Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

Many radiologists believe that at initial presentation an antral adenocarcinoma does not spread across the pylorus into the duodenal bulb, the exceptions being if a patient is immunocompromised or during late presentation with extensive tumor spread when bulbar involvement is common. Radiologists use this observation to differentiate among an antral cancer, lymphoma, and peptic ulcer disease, with the latter two readily extending across the pylorus. In a minority of patients, however, transpyloric spread occurs with an antral carcinoma. Duodenal invasion is either direct, through deep muscle layers or lymphatics, or through venules. Barium studies reveal a rigid, open, and eccentric pylorus and a deformed bulb. Computed tomography identifies infiltration extending across the pylorus.

A retrospective review found perceptual error to be the reason for not detecting 41% of 27 carcinomas by upper gastrointestinal radiography performed within 3 years prior to tumor diagnosis (34); the most common overlooked finding was an intramural depression.

Computed tomography tumor detection is aided by gastric distention with water (called hydro-CT). A hypotonic agent helps maintain gastric distention.

In a collection of advanced gastric cancers, a triphasic CT technique achieved a 98% detection accuracy (35); of these, 28% were best seen on parenchymal phase during gradual enhancement from the mucosal side. Computed tomography of a typical gastric carcinoma reveals focal gastric wall thickening and mild-to- moderate arterial phase enhancement. Extensive gastric wall thickening develops with tumor growth, with some tumors exhibiting marked enhancement. Computed tomography can miss early linitis plastica, although when it is well established, CT reveals diffuse gastric wall thickening. When extensive, scirrhous carcinomas result in extensive circumferential infiltration of the stomach, although some of these tumors are plaque-like in appearance. A less common linitis plastica cancer appearance is diffusely thickened gastric rugal folds with no apparent overlying ulcerations; the appearance mimics diffuse lymphoma, less often Ménétrier’s disease. Dynamic CT shows most nonscirrhous carcinomas to have either homogeneous contrast enhancement or at least a thin outer layer, whereas scirrhous ones tend toward

ADVANCED IMAGING OF THE ABDOMEN

a gastric wall consisting of two layers: a lower attenuation thick outer layer and a higher attenuation thick inner layer.

Computed tomography arteriography, performed by injecting contrast through a catheter in the celiac trunk, correctly detected seven of eight early gastric cancers and all 13 advanced gastric cancers (36). Computed tomography arteriography is not used as a primary tool for gastric cancer detection, but it is of potential use for cancer staging.

Endoscopic US shows promise in detecting gastric malignancies, achieving sensitivities over 95%. It delineates gastric wall and adjacent structure infiltration. Mucinous carcinomas result in a hyperechoic endoscopic US appearance due to mucinous content and surrounding fibrosis. Endoscopic US of scirrhous carcinomas reveals irregular hypoechoic thickening of submucosa and muscularis propria.

Some scirrhous carcinomas are hypointense on both T1and T2-weighted images, presumably due to the often associated fibrosis. Such a pattern is seen with linitis plastica. Little enhancement is evident with linitis plastica. Magnetic resonance reveals most gastric cancers as early enhancing thickened gastric wall tumors.

Both intestinal adenocarcinomas and various endocrine neoplasms contain receptors for vasoactive intestinal peptide (VIP) and scintigraphy with I-123 VIP appears advantageous. Binding of labeled VIP by primary tumors and metastases is visible for up to 24 hours in primary and recurrent gastric adenocarcinomas.

Endoscopy

Accuracy of gastric cancer detection with endoscopy varies considerably, especially with diffusely infiltrating cancers that are covered by intact mucosa. Even some biopsies are noncontributory, with cancer cells being dispersed in a fibrous matrix. Tumor location is better appreciated on an imaging study than with endoscopy.

In general, endoscopy of a polyp revealing normal overlying gastric mucosa suggests a mesenchymal tumor. Nevertheless, on rare occasion an adenocarcinoma will be mostly intramural and even invade to the serosa and not be detected by endoscopy or even biopsies.

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Early Gastric Cancer

Clinical

Controversy surrounds the definition of an early gastric cancer. In Japan, a diagnosis of an early gastric cancer is made considerably more often than in the West. Some observers believe that an early gastric cancer is simply an early stage of a conventional gastric cancer rather than a different disease entity, an opinion not universally shared. Early gastric cancers tend to be intestinal type rather than diffuse type. In Japan about 10% of early gastric cancers are multiple.

