Small intestinal stromal tumors with skeinoid fibers. Clinicopathological, immunohistochemical, and ultrastructural investigations.

Microscopic appearances of spindle cell tumors of the gastrointestinal tract are suggestive of smooth muscle origin; however, they usually lack specific muscle cell features by electron microscopy and immunohistochemistry, thus