Mesenteric fibromatosis with involvement of the gastrointestinal tract. A GIST simulator: a study of 25 cases.

Mesenteric fibromatosis (MF) and gastrointestinal stromal tumors (GISTs) are distinct lesions, but they often are confused with each other. Correct identification is essential because of their vastly different therapeutic and prognostic connotations. We reviewed 25 cases of MF with involvement of the wall of the gastrointestinal tract and found that GIST was the most common misdiagnosis (13 [52%]). MF was characterized by a spatially homogeneous proliferation of wavy spindle cells without atypia, associated with collagen deposition (often of the keloidal type), and an infiltrative border. Most cases displayed prominent muscular arteries and dilated, thin-walled veins. The mitotic count was relatively low, and no atypical mitotic figures were identified. These features are sufficiently characteristic of MF to permit distinction from GIST on the basis of routinely stained sections in the large majority of the cases, but immunohistochemical analysis provides a supporting role in the few equivocal cases.