Metastasis to lymph nodes is rare if a gastric cancer is limited to the mucosa; nodal involvement is more common with submucosal tumor spread. In the occasional patient with a recurrence after curative resection for early gastric cancer, recurrence is believed to be secondary to occult metastases in perigastric lymph nodes.

Endoscopic resection of early gastric cancer is performed in some countries for elevated mucosal cancers <2cm in size and depressed mucosal cancers without ulceration <1cm in size. Although endoscopic resection is possible for some tumors with submucosal extension, those with deep submucosal invasion are not cured with this approach.

Imaging

Detection and staging cannot be discussed separately for early gastric cancers because the definition already implies that a tumor is limited to the mucosa or at most submucosa. Computed tomography has a limited role in detecting early gastric cancer. Among patients with an early gastric cancer, axial CT identified 64% of tumors, whereas 3D CT images revealed 94% (37). On the other hand, using a triphasic CT technique and a water-filled stomach, another study detected only 23% and achieved a staging accuracy of 15% (35); those early cancers detected were best seen during the arterial or parenchymal phases.

Early gastric cancers detected by CT are polypoid, elevated, or invade submucosa. Wall thickening with early gastric cancers is limited to the inner layer only. These cancers show CT contrast enhancement. Advanced cancers also have con-

trast enhancement, but gastric wall thickening generally is more diffuse.

Postcontrast CT (mucosal phase, 38 to 45 seconds after the start of contrast injection) in patients with early gastric cancer revealed three patterns (38): (1) localized thickening of an inner hyperenhancing layer; (2) focal interruption of an inner hyperenhancing mucosal layer; and (3) focal protrusion of the inner hyperenhancing layer. The lesions became less distinct on a delayed phase. Overall, CT early gastric cancer detection rate was 57%. Of note is that some advanced gastric cancers had an appearance similar to early gastric cancer. Considerable CT difficulty exists differentiating between T1 cancers with submucosal invasion and more advanced cancers.

Conventional endoscopy and endoscopic US have roughly similar accuracy in determining the depth of invasion with early gastric cancers.

Staging

The TNM staging classification is outlined in Table 2.4. Preoperative staging of gastric cancer is of obvious importance but is fraught with uncertainty. Intraoperative surgical assessment tends to overstage early invasion and understage deep invasion. Microcarcinosis, defined as scattered carcinoma cells within lymph node sinuses or pulp without surrounding stromal reaction, is common in otherwise pN0 tumors; the number of detected tumor cells and involved lymph nodes carries prognostic significance, a significance differing from that found with gross lymph node metastasis.

Primary gastric cancer typically spreads to adjacent lymph nodes and liver. In a prospective study of regional lymph nodes from consecutive patients with primary gastric cancer, the mean diameter of tumor-free lymph nodes was 4.1mm and those infiltrated by metastases was 6.0mm (39); a practical problem is that although 80% of tumor-free lymph nodes were <5mm in diameter, 55% nodes containing metastases were also <5mm in diameter and the authors concluded that lymph node size is not a reliable indicator for lymph node metastasis. In spite of such findings, many authors assume an arbitrary boundary between normal and metastatic nodes. Currently no imaging modality is sufficiently accurate to reliably predict lymph node involvement. Perigastric lymph

Table 2.4. Tumor, node, metastasis (TNM) staging of gastric carcinoma

Primary tumor:

Lymph nodes:

Nx Regional nodes cannot be assessed

N0 No metastases in regional lymph nodes

N1 Metastases in 1 to 6 regional lymph nodes

N2 Metastases in 7 to 15 regional lymph nodes

N3 Metastases in more than 15 regional lymph nodes

Distant metastasis:

Mx Distant metastases cannot be assessed M0 No distant metastases

M1 Distant metastases

Source: From the AJCC Cancer Staging Manual, 6th edition (2002), published by Springer-Verlag, New York, NY, used with permission of the American Joint Committee on Cancer (AJCC), Chicago, IL.

nodes can be infiltrated by tumor but still be normal in size; conversely, some enlarged nodes do not contain tumor. Compounding the issue is that a nonneoplastic reactive infiltrate develops in some lymph nodes close to a cancer. Such sarcoid-like epithelioid cell lesions are detected in regional lymph nodes in about one third of gastric cancers; they are independent of tumor stage, type, and tumor grade, and do not aid in prognosis.

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Fluoroscopic guidance of nasogastric biopsies is feasible to establish boundaries of a known gastric carcinoma, but this technique has achieved only limited acceptance.

