Gastrointestinal stromal tumours

Small bowel GISTs resemble those of the stomach histologically, although epithe-lioid lesions are uncommon. Globoid extracellular collagen accumulations (so-called skeinoid fibers) are frequently observed, especially in benign small intestinal GISTs {1235}. Factors that correlate with malignancy are tumour size > 5 cm, mitotic count > 5 per 50 HPF dense cellularity, and mucosal invasion (rarely observed). Even with low or absent mitotic activity, tumours larger than 5 cm are considered to have malignant potential. Small intestinal GISTs are positive for KIT (CD117) and usually for CD34, and a subset (30-50%) are positive for a-smooth muscle actin; most tumours are negative for desmin and almost all are negative for S100-protein.

Leiomyomas and leiomyosarcomas are rare in the small intestine, and can be identified immunohistochemically by their smooth muscle actin and desmin expression and lack of KIT.

Angiosarcomas are recognized by an anastomosing proliferation of atypical endothelial cells. Immunohistochemical demonstration of CD31, less consistently von Willebrand factor, is diagnostically useful {1904}.

Fig. 4.25 A Stromal tumour of small intestine. B Cut surface of lesion illustrated in A.
Fig. 4.26 Small intestinal stromal tumour. Extracellular accumulation of skeinoid fibres produces eosinophilic globules.

Kaposi sarcomas may involve small intestine, either the mucosa alone or more extensively. Histologically typical are elongated spindle cells with vascular slits. Cytoplasmic PAS-positive hyaline globules are present in some tumour cells. Immunohistochemically, the lesion-al cells are positive for CD31 and CD34. Human herpesvirus 8 can be demonstrated by PCR.

Lipomas exhibit the same morphological features as their colonic counterparts.

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