Introduction

Gastrointestinal stromal tumor (GIST) has recently been classified as a specific pathological entity, and the specific expression of c-kit (CD117), a tyrosine kinase-dependent transmembrane receptor, has enabled its distinction from leiomyoma, leiomyosarcoma, schwannoma and neurofibroma <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 25Mazur M.T., Clark H.B. Gastric stromal tumors. Reappraisal of histogenesis Am J Surg Pathol 1983 ;  7 : 507-519 [cross-ref]

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

While GIST is the most common mesenchymal tumor of the gastrointestinal (GI) tract, it accounts for less than 1% of all GI malignant tumors, with an estimated incidence in France of 1000 cases per year <sup>[5Karila-Cohen P., Petit T., Teissier J., Merran S. Tumeur stromale digestive J Radiol 2004 ;  85 : 1911-1914 [inter-ref]

Cliquez ici pour aller à la section Références]</sup>.

GISTs that measure less than 2cm are generally considered benign and have a low rate of recurrence. Nevertheless, the risk of malignancy can never be completely ruled out. On histology, GIST presents with at least one of the three characteristic features:

Therefore, to confirm the diagnosis of c-kit-negative GIST, tumor genotyping must be performed to look for mutations of the KIT and PDGFR genes.

It is particularly important to confirm the diagnosis of GIST, as even advanced-stage tumors can be treated with tyrosine kinase inhibitors. Imatinib mesylate (Glivec^®; Novartis Pharma, Basel, Switzerland) has provided good results in terms of tumor response and patient survival <sup>[20DeMatteo R.P., Heinrich M.C., El Rifai W.M., Demetri G. Clinical management of gastrointestinal stromal tumors: before and after STI-571 Hum Pathol 2002 ;  33 : 466-477 [cross-ref]

Cliquez ici pour aller à la section Références and 21Dagher R., Cohen M., Williams G., Rothmann M., Gobburu J., Robbie G., et al. Approval summary: imatinib mesylate in the treatment of metastatic and/or unresectable malignant gastrointestinal stromal tumors Clin Cancer Res 2002 ;  8 : 3034-3038

Cliquez ici pour aller à la section Références]</sup>.

However, preoperative evaluation using biopsy material is highly controversial because of the risk of bleeding. For this reason, in most cases, the diagnosis has to be based on imaging features.

The purpose of this study was to describe the initial computed tomography (CT) findings in 45 patients known to have GIST.

	Patients and methods

This retrospective study used the files of all patients who underwent surgery for GIST between January 1990 and March 2006 at the Poitiers University Hospital (Table 1).

CT scans were obtained with two machines:

Surgery had been performed in all patients. For the purposes of this study, two radiologists specializing in GI imaging read all images and established a consensus in the interpretation.

Contrast uptake was considered significant if tumor enhancement was more than 10 Hounsfield units (HU) after contrast injection. Tissue density was defined as 30–50HU before injection and contrast uptake after injection. Necrotic density was defined as 0–30HU in a zone devoid of contrast uptake. Cystic density was defined as –10 to +20HU in a zone devoid of contrast uptake. Hemorrhagic density was defined as 60–80HU without contrast injection and no contrast uptake.

The pathology slides were read by pathologists who were experienced in gastroenterological diseases. Histological diagnosis was based on the gross appearances, pathological features and immunolabeling of CD117-positive tumor cells.

	Results

Forty-five patients (23 male, 22 female) were included in the study; their mean age was 64 years, range 19–93 years. Time from symptom onset to CT diagnosis ranged from one month to five years.

Patients’ symptoms varied in terms of tumor size and location. The most common symptoms were abdominal pain and GI bleeding (hematemesis, anal bleeding, melena, iron-deficiency anemia, occult bleeding). Rarely, the inaugural sign was an abdominal mass found on digital rectal palpation. For two patients, the tumor was a fortuitous discovery on a CT scan performed for other purposes.

The stomach was the leading site of tumor (n =28, 63%) and involved the body of the stomach in 19 patients (43%), the antrum in five (11%) and the fundus in four (9%). The second most frequent location was the small intestine (n =13, 28%), with six tumors (13%) involving the jejunum, four (9%) in the duodenum and three (7%) in the ileum. There were two cases each (4%) involving the small bowel mesentery and the rectum. Colonic, esophageal and rectoperitoneal sites were not observed in this series.