Comparing published CT, US, and MR gastric cancer staging results is fraught with difficulty. Sensitivities vary markedly, depending on the basic assumptions used. As a gross approximation, helical CT and endoscopic US appear to provide similar T and N staging accuracies. Most problems arise in differentiating T2 and T3 tumors.

Computed Tomography

Computed tomography staging of gastric cancer has been rather disappointing and continues to be controversial. Some patients have obliteration of adjacent fat planes due to cachexia or inflammation. Some CT studies have achieved a tumor (T) and node (N) staging accuracy of only about 50% (40), although multislice triphasic CT results are more accurate. In general, CT differentiation between T3 and T4 cancers is not sufficiently accurate to predict resectability. It is accurate, however, in detecting distant metastases, and some surgeons rely on CT primarily to detect these metastases. Computed tomography more often suggests correctly unresectability rather than resectability.

Depth of tumor invasion and serosal invasion are believed to be difficult to determine even with helical CT. Yet in establishing serosal invasion, use of hydro-CT, induced hypotonia, and prone patient position has achieved a sensitivity of 100% and specificity of 80% to 87% (41).

The ability of multislice CT to detect lymph nodes depends on node size; roughly half of nodes <10mm and about 75% of those >10mm are detected. Detecting tumor in a lymph node is another matter; intrinsically, nothing in the CT appearance suggests that a particular lymph node is involved by tumor.

Staging accuracy of CT arteriography in patients with gastric cancer was 77% for serosal invasion and 76% for regional lymph node metastasis (36); of note is that accuracy for T staging of early gastric cancers was only 50%.

Whether multislice 3D images aid in staging gastric cancers remains to be determined.

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Ultrasonography

Ultrasonography detection of motion (the sliding sign) between a gross gastric cancer and pancreas aids in assessing pancreatic invasion. In patients with advanced gastric cancer, the presence of such a US sliding sign yielded 80% sensitivity and 96% specificity in detecting pancreatic invasion (42); the presence of sliding motion by a tumor against the pancreas on respiration or on extrinsic transducer compression and a preserved fat plane between the two structures signified the lack of pancreatic invasion.

Theoretically, endoscopic US with its high resolution should be able to detect submucosal or deeper invasion and thus aid in differentiating early gastric cancer from an advanced one. Published studies suggest that endoscopic US is superior to both CT and intraoperative surgical staging, achieving a T-stage accuracy of about 60% to 90% and an N-stage accuracy of 65% to 70%. Endoscopic US-guided fine needle aspiration cytology is an option for evaluating gastrointestinal wall infiltration and even lymph node staging. Staging is limited for large carcinomas extending beyond the stomach. The most frequent cause of understaging depth of invasion is microscopic tumor spread, whereas surrounding inflammation leads to overstaging. Nodal staging by endoscopic US should be approached cautiously; most published results are on a per patient basis; expressed on a per node basis, accuracy falls considerably, similar to other imaging modalities.

Magnetic Resonance Imaging

Limited data are available about the role of MRI in staging gastric cancer. Gastric distention with an oral contrast agent appears advantageous in these patients, but no major difference between water and gadolinium is apparent (43).

Current evidence suggests no significant accuracy differences in either T or N staging between preoperative MRI and CT. For example, the T stage was overstaged with MRI in 7% and CT in 10% and understaged with MRI in 20% and CT in 23%. The N stage was overstaged with MRI in 10% and CT in 7% and understaged with both MRI and CT in 34% (44). Comparable results have been achieved in other studies. Relative accuracy of lymph node metastases varies considerably between MR studies, ranging from

only about 50% to over 90% with some techniques. In general, MRI tends to be superior to CT in T staging, but CT is superior in N staging.

Compared to surgical staging, MRI staging is accurate for 75% to 80% of tumors, being higher for more extensive tumors (45); an interrupted hypointense band or enhancing tumor penetrating through the gastric wall signifies tumor penetrating serosa.

Therapy

For Cure

The highest cure rates are reported from Japan, where early gastric cancer detection is greatest. Outside of Japan, advanced disease is common. In many Western centers most of these cancers are already advanced, and cure rates are dismal. Typical statistics are that only two thirds of gastric cancer patients undergo surgery and of these in only half is tumor resection feasible. Generally age is no barrier, and in patients aged 80 years or older with gastric cancer some investigators achieve crude 5-year survival rates of over 40% in those amenable to curative resection (46). Parameters predicting survival in patients with advanced gastric cancer undergoing potentially curative resection are depth of tumor invasion, lymph node invasion, and tumor histologic classification (Fig. 2.14).

