Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

STOMACH

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.

ADVANCED IMAGING OF THE ABDOMEN

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,

STOMACH

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-

ADVANCED IMAGING OF THE ABDOMEN

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