	Gastric tumors

In the stomach, tumor sizes ranged from 2.5 to 30cm, with an average size at diagnosis of 8cm. An intraparietal component was observed in all cases, but extraparietal extension was the rule (n =38, 85%), usually involving the gastrohepatic ligaments, gastrosplenic ligaments or omental bursa—in other words, tumor extension was mostly extragastric (Figure 1 and Figure 2). The adjacent wall was thickened in 36cases (80%) (Figure 1, Figure 2 and Figure 3), and mucosal ulcerations were observed in 23 (50%) (Figure 3). Also, a mostly minor intraluminal component was found in nine cases (20%) (Figure 3), and the tumor was pedunculated in three (7%).

Figure 1
	Figure 1. Abdomen-pelvic scan. Axial slices without injection (a, b) and after injection during the portal time (c). Gastric slices show a stromal tumor: a and b: voluminous exophytic mass with extraparietal extension to the gastrohepatic ligament and the omental bursa; c and d: heterogeneous enhancement with peripheral contrast uptake and central necrotic and cystic remodeling. Scanner abdominopelvien en coupes axiales sans injection (a, b) et après injection au temps portal (c). Tumeur stromale du corps de l’estomac : a et b : volumineuse masse exophytique à extension extrapariétale vers le ligament gastrohépatique et l’arrière cavité des épiploons ; c et d : rehaussement hétérogène avec prises de contraste périphériques et remaniements kystiques et nécrotiques centraux . Zoom

Figure 2
	Figure 2. Abdomen-pelvic scan. Axial slices without injection (a) and after injection during portal time (b), gross aspect (c). Stromal tumor of the body of the stomach: a and b: exophytic mass with extension to the gastrohepatic ligament, heterogeneous enhancement showing cystic and hemorrhagic central remodeling; c: exphytic mass showing the peripheral tissue and the necrotic-hemorrhagic central area. Scanner abdominopelvien en coupes axiales sans injection (a) et après injection au temps portal (b), avec confrontation macroscopique (c). Tumeur stromale du corps de l’estomac : a et b : masse exophytique à extension vers le ligament gastrohépatique, au rehaussement hétérogène montrant des remaniements kystiques et hémorragiques centraux ; c : masse exophytique montrant des portions charnues périphériques et un centre nécroticohémorragique. Zoom

Figure 3
	Figure 3. Abdomen-pelvic scan. Axial slices after injection during portal time (a, b) and gross aspect (c). Stromal tumor of the body of the stomach: a and b: endoluminal mass with homogeneous enhancement showing a large mucosal ulcer; c: endoluminal polypoid formation with mucosal ulceration. Scanner abdominopelvien en coupes axiales après injection au temps portal (a, b) et confrontation macroscopique (c). Tumeur stromale du corps de l’estomac : a et b : masse endoluminale au rehaussement homogène montrant une large ulcération muqueuse ; c : formation polypoïde endoluminale surmontée d’une ulcération muqueuse . Zoom

On contrast injection, tumor enhancement was heterogeneous in 36 cases (80%), with peripheral contrast uptake surrounding central zones with hemorrhagic, necrotic or cystic density (Figure 1 and Figure 2). Homogeneous enhancement was less frequently seen (n =9, 20%, Figure 3). In some cases, large necrotic zones formed cystic cavities that communicated with the GI lumen, resulting in air-filled cavities or air–fluid levels within the tumor (Figure 1).

When predominant, exophytic expansion led to a mass effect with displacement of neighboring organs such as the pancreas and liver (Figure 1 and Figure 2).Intraperitoneal effusion due to peritoneal carcinomatosis and associated liver metastases was observed in three patients (10%) (Figure 4). No intratumor calcification, lymphadenopathy, vascular extension or venous thrombosis was noted in this series.