Some patients with an unresectable gastric cancer undergo chemotherapy in an attempt to downstage the cancer. Results of chemotherapy, evaluated by CT, have been mixed. Some gastric hepatoid adenocarcinomas respond to adjuvant chemotherapy.

Palliation

Whether patients with metastatic gastric cancer should undergo palliative resection, even laparoscopic, is questionable. But before we adopt a fatalistic outlook, however, it should be pointed out that although most patients with stage IV gastric cancer have a poor prognosis, some of these patients with a resectable cancer do survive for 5 years or longer.

Palliation with self-expanding flexible metallic stents is an option for inoperable malignant gastric outlet obstruction; both peroral and percutaneous gastrostomy approaches are used. In

Figure 2.14. Recurrent gastric carcinoma after a Billroth II resection 6 months previously for antral cancer. Most of the residual stomach is now infiltrated.

general, results are less favorable than in the esophagus. Successful palliation of patients with malignant gastric or duodenal obstruction using self-expandable covered metallic stents can be achieved in over 75% (47,48). Complications consist of stent migration and fracture. Overall, a stent improves quality of life, restores oral food intake, and relieves vomiting in these patients with a limited life expectancy.

Metastasis/Recurrence

Following gastric cancer resection, the most common recurrence is either in regional lymph nodes or in distal organs. Metastasis to hepatoduodenal nodes leads to obstructive jaundice. Duct obstruction is due to not only compression by surrounding nodes but also direct bile duct invasion from nodes. Patient survival with liver metastasis is short, although an occasional longterm survivor is reported.

Metastasis to cervical lymph nodes currently is seldom encountered. The presence of cervical lymphadenopathy, however, is not pathognomonic of cancer spread, and a biopsy is necessary.

Bone metastases are evaluated by Tc-99m bone scans. Common metastatic sites are spine, then ribs, pelvis, femur, and skull. These sites are also common for other malignancies. A direct correlation exists between bone scan results and serum alkaline phosphatase levels.

Occasionally a gastric carcinoma extends as a tumor thrombus via gastric veins into the portal vein; intraoperative US should be considered when such a tumor thrombus is suspected.

Gastric scirrhous carcinomas readily metastasize to other bowel. Direct spread and invasion to transverse colon via the gastrocolic ligament is a feature of advanced gastric cancers.

More unusual metastases are to pericardium, gallbladder, and skin. Solitary metastases have been to the spleen and bladder. Even leptomeningeal carcinomatosis has developed before a primary gastric cancer became symptomatic.

Adenosquamous/Squamous Cell Carcinoma

Both adenosquamous and squamous cell gastric carcinomas are rare. These tumors have a predilection for the proximal part of the stomach. Squamous metaplasia, presumably from prior mucosal injury, appears to play a role in the origin of squamous cell carcinomas.

Adenosquamous carcinomas tend to invade extensively and even if limited to the submucosa can already have metastasized.

Small Cell Carcinoma

Small cell carcinomas are rare. Their prognosis is very poor compared with that of more common types of gastric cancers.

The imaging appearance of gastric small cell carcinomas is variable and nonspecific; described CT features consist of a bulky exophytic tumor showing little peritumoral infiltration and having little contrast enhancement (49).

A gastric small cell carcinoma developed in a man with a 5-year history of progressive systemic sclerosis (50); serum CEA and neuronspecific enolase levels were elevated, and imaging revealed a sharply marginated and ulcerated tumor.Although esophageal and some

sion, they infiltrate extensively and develop into advanced gastric lymphomas. Histologically, these tumors are difficult to diagnose, with a correct initial biopsy diagnosis made in only about two-thirds of patients; a biopsy diagnosis can be improved with immunohistochemical and molecular biology techniques.

Low-grade primary gastric lymphomas involve Peyer’s patches rather than lymph nodes. Confusing the issue is that reactive lymphoreticular hyperplasia is common in chronic gastritis, especially if associated with H. pylori infection, with this lymphoid tissue presumably having a role in a local immune response. Previously called pseudolymphoma by pathologists, extensive lymphocytic infiltration or the presence of large lymphoid follicles should raise the suspicion of MALT lymphoma.

Follow-up of patients with gastric lesions difficult to distinguish between reactive lymphoreticular hyperplasia and MALT lymphoma has led to an eventual diagnosis of MALT lymphoma in one half (52); metastases were evident in several patients.

Imaging

The most common imaging appearance of a primary non-MALT gastric lymphoma is that of an ulcerating tumor, an appearance less often seen with small bowel and colon lymphomas.

