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Small intestinal stromal tumors with skeinoid fibers in neurofibromatosis: report of four cases with ultrastructural study of skeinoid fibers from paraffin blocks.

Small intestinal stromal tumors (SIST) with skeinoid fibers (SF) are reported in four patients with neurofibromatosis. SF appeared as intercellular eosinophilic globules that stained positively on periodic acid-Schiff stain and bluish on Masson trichrome stain, like those reported previously. Electron microscopic confirmation of SF was carried out on tissue retrieved from paraffin blocks, which revealed diagnostic ultrastructural features of SF. The association of SIST with SF and neurofibromatosis enhances the diagnostic value of SF as a marker of neurogenic spindle cell tumors. This study also underscores the importance of electron microscopy for differentiation of SF from pseudoskeinoid structures, which may mimic SF on light microscopic examination.