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Original Report |
1 Department of Radiology, Seoul National University College of Medicine,
Institute of Radiation Medicine, SNUMRC, and Clinical Research Institute,
Seoul National University Hospital, 28 Yongon-dong, Chongno-gu, Seoul 110-744,
Korea.
2 Department of Radiology, Asan Medical Center, University of Ulsan College of
Medicine, Seoul, Korea.
3 Department of Pathology, Seoul National University College of Medicine, Seoul
National University Hospital, Seoul, Korea.
Received September 26, 2003;
accepted after revision November 20, 2003.
Supported in part by a grant from the 2003 BK21 Project for Medicine,
Dentistry, and Pharmacy.
Abstract
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CONCLUSION. On contrast-enhanced CT, primary gastrointestinal stromal tumors in the omentum and mesentery are usually well-defined, huge masses that contain large areas of low-attenuation necrosis and hemorrhage and that lack central gas.
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Gastrointestinal stromal tumors are most frequently found in the stomach (60–70%), followed by the small intestine (20–30%), colorectum (10%), and esophagus (< 5%) [4]. Although stromal tumors in the gastrointestinal tract commonly metastasize to the omentum and mesentery, they may also occur as primary tumors outside the gastrointestinal tract, especially in the omentum and the mesentery [5]. A number of studies of gastrointestinal stromal tumors have been published in the radiology literature [6–9]; however, few case reports are available on the imaging appearances of primary gastrointestinal stromal tumors in the omentum and mesentery [10, 11]. The purpose of our study was to describe the CT findings of primary gastrointestinal stromal tumors in the omentum and mesentery and to identify any pathologic correlation.
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CT data were available on a PACS (picture archiving and communications system, Marotech) in all patients. CT examinations were performed with a Somatom Plus-4 (Siemens Medical Solutions) or a Hi-Speed Advantage (General Electric Medical Systems) scanner. Each patient received 120 mL of nonionic contrast material ([iopromide] Ultravist 370, Schering) through an 18-gauge angiographic catheter inserted into a forearm vein. The contrast material was injected at a rate of 2.5 mL/sec using an automated injector. Helical CT was performed in each case with the following parameters: 7-mm collimation, 1:1 table pitch, and 7-mm reconstruction interval. The delay between contrast material administration and scanning was 60–70 sec.
Three radiologists retrospectively reviewed all the radiologic studies, and final interpretations were reached by consensus. All images were reviewed on a 2,000 x 2,000 PACS monitor. Tumor margins were categorized as well defined (i.e., a smooth or lobular contour with no surface projections), irregular (i.e., with surface projections), or clearly invasive (i.e., when soft tissue of a similar attenuation to that of the tumor breached an adjacent organ). We assessed the size, areas of low attenuation, intratumoral gas, and calcification of the tumors. CT findings were also evaluated for engorged vessels, bowel obstruction, abdominal lymphadenopathy, ascites, and distant metastasis. Tumor attenuation of the soft-tissue elements was visually compared with that of the liver.
For objective analysis, CT attenuation values were measured in a circular region of interest of 10 mm in diameter on tumor images using a 3D software program (Rapidia, INFINITT). CT attenuation values were measured in areas of high and low attenuation by one radiologist.
Follow-up analysis included a review of all postoperative CT scans. For all patients with follow-up images, the time to detection and location of any disease relapse was recorded. Ascites, liver metastasis, peritoneal mass, lymphadenopathy, and lung metastasis were recorded from the time of the scan on which they were first observed.
The pathology records of each patient were reviewed to establish mitotic activity and immunoreactivity with CD117. The histopathologic findings in surgical specimens were retrospectively reviewed by one gastrointestinal pathologist, with a special emphasis on the presence of hemorrhage, necrosis, or cystic degeneration. A direct comparison between imaging and histopathologic findings was performed by one pathologist; and three radiologists reviewed the pathology reports, photographs of gross specimens, and microscopic examinations.
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On contrast-enhanced CT, the tumor margin was well defined in all patients (Figs. 1, 2A, 2B, 3A, 3B, 3C, 4A, 4B). All tumors had a lobulated contour, and two (25%) showed a bilobular appearance (Fig. 2A, 2B). Tumor sizes ranged from 10 to 27 cm (mean ± standard deviation, 16.6 ± 6.1 cm) in the greatest dimension. Central low attenuation was present in all eight tumors, whereas central gas or mural calcification was not seen in any tumor. Areas of central low attenuation on CT occupied more than half the tumor in all patients.
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Vessels were often engorged and stretched over large tumors. In one patient whose tumor occurred in the transverse mesocolon, the colic vein was engorged despite the fact that it adjoined the second portion of the duodenum and the head of the pancreas (Fig. 3A, 3B, 3C). No cases of bowel obstruction or lymphadenopathy were seen. Metastases were present in four (50%) of the eight patients. Peritoneal metastases were present in two patients (Fig. 4A, 4B), and the liver was involved in another two. The liver metastasis was of lower attenuation than the normal surrounding parenchyma. Three patients had ascites, and one of these three had no evidence of peritoneal implants either on CT or at surgery.
