false-positive diagnoses of polyps with use of only the axial CT images. In this study and in routine clinical practice, we review both the virtual colonoscopic images and axial CT images in combination to use all of the available CT information and to improve lesion detection.
Several authors (33–36) have reported on the use of CT in the examination of patients with obstructing lesions of the small and large bowels. Frager et al (36) reported a greater sensitivity for CT (sensitivity of 96%) in depicting colonic obstruction compared with that for the barium enema examination (sensitivity of 80%), with overall accuracies of 95% and 81%, respectively. The technique described by Frager et al (36) involved CT acquisitions obtained with the patient supine at 8–10-mm section thickness with additional 3–5-mm sections as required. All patients received oral contrast medium, selected patients received intravenous contrast medium, and a proportion of patients were also examined in a decubitus or prone position. Rectal air was administered in only two of the 75 patients examined. To our knowledge, the current study is the first to address the use of reformatted thin-section helical CT images of the air-distended colon (virtual colonoscopy) to examine patients with occlusive colorectal carcinomas and to specifically search for proximal synchronous disease. Despite the presence of a distal occlusive carcinoma that precluded adequate evaluation of the proximal colon with either colonoscopy or barium enema examinations in all patients, virtual colonoscopy enabled a complete colon evaluation in 26 of the twenty-nine patients examined. Retained stool proximal to the tumor was not a prominent feature. Possible explanations for this are patient anorexia and "overflow" diarrhea. In addition, acquisition of both prone and supine CT data sets helped differentiate mobile intraluminal fluid and stool from fixed neoplasms, such as polyps and cancers.
Apart from its ability to demonstrate the entire colon in the majority of patients with distal occlusive carcinomas and its apparent accuracy in depicting synchronous colorectal neoplasms, virtual colonoscopy before surgery provides additional benefits in patients with colorectal cancer. By reference to adjacent osseous and soft-tissue landmarks on the axial CT images and multiplanar reconstructions, it is possible to predict tumor location more accurately at virtual colonoscopy than at conventional colonoscopy. This may influence surgical conduct, such as the location of the incision, the level of placement of an epidural catheter for perioperative analgesia, the vigor with which the colon is manipulated, the extent of the resection, and even stoma site planning. Compared with preoperative barium enema examinations, virtual colonoscopy is not associated with an increased risk of preoperative obstruction because of barium inspissation proximal to a cancer, and it does not necessitate a delay before surgery to cleanse the colon of residual barium.
Virtual colonoscopy is an effective method for evaluating the entire colon before surgery in patients with distal occlusive colorectal carcinomas. It is preferable to preoperative barium enema examinations in terms of the extent of the proximal colon visualized and in the depiction of synchronous lesions, and it avoids the risks of colon obstruction from barium inspissation and intraoperative barium contamination of the peritoneum. It eliminates the need for and the ambiguities associated with intraoperative colon palpation and intraoperative colonoscopy and is more sensitive than intraoperative palpation in the detection of subcentimeter polyps. By depicting synchronous cancers before surgery, virtual colonoscopy helps optimize the surgical approach and, by demonstrating synchronous polyps, helps identify those patients who may benefit from early postoperative colonoscopy.
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Footnotes |
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Address reprint requests to H.M.F.
From the 1997 RSNA scientific assembly.
Author contributions: Guarantor of integrity of entire study, J.T.F.; study concepts and design, H.M.F., D.P.N., J.T.F.; definition of intellectual content, D.B.M., J.T.F.; literature research, H.M.F., D.P.N.; clinical studies, H.M.F., D.B.M., D.P.N., P.D.C.; data acquisition and analysis, H.M.F., P.D.C.; manuscript preparation, H.M.F., D.B.M., D.P.N.; manuscript editing, D.B.M., D.P.N.; manuscript review, J.T.F.
Received March 25, 1998;
revision requested June 19, 1998; revision received July 6, 1998;
accepted September 8, 1998.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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