Figure 4
	Figure 4. Abdomen-pelvic scan. Axial slices after injection during the portal time (a–f): a and b: typical pattern of malignant stromal tumor of the body of the stomach: large size, major extragastrointestinal development, heterogeneous peripheral enhancement with central necrotic and hemorrhagic remodeling; c and d: two types of liver metastasis from a gastric stromal tumor: homogeneous, hypodense (c) and hypervasculair, heterogeneous (d); e and f: mesenteric and omental metastases of a gastric stromal tumor. Scanner abdominopelvien en coupes axiales après injection au temps portal (a–f) : a et b  : « profil » de tumeur stromale gastrique maligne du corps de l’estomac : taille importante, développement extradigestif majeur, rehaussement hétérogène et périphérique avec remaniements nécrotiques ou hémorragiques centraux ; c et d : deux types de métastases hépatiques d’une tumeur stromale gastrique : homogène, hypodense (c) et hypervasculaire, hétérogène (d) ; e et f : métastases mésentériques et omentales d’une tumeur stromale gastrique. Zoom

	Small bowel tumors

Small bowel GIST ranged in size from 2.2 to 20cm (mean 9cm). In most cases (n =27, 60%), the tumor was a large extraserous mass that could not be readily assigned to an intestinal origin (Figure 5 and Figure 6). The other tumors appeared to have a thickened wall with a more or less voluminous intraluminal component (n =18, 40%), of which seven (15%) were causing considerable luminal dilatation.

Figure 5
	Figure 5. Abdomen-pelvic scan. Axial slices without injection (a, b), then after injection during portal time (c, d) and gross aspect. Jejunal stromal tumor: a and b: voluminous lobulated mass with a major extraparietal component extending to the mesentry; c and d: heterogeneous enhancement showing partitioning, peripheral tissue and cystic and necrotic areas; e: voluminous heterogeneous tumor with tissue budding and several areas of necrotic-hemorrhagic remodeling. Scanner abdominopelvien en coupes axiales sans injection (a, b), puis après injection au temps portal (c, d) et confrontation macroscopique. Tumeur stromale jéjunale : a et b : masse volumineuse aux contours lobulés, à composante extrapariétale majeure envahissant le mésentère ; c et d : rehaussement hétérogène montrant des cloisons, des portions charnues et des plages kystiques et nécrotiques ; e : volumineuse tumeur hétérogène avec bourgeons charnus et nombreux remaniements nécroticohémorragiques . Zoom

Figure 6
	Figure 6. Abdomen-pelvic scan. Axial slices after injection during portal time (a, b), late time (c) and upper gastrointestinal opacification. Duodenal stromal tumor: a and b: major extraparietal component with heterogeneous enhancement; c: the stromal tumor communicates with the gastrointestinal lumen (opacification agent leaks into the tumor). Scanner abdominopelvien en coupes axiales après injection au temps portal (a, b), au temps tardif (c) et opacification digestive haute. Tumeur stromale duodénale : a et b : masse à composante extrapariétale majeure au rehaussement hétérogène ; c : communication de la tumeur stromale avec la lumière digestive (produit d’opacification digestive au sein de la masse tumorale) . Zoom

As with gastric tumors, contrast injection produced heterogeneous enhancement in most of these tumors (n =38, 85%), including central cystic or necrotic areas that communicated with the intestinal lumen to form air-filled cavities or air–fluid levels within the tumor (Figure 5 and Figure 6).

Twenty-seven (60%) exophytic stromal tumors had infiltrated or displaced the mesentery as well as the adjacent small bowel or colonic segments (Figure 5). However, no extension was observed to the ureters, bile ducts, abdominal wall or peritoneal cavity. Also, there were no cases of calcification, lymphadenopathy, venous thrombosis or vascular extension.

	Rectal tumors

There were two rectal tumors—measuring 4.5 and 4cm, respectively—both of which had an intramural component causing discrete distention of the rectal wall and extending into the ischiorectal fossae. After contrast injection, they presented with heterogeneous enhancement similar to that of stomach and small bowel tumors.

	Primary mesenteric tumors

There were two primary mesenteric tumors, measuring 3cm and 12cm, respectively. These were well-delineated, lobulated masses devoid of calcification or a central cavity. Enhancement was heterogeneous in both cases. The origin of the larger tumor was difficult to establish (Figure 7); it was displacing small bowel loops and the right ureter. Neither patient presented with ascites or peritoneal metastatic spread.