The attenuation of the soft-tissue tumor elements on CT scans was less than that of the normal liver parenchyma in three tumors (37.5%), the same in three (37.5%), and greater in two (25%). The attenuation of areas of high attenuation ranged from 80 to 170 H (mean, 105 H), and those of low attenuation, from 8 to 30 H (mean, 21 H).
Six patients had follow-up images available for review. Five of these six patients developed metastases at a mean of 16 months (range, 12–23 months). The locations of metastases were the liver and peritoneum (n = 2); the liver alone (n = 1); the peritoneum alone (n = 1); and in one patient, the liver, peritoneum, lymph nodes, and lung. The single remaining patient was disease-free after 34 months of follow-up.
The mitotic rate was one to 40 mitoses (mean, 18.4 ± 12.7 mitoses) per 50 high-power fields. All eight tumors were positive for KIT by immunohistochemical staining. In six patients, immunoreactivity to KIT was prospectively verified after its introduction. In two patients who were diagnosed before the introduction of the technique, immunoreactivity to KIT was shown on a biopsy or a surgical specimen of the metastatic lesion during the follow-up period.
Correlation between CT and pathology findings was studied in seven patients whose tumors were surgically removed. Areas of low attenuation were found to be due to hemorrhage and necrosis in five tumors (71.4%) (Figs. 3A, 3B, 3C and 4A, 4B), and to hemorrhage, necrosis, and cystic degeneration in one. In the remaining tumor, areas of low attenuation were found to correspond to solid tumor containing multiple foci of hemorrhage, necrosis, and cystic degeneration; and areas of low attenuation had lower cellularity and vascularity than areas of high attenuation at microscopic examination (Fig. 2A, 2B).
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Stromal tumors in the gastrointestinal tract may be ulcerated, and areas of cavitation may contain air or oral contrast medium [6, 7]. The most common clinical manifestation of symptomatic stromal tumors in the gastrointestinal tract is gastrointestinal bleeding caused by mucosal ulceration [6]. Patients may present with hematemesis, melena, hematochezia, or signs and symptoms of anemia caused by occult bleeding. In our series, however, no patient presented with gastrointestinal bleeding, and no tumor contained air or showed a cavitary mass. The explanation of this apparent discrepancy is straightforward because cavitation means that a necrotic center has discharged its contents into the bowel and been replaced by air, but gastrointestinal stromal tumors in our series occurred outside the tubular gastrointestinal tract. In fact, we believe that cavitation and gas content can militate against the diagnosis of gastrointestinal stromal tumor in an extraintestinal mass.
Areas of central low attenuation in gastrointestinal stromal tumors usually correspond to hemorrhage, necrosis, or cystic degeneration [6–8]. Extensive areas of low attenuation were present in all cases, and in all but one of those cases corresponded to hemorrhage, necrosis, or cystic degeneration. In the one exception, areas of low attenuation primarily corresponded to solid tumor, although the tumor contained multiple foci of hemorrhage, necrosis, and cystic degeneration. Regional differences of cellularity and vascularity were observed in that tumor, which might have been responsible for this phenomenon.
Primary gastrointestinal stromal tumors in the omentum and mesentery are usually so large that they are often inseparable from the wall of the stomach or intestine. Although in our experience, primary gastrointestinal stromal tumors in the omentum and mesentery seem to have more extensive central low attenuation than do gastrointestinal tumors, the determination of tumor origin can be difficult unless the tumor contains an ulcer, a cavity, or air.
The CT appearance of primary gastrointestinal stromal tumors in the omentum and mesentery seems to be indistinguishable from that of other sarcomas that may arise in these locations, such as leiomyosarcoma, malignant fibrous histiocytoma, fibrosarcoma, and liposarcoma [6]. Although no comparative study has been performed because of the rarity of these tumors, we believe that gastrointestinal stromal tumors have a tendency to have a less invasive appearance on CT than other sarcomas. Desmoid tumors typically present as mesenteric masses that show homogeneous attenuation on CT images. The common mesenteric and omental cystic masses are lymphangioma, enteric duplication cyst, enteric cyst, and mesothelial cyst [12]. These benign cystic masses usually have a thin wall and lack solid enhancing components, which can help discriminate them from gastrointestinal stromal tumors.
In conclusion, we present a series of gastrointestinal stromal tumors occurring as primary tumors in the omentum and mesentery. Primary gastrointestinal stromal tumor in the omentum and mesentery can be suggested as a diagnosis in a patient with a well-marginated, lobulated mass that contains large areas of low attenuation and lacks central gas.
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