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Some of these ulcerating tumors consist of a huge ulcer surrounded by a sparse soft tissue mass. Less common is intramural infiltration. Growing intramurally, these lymphomas tend either to thicken rugae massively or to result in a linitis plastica appearance. Computed tomography in these patients reveals marked circumferential gastric wall thickening (Fig. 2.15).

Lymphoma is more common in the distal half of the stomach. Some extend through the pylorus into the duodenum, but gastric outlet obstruction is not common and helps differentiate lymphoma from an antral adenocarcinoma. Associated adenopathy is common and tends to be more extensive than seen with gastric adenocarcinoma. A rare gastric lymphoma results in a fistula to an adjacent organ.

Endoscopic and imaging findings of lowgrade MALT gastric lymphoma ranges from a grossly normal appearance, erosions (flat type), ulcerations (depressed type), and a nodular cobblestone appearance due to lymph follicle prominence, to thickened rugae (elevated type). Only a minority of these lymphomas have focal gastric wall thickening. The appearance thus mimics that of gastritis, ulcer, early gastric carcinoma, or even a metastasis, such as a malignant melanoma or, in an AIDS patient, Kaposi’s sarcoma. Multiple nodules mimic a polyposis syndrome. Thickened rugae suggest Ménétrier’s disease. The presence of disorganized rugae,

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poorly defined ulcer margins, and multiple lesions suggests lymphoma. Low-grade MALT lymphomas tend toward a nodular, almost confluent appearance on double-contrast barium studies, and high-grade ones tend toward a more polypoid, ulcerated tumor.

A double-contrast gastrointestinal examination is the preferred imaging test to detect these tumors. Computed tomography is of limited help in detecting low-grade MALT lymphoma. With high-grade lymphoma, CT detects not only a gross tumor but also any associated adenopathy. Computed tomography shows poor enhancement of those tumors with a noticeable intramural component.

Endoscopic US is of limited value in detecting low-grade MALT lymphomas because in some of these tumors the gastric wall is not thickened. Wall thickening, when present, consists predominantly of mucosa alone and/ or submucosa. In those with a thickened wall, however, endoscopic US may document regression to a normal-thickness wall, keeping in mind that in some patients the mucosa and submucosa remain thickened even in remission.

After radiation therapy a gastric lymphoma often regresses to a normal appearance. Occasionally seen are radiating folds to a centrally located ulcer or scar.

Computed tomography in a 54-year-old man with primary gastric Burkitt’s lymphoma showed marked gastric wall thickening but lymph nodes tend not to be enlarged (53).

Figure 2.16. Gastric involvement with chronic lymphocytic leukemia results in thick, irregular folds (arrows) and ulcerations.

ance ranging from a depressed tumor pathologically representing a plasma cell granuloma; a nodular, submucosal tumor containing atypical plasma cells; a sessile polypoid appearance; to extensive gastric wall infiltration. Plasmacytomas appear similar to carcinomas and lymphomas. Aside from confirming intramural infiltration, CT is noncontributory. Plasmacytomas tend to enhance poorly, similarly to lymphomas. Associated adenopathy is not common.

Leukemia

Leukemic infiltration of the stomach generally does not lead to bleeding.

The imaging appearance of leukemia involving the stomach ranges from normal to the occasional massive rugal thickening (Fig. 2.16). A rare gastric leukemia presents with intraluminal polyps.

Plasmacytoma

Some plasmacytomas grow rapidly. Weight loss and upper gastrointestinal bleeding are typical presentations.

Barium studies of the few reported primary gastric plasmacytomas reveal a varying appear-

Sarcoma

Some sarcomas manifest by their bulk, whereas others result in chronic blood loss. An occasional one presents with an acute bleed, usually into the stomach but at times intraperitoneally. Hypoglycemia is an occasional presentation.

Presence of a gastric stromal tumor (or an autonomic nerve origin tumor), extraadrenal paraganglioma, and a pulmonary chondroma signifies Carney’s triad, with at least two of these findings needed for a presumptive diagnosis. Some evidence suggests that Carney’s triad is a disorder of the autonomic nervous system rather than a multiple endocrine neoplasia syndrome or multiple hamartoma syndrome.