Figure 7
	Figure 7. Abdomen-pelvic scan. Axial slices after injection during portal time (a, b). Mesenteric stromal tumor: a and b: well-delimited mass with discretely lobulated contour and heterogeneous enhancement. Scanner abdominopelvien en coupes axiales après injection au temps portal (a, b). Tumeur stromale mésentérique : a et b : masse bien limitée aux contours discrètement lobulés et au rehaussement hétérogène . Zoom

	Discussion

The most common mesenchymal tumor of the GI tract, GIST is characterized by its main immunopathological feature—expression of c-kit (CD117), a tyrosine kinase-dependent transmembrane receptor <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 25Mazur M.T., Clark H.B. Gastric stromal tumors. Reappraisal of histogenesis Am J Surg Pathol 1983 ;  7 : 507-519 [cross-ref]

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

GIST arises as a result of proliferation of primary stem cells derived from interstitial cells of Cajal, the CD117 (c-kit)-positive intestinal pacemaker cells. Permanent activation of the c-kit receptor and, in turn, tyrosine kinase by a ligand contribute to the pathogenesis of GIST. These tumors are composed of epithelioid, spindle or signet cells that express c-kit (CD117), sometimes CD34 (70%) and, more rarely, smooth muscle actin or S-100 protein. They also express a mutation of tyrosine kinase PDGFR . These cells form an intertwining network or a palisading nuclear pattern. They generally develop from the outermost layer of the gastric or intestinal mucosa, explaining the propensity for exophytic growth (Figure 8), from where they then spread to the serous membrane to form a voluminous exophytic mass, or to the submucosa and mucosa, sometimes generating hemorrhagic mucosal ulcers that bulge into the lumen <sup>[3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références, 4Pidhorecky I., Cheney R.T., Kraybill W.G., Gibbs J.F. Gastrointestinal stromal tumors: current diagnosis, biologic behavior, and management Ann Surg Oncol 2000 ;  7 : 705-712 [cross-ref]

Cliquez ici pour aller à la section Références, 17Sarlomo-Rikala M., Kovatich A.J., Barusevicius A., Miettinen M. CD117: a sensitive marker for gastrointestinal stromal tumors that is more specific than CD34 Mod Pathol 1998 ;  11 : 728-734

Cliquez ici pour aller à la section Références and 18Kindblom L.G., Remotti H.E., Aldenborg F., Meis-Kindblom J.M. Gastrointestinal pacemaker cell tumor (GIPACT): gastrointestinal stromal tumors show phenotypic characteristics of the interstitial cells of Cajal Am J Pathol 1998 ;  152 : 1259-1269

Cliquez ici pour aller à la section Références]</sup>.

Figure 8
	Figure 8. Histology slides of gastric stromal tumors (a and b: HES stain×50 magnification; c: HES stain ×10 magnification; d: c-kit immunolabeling ×10 magnification): a: proliferation of spindle cells in intertwined network and presence of nuclear palisade; b: proliferation of epithelioid cells; c: initial development of tumor cells from the outer layer of the gastric muscuosa; d: tumor cell expression (brown color) of c-kit immunolabeling. Lames histologiques de tumeurs stromales gastriques (a, b : coloration HES grossissement ×50 ; c : coloration HES grossissement ×10 ; d : immunomarquage au c-kit grossissement ×10) : a : prolifération de cellules fusiformes agencées en faisceaux entrelacés et présentant un agencement nucléaire palissadique ; b : prolifération de cellules épithélioïdes ; c : développement initial des cellules tumorales à partir de la couche externe de la musculeuse de la paroi gastrique ; d : expression par les cellules tumorales (couleur marron) de l’immunomarqueur c-kit . Zoom

