Gastrointestinal Stromal Tumors GISTs

Histologically, GISTs are typically spindle cell tumors that have a prominent, nerve sheath tumor-like nuclear palisading pattern. Other GISTs may show prominent perinuclear vacuoles. GISTs may also have an epithelioid appearance, containing cells with round nuclei and abundant cytoplasm. It is now believed that GISTs are derived from the intestinal cells of Cajal. Recent application of immunohistochemical studies has revealed strong and uniform expression of the KIT (CD117, stem cell factor receptor) protein and CD34 in GISTs. This offers the possibility to accurately diagnose these tumors and separate them from other mesenchymal tumors of GI tract.

Benign GIST is the most common symptomatic benign neoplasm. Its type of growth is reflected in its radiological appearances. A broad-based, round or semi-lunar filling defect is usually seen with intraluminal tumors, while a mass effect on neighboring loops is seen with ex-traluminal benign GISTs. Dumb-bell type tumors combine features of both. Deformity of the intestinal wall, mucosal ulceration and signs of intussusception may also be seen. Despite their distinctive tendency to bleed, ulceration is rather infrequently demonstrated in intestinal benign GISTs, on enteroclysis. When seen, ulcerations are usually single, small, well-defined and round or linear in shape. Intussusception may be an additional feature of a benign GIST, easily depicted by either CT or enteroclysis. Besides enteroclysis, CT may contribute to the preoperative diagnosis of such neoplasms, by detecting unsuspected pathology and localizing it within the small bowel, or additionally characterizing pathology detected on barium studies. On CT, benign GISTs usually present as round, smoothly-outlined, homogeneous soft tissue masses, associated with the intestinal wall, while showing marked contrast enhancement. When large, such le sions may displace or deform adjacent small bowel loops.

Malignant GISTs grow slowly, predominantly extra-luminally and eccentrically, and are prone to develop degenerative changes such as necrosis, hemorrhage, calcification, fistula or secondary infection. Determination of the malignant potential of GISTs is based on factors such as location, tumor size, degree of cellularity and pleo-morphism, and presence or absence of necrosis. Small intestinal GISTs sized less than 5 cm are usually benign, regardless of their cellularity. However, GISTs greater than 10 cm in size and/or with mitotic counts greater than 5/50 high power field usually behave in a malignant fashion. Such tumors have a high risk for liver metastases and/or diffuse intra-abdominal spread. Bone and lung metastases are rare.

The radiologic appearances of malignant GISTs are fairly characteristic. On barium studies the main feature is frequently a large, extrinsic, non-obstructing mass displacing or distorting adjacent barium-filled loops of intestine. This may be associated with ulceration, cavitation or fistula formation. Less often, a GIST may appear as a large cavity filled with barium and it may be difficult to identify the connection between the small intestine and the cavity.

A CT scan may add considerably to the pre-operative evaluation of these tumors. CT can accurately demonstrate the size, shape and extent of the lesion, uniformity of densities and enhancing patterns, and it can depict the presence of liver, peritoneal or other metastases. CT is useful in the differentiation from other malignant tumors that often have a predominantly submucosal location and/or appear largely excavated, such as lymphoma or metastatic melanoma. The main differential diagnosis of malignant GISTs, however, includes their benign counterparts, benign smooth muscle or neurogenic tumors. CT criteria favoring malignancy include an irregular, lobu-lated, large-sized mass, heterogeneous tissue density, central liquefactive necrosis, seen as water density with or without air fluid level, ulceration or fistula formation. Liver metastases from malignant GISTs are large, necrot-ic or cystic in nature with peripheral or 'rim' enhancement, whereas peritoneal metastases may appear as widely distributed, multiple, round, smoothly-outlined, homogeneous satellite masses.

The signal intensity on MRI suggests that GISTs usually have no fibrotic component, and that the inner portion of these tumors does not usually contain blood products. Their signal intensity on MRI has been reported to be hyperintense compared to fat on Tl-weighted image. They also show intense enhancement after gadolinium-chelate injection (Fig. 5).

GISTs are distinctive compared to other malignant small intestinal neoplasms as they have a greater tendency to grow extra-luminally, to develop large ulcers and therefore to bleed, and attain a large size without obstruction. They do not normally metastasize to the regional lymph node and they have a larger survival rate, even despite metastases.

Magnetic Resonance Metastatic Intestinum

Fig.5. MR enteroclysis showing jejunal intussusception due to a benign GIST tumor. A round well-circumscribed mass of intermediate signal intensity is shown in a coronal true FISP image (a). Post gadolinium coronal 3D FLASH image (b) demonstrates marked homogeneous enhancement of both the tumor mass and the viable intestinal wall

Fig.5. MR enteroclysis showing jejunal intussusception due to a benign GIST tumor. A round well-circumscribed mass of intermediate signal intensity is shown in a coronal true FISP image (a). Post gadolinium coronal 3D FLASH image (b) demonstrates marked homogeneous enhancement of both the tumor mass and the viable intestinal wall

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