Gastrointestinal Stromal Tumors (GISTs)

The terms stromal tumors, smooth muscle tumors, and mesenchymal tumors are often used interchangeably, although not all mesenchymal tumors are stromal in origin.Also, some authors lump all stromal tumors into one category and simply refer to them as GISTs, others define GISTs as soft tissue sarcomas originating from gastrointestinal tract mesenchymal cells, and still others limit this term primarily to undifferentiated sarcomas. Most sporadic GISTs contain somatic c-kit gene mutations, believed to be pathogenetic for stromal tumors originating from interstitial cells of Cajal, and thus GISTs can be defined by the presence of appropriate immunoreactivity, an approach preferred by a number of experts in this field. On the other hand, stromal tumors in neurofibromatosis type 1 patients do not contain detectable c-kit gene mutations, and in these patients presumably develop by a different mechanism (54). The term pure stromal tumor as a synonym for an undifferentiated tumor appears inappropriate. The major significance of GISTs is their particular sensitivity to imatinib mesylate (Gleevec), a selective tyrosine kinase inhibitor and a cancer therapeutic agent. Most stromal tumors contain mutations in the KIT receptor tyrosine kinase or mutations in the platelet-derived growth factor receptor a (55).

Based on immunohistochemical and other techniques, stromal tumors can be subdivided into (1) those differentiating toward smooth muscle, (2) those differentiating toward neural elements, (3) tumors containing both cell types, and (4) undifferentiated stromal tumors.

Undifferentiated stromal tumors signify those undifferentiated or poorly differentiated tumors of stromal origin, which are difficult to subclassify further.

Gastrointestinal stromal tumors occur most commonly in the stomach, followed by small bowel, then rectum, colon, and least often in the esophagus. The most common stromal tumor in the esophagus is a leiomyoma, and for clinical and prognostic purposes, whenever possible, it should be classified as such. Most stromal tumors originate from muscularis propria, with only a small minority from muscularis mucosa; most of the latter are in the colon and present as intraluminal polyps, although current evi-

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dence suggests that these colonic tumors are leiomyomas and not GISTs. Histologically, most GISTs are spindle cell in appearance, with a minority being epithelioid. In the past, many of the latter were classified as leiomyoblastomas or epithelioid leiomyosarcomas (these are discussed in a subsection below). A minority of stromal tumors contain a prominent myxoid component and some of these are at the borderland of being considered GISTs.

Gastrointestinal stromal tumors’ malignant potential is estimated based on their mitoses per high power field. Patient survival is significantly related to number of mitoses.

The imaging appearance of GISTs varies markedly; they range from mostly exophytic, intramural, intraluminal, to a dumbbell-shaped appearance (56). They predominate in the gastric fundus and body. Small tumors have a sharp margin, tend to be mostly intraluminal and have a homogeneous density on unenhanced and contrast enhanced images (57); with growth, they become irregular in outline, have a larger extraluminal component and are more inhomogeneous in density. Large ones infiltrate adjacent organs. Presence of metastases establishes malignancy. When detected with barium or CT, many appear large, bulky, and ulcerated (Fig. 2.17). Stromal sarcomas tend to be solitary, multilobulated and mostly exophytic. Necrosis and hemorrhage are common. Interestingly, GISTs rarely obstruct bowel lumen. Ulceration of overlying mucosa is common with the larger ones, accounting for their propensity to bleed. Computed tomography and MRI readily outline the extraserosal extent of both benign and malignant varieties. Coronal and sagittal CT and MRI reconstructions are at times useful in establishing the organ of origin of large GISTs. Postcontrast, these tumors have variable enhancement except in regions of necrosis. Calcifications are rare. These stromal tumors rarely metastasize to lymph nodes, and the presence of enlarged adjacent nodes should suggest another diagnosis. Except when metastases are present, imaging cannot differentiate benign stromal tumors from malignant ones or other solid tumors such as neuroendocrine ones. Endoscopic US findings in 35 benign and malignant stromal cell tumors suggest that a tumor diameter of >4cm and an irregular extraluminal border, hyperechoic foci, and cystic regions point toward a malignancy (58).

Preoperatively, a pathologic diagnosis is often not made; endoscopic cytology is generally negative and even endoscopic biopsy is often negative.

Leiomyosarcoma

Most gastric leiomyosarcomas discussed in the older literature would now be classified as GISTs.

Multiple gastric leiomyosarcomas are rare. Imaging readily identifies these tumors (Fig. 2.18). In children, some gastric leiomyosarcomas reveal regions of necrosis and calcification, findings less common in adults.

Prognosis depends on tumor size, degree of mitotic activity, and nuclear grading. A poor prognosis can be predicted if four or more mitoses are identified per 20 high power fields or if severe nuclear atypia is present. Ulceration