GIST accounts for 1% of all malignant GI tumors. There is no bias related to gender, geographical residence or ethnic origin. Mean age at the time of discovery is about 60 years, and they are rarely diagnosed before the age of 40. However, the risk of stromal tumor formation is increased in patients with type 1 neurofibromatosis, and there may be an association with the Carney triad (epithelioid leiomyosarcoma, paraganglioma and pulmonary chondroma) <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références, 4Pidhorecky I., Cheney R.T., Kraybill W.G., Gibbs J.F. Gastrointestinal stromal tumors: current diagnosis, biologic behavior, and management Ann Surg Oncol 2000 ;  7 : 705-712 [cross-ref]

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

The clinical diagnosis of GIST is usually established late, often only when it has become a large abdominal mass. Early clinical signs are minimal to silent, presenting as non-specific abdominal pain or GI bleeding (hematemesis, anal bleeding, melena, iron-deficiency anemia, occult bleeding). Fibroscopic biopsy is often negative as the tumor is essentially found in the submucosal layer <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

In order of decreasing frequency, the distribution of GIST in the present series was in the stomach (60–70%), small intestine (30%), colon and rectum (5%), esophagus (2%) and, more rarely, in the mesentery or omentum <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

CT imaging features are relatively homogeneous in terms of tumor location and size. These tumors appear as large, well-circumscribed, lobulated masses that are usually free of calcification and sometimes with spontaneous central areas of hemorrhagic hyperdensity. The parietal component, always seen as tissue thickening, is rarely a predominating feature. GIST is essentially exophytic, invading or displacing the fascia, mesentery and adjacent organs. The GI origin of the tumor is sometimes difficult to establish. Rarely, the tumor has an endoluminal polypoid appearance, sometimes with an ulcerated mucosa. These rapidly growing tumors can cause major dilatation of the intestinal lumen with destruction of the myenteric plexus.

Iodine contrast injection during portal time almost always reveals heterogeneous enhancement, with peripheral contrast uptake and non-enhancing central cystic, necrotic or hemorrhagic areas. These necrotic areas can communicate with the GI lumen, explaining the presence of air or air–fluid levels within the tumor. Homogeneous contrast uptake is less common. Metastatic dissemination to the peritoneal cavity or liver is possible but, as a rule, the tumor does not spread to lymph nodes, and venous thrombosis or vascular extension is highly unusual <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 6Fletcher C.D., Berman J.J., Corless C., Gorstein F., Lasota J., Longley B.J., et al. Diagnosis of gastrointestinal stromal tumors: a consensus approach Hum Pathol 2002 ;  33 : 459-465 [cross-ref]

Cliquez ici pour aller à la section Références, 7Kim H.C., Lee J.M., Son K.R., Kim S.H., Lee K.H., Kim K.W., et al. Gastrointestinal stromal tumors of the duodenum: CT and barium study findings AJR Am J Roentgenol 2004 ;  183 : 415-419 [cross-ref]

Cliquez ici pour aller à la section Références, 8Hama Y., Okizuka H., Odajima K., Hayakawa M., Kusano S. Gastrointestinal stromal tumor of the rectum Eur Radiol 2001 ;  11 : 216-219 [cross-ref]

Cliquez ici pour aller à la section Références, 9Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Anorectal gastrointestinal stromal tumors: CT and MR imaging features with clinical and pathologic correlation AJR Am J Roentgenol 2003 ;  180 : 1607-1612

Cliquez ici pour aller à la section Références, 10Miettinen M., Sarlomo-Rikala M., Sobin L.H., Lasota J. Esophageal stromal tumors: a clinicopathologic, immunohistochemical, and molecular genetic study of 17 cases and comparison with esophageal leiomyomas and leiomyosarcomas Am J Surg Pathol 2000 ;  24 : 211-222 [cross-ref]

Cliquez ici pour aller à la section Références and 11Miettinen M., Sarlomo-Rikala M., Sobin L.H., Lasota J. Gastrointestinal stromal tumors and leiomyosarcomas in the colon: a clinicopathologic, immunohistochemical, and molecular genetic study of 44 cases Am J Surg Pathol 2000 ;  24 : 1339-1352 [cross-ref]

Cliquez ici pour aller à la section Références]</sup>. Our present findings were consistent with those previously reported.

Less frequently, GIST can be found in the distal third of the esophagus. Mesenteric or omental sites are even more unusual. These esophageal tumors have the same pathological features as those in the GI tract, but without the central air cavity <sup>[12Kim H.C., Lee J.M., Kim S.H., Kim K.W., Lee M., Kim Y.J., et al. Primary gastrointestinal stromal tumors in the omentum and mesentery: CT findings and pathologic correlations AJR Am J Roentgenol 2004 ;  182 : 1463-1467

Cliquez ici pour aller à la section Références]</sup>.

Malignant GIST must be distinguished from benign GIST in terms of both the imaging features and pathological appearances. On CT scans, malignant tumors tend to be larger (although a cut-off dimension has yet to be established), with major extra-GI growth, peripheral heterogeneous enhancement and central necrotic or hemorrhagic remodeling. The diagnosis of malignancy is straightforward in patients with hepatic, mesenteric or omental metastases. Metastatic spread is usually seen as small hypodense nodules with discrete heterogeneous enhancement (Figure 4). Other features, such as the presence of ulcers, extension into mesenteric fat and displacement of neighboring organs, are not particularly distinctive. On the other hand, a small round or ovoid mass measuring less than2cm in an intraluminal location and presenting with homogeneous enhancement is strongly suggestive of a benign GIST (Figure 9) <sup>[3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références, 13DeMatteo R.P., Lewis J.J., Leung D., Mudan S.S., Woodruff J.M., Brennan M.F. Two hundred gastrointestinal stromal tumors: recurrence patterns and prognostic factors for survival Ann Surg 2000 ;  231 : 51-58 [cross-ref]

Cliquez ici pour aller à la section Références, 14Crosby J.A., Catton C.N., Davis A., Couture J., O’Sullivan B., Kandel R., et al. Malignant gastrointestinal stromal tumors of the small intestine: a review of 50 cases from a prospective database Ann Surg Oncol 2001 ;  8 : 50-59 [cross-ref]

Cliquez ici pour aller à la section Références, 15Burkill G.J., Badran M., Al Muderis O., Meirion T.J., Judson I.R., Fisher C., et al. Malignant gastrointestinal stromal tumor: distribution, imaging features, and pattern of metastatic spread Radiology 2003 ;  226 : 527-532 [cross-ref]

Cliquez ici pour aller à la section Références and 16Kim H.C., Lee J.M., Kim K.W., Park S.H., Kim S.H., Lee J.Y., et al. Gastrointestinal stromal tumors of the stomach: CT findings and prediction of malignancy AJR Am J Roentgenol 2004 ;  183 : 893-898 [cross-ref]

Cliquez ici pour aller à la section Références]</sup>.

Figure 9
	Figure 9. Abdomen-pelvic scan. Axial slices after injection during the portal time, upper gastrointestinal opacification (a) and fibroscopic aspect (b): a and b: typical pattern of a benign stromal tumor: small ovoid intraluminal ovoid mass with homogeneous enhancement. Scanner abdominopelvien en coupes axiales après injection au temps portal, opacification digestive haute (a) et confrontation fibroscopique (b) : a et b : « profil » de tumeur stromale bénigne : petite masse ovoïde, intraluminale au rehaussement homogène . Zoom

On histology, a poor prognosis is suggested by large tumor size (greater than 10cm for gastric tumors and greater than 5cm for intestinal tumors), a high tumor cell mitotic index (greater than 5 mitoses at 50 times magnification) and positive surgical margins.

There are several differential diagnoses. Lymphoma is the primary diagnosis to rule out because of its different therapeutic strategy (biopsy and chemotherapy vs. surgery for GIST). Whether it is a systemic lymphoma of the GI tract or a mucosa-associated lymphoid tissue (MALT), lymphoma can occur in the submucosal layer, although the diagnosis of lymphoma is relatively easy if the wall thickening is circumferential. However, the “aneurysmal” form of lymphoma is difficult to distinguish from GIST. Such a lymphoma presents as a voluminous exophytic parietal mass, and may have ulcerations or a central cavity leading to intestinal dilatation. Nevertheless, the presence of multiple nodal enlargements is suggestive of lymphoma <sup>[3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

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Connective tissue tumors (leiomyoma, leiomyosarcoma, schwannoma, neurofibroma) are much less frequently seen in the GI tract than GIST, but can present with similar features on CT; however, surgery may be necessary to establish the diagnosis <sup>[11Miettinen M., Sarlomo-Rikala M., Sobin L.H., Lasota J. Gastrointestinal stromal tumors and leiomyosarcomas in the colon: a clinicopathologic, immunohistochemical, and molecular genetic study of 44 cases Am J Surg Pathol 2000 ;  24 : 1339-1352 [cross-ref]

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GI adenocarcinoma is usually easy to distinguish from GIST. There is wall thickening, but without an exodigestive mass, and it is generally small, irregular, stenosing and associated with an inflammatory reaction or tumor extension into peritoneal fat. An upstream organic obstruction and perigastric, celiac, or hepatoduodenal or perirectal lymph node enlargement are also useful differentiating signs.

Parietal metastasis is another differential diagnosis, particularly in cases of malignant melanoma, which is often seen as multiple, highly vascularized, nodular seedings, sometimes complicated by intestinal intussusception <sup>[1Miettinen M., Lasota J. Gastrointestinal stromal tumors – definition, clinical, histological, immunohistochemical, and molecular genetic features and differential diagnosis Virchows Arch 2001 ;  438 : 1-12 [cross-ref]

Cliquez ici pour aller à la section Références, 2Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Gastrointestinal stromal tumors: radiologic features with pathologic correlation Radiographics 2003 ;  23 : 283-304456.  [cross-ref]

Cliquez ici pour aller à la section Références, 3Sandrasegaran K., Rajesh A., Rydberg J., Rushing D.A., Akisik F.M., Henley J.D. Gastrointestinal stromal tumors: clinical, radiologic, and pathologic features AJR Am J Roentgenol 2005 ;  184 : 803-811

Cliquez ici pour aller à la section Références, 8Hama Y., Okizuka H., Odajima K., Hayakawa M., Kusano S. Gastrointestinal stromal tumor of the rectum Eur Radiol 2001 ;  11 : 216-219 [cross-ref]

Cliquez ici pour aller à la section Références and 9Levy A.D., Remotti H.E., Thompson W.M., Sobin L.H., Miettinen M. Anorectal gastrointestinal stromal tumors: CT and MR imaging features with clinical and pathologic correlation AJR Am J Roentgenol 2003 ;  180 : 1607-1612

Cliquez ici pour aller à la section Références]</sup>.

GIST has to also be differentiated from intestinomesenteric carcinoid tumors, which present as a characteristically spiculated mesenteric mass with progressive late enhancement and significant desmoplastic reaction, giving the mesentery a retractile appearance. In such cases, a highly vascularized parietal carcinoid nodule after iodine bolus during arterial time and satellite nodes can be seen <sup>[19Barbary C., Corby S., Tissier S., Michel N., Meyer-Bish L., Régent D. Les tumeurs carcinoïdes du tube digestif : points clés et images pièges de l’imagerie en coupe Feuillet Radiol. 2005 ;  45 : 49-60 [inter-ref]

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At a mesenteric or omental site, the differential diagnosis includes leiomyosarcoma, malignant histiocytofibroma, fibrosarcoma and liposarcoma. Imaging is rarely sufficient to make the distinction.

In a context of familial rectocolonic adenomatous polyposis, or Gardner syndrome, the diagnosis of a mesenteric desmoid tumor must be considered. As a rule, this tumor arises as a unique fibrous mass with homogeneous enhancement.

Finally, cystic mesenteric formations (lymphangioma, mesothelial cysts, cystic digestive duplications) are easily distinguished because of their solely liquid imaging density <sup>[12Kim H.C., Lee J.M., Kim S.H., Kim K.W., Lee M., Kim Y.J., et al. Primary gastrointestinal stromal tumors in the omentum and mesentery: CT findings and pathologic correlations AJR Am J Roentgenol 2004 ;  182 : 1463-1467

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The esophagus is the only GI location where GIST is not the most frequent mesenchymal tumor. At this site, the leading diagnosis is leiomyoma, a small homogeneous nodular tumor confined to the esophageal wall <sup>[10Miettinen M., Sarlomo-Rikala M., Sobin L.H., Lasota J. Esophageal stromal tumors: a clinicopathologic, immunohistochemical, and molecular genetic study of 17 cases and comparison with esophageal leiomyomas and leiomyosarcomas Am J Surg Pathol 2000 ;  24 : 211-222 [cross-ref]

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The standard treatment for GIST is surgical resection. Medical management with specific inhibition of c-kit mutation with STI-571 (imatinib mesylate, Glivec^®) can be proposed for patients with non-resectable tumor or metastases. Despite technical advances with perfusion MRI, CT remains the imaging method of choice to monitor tumor response to medical treatment. A positive response to Glivec^® can lead to several changes in the CT presentation of the primary tumor or secondary localizations. Reduction of tumor size has been observed, but generally late—several months after treatment onset. Frequently, the tumor becomes necrotic, presenting myxoid, amorphous and/or avascular involution. Myxoid degeneration provokes homogeneous degradation of the tumor with a clear decrease in tumor density and a paradoxical transient increase in tumor size. Rarely, the tumor response to treatment is seen as an intratumor calcification <sup>[4Pidhorecky I., Cheney R.T., Kraybill W.G., Gibbs J.F. Gastrointestinal stromal tumors: current diagnosis, biologic behavior, and management Ann Surg Oncol 2000 ;  7 : 705-712 [cross-ref]

Cliquez ici pour aller à la section Références, 20DeMatteo R.P., Heinrich M.C., El Rifai W.M., Demetri G. Clinical management of gastrointestinal stromal tumors: before and after STI-571 Hum Pathol 2002 ;  33 : 466-477 [cross-ref]

Cliquez ici pour aller à la section Références, 21Dagher R., Cohen M., Williams G., Rothmann M., Gobburu J., Robbie G., et al. Approval summary: imatinib mesylate in the treatment of metastatic and/or unresectable malignant gastrointestinal stromal tumors Clin Cancer Res 2002 ;  8 : 3034-3038

Cliquez ici pour aller à la section Références, 22Hong X., Choi H., Loyer E.M., Benjamin R.S., Trent J.C., Charnsangavej C. Gastrointestinal stromal tumor: role of CT in diagnosis and in response evaluation and surveillance after treatment with imatinib Radiographics 2006 ;  26 : 481-495 [cross-ref]

Cliquez ici pour aller à la section Références, 23Warakaulle D.R., Gleeson F. MDCT appearance of gastrointestinal stromal tumors after therapy with imatinib mesylate AJR Am J Roentgenol 2006 ;  186 : 510-515 [cross-ref]

Cliquez ici pour aller à la section Références, 24Chen M.Y., Bechtold R.E., Savage P.D. Cystic changes in hepatic metastases from gastrointestinal stromal tumors (GISTs) treated with Gleevec (imatinib mesylate) AJR Am J Roentgenol 2002 ;  179 : 1059-1062 [cross-ref]

Cliquez ici pour aller à la section Références and 26Demetri G.D., Benjamin R., Blanke C.D., Blay J.Y., Casali P., Choi H., et al. NCCCN task force report: optimal management of patients with stromal tumors (GIST) expansion and update of NCNN clinical practice guidelines J Natl Compr Cancer Netw 2004 ;  2 : S1-S26

Cliquez ici pour aller à la section Références]</sup>.

Chemotherapy or adjuvant chemoembolization may be proposed as a last resort.

	Conclusion

A recently defined nosological entity, GIST is currently the most common mesenchymal tumor of the GI tract. Specific expression of c-kit (CD117) has enabled the development of a specific effective treatment (imatinib mesylate) and, consequently, the need for a confirmed diagnosis. The risks of diagnostic biopsy emphasize the importance of recognizing GIST from CT scans: a voluminous exophytic mass with heterogeneous enhancement, characterized by peripheral uptake surrounding hemorrhagic, cystic or necrotic central areas. Circumferential parietal thickening with homogeneous enhancement, tumor calcification, a spiculated mass, lymph node enlargement or obstruction are elements suggestive of a differential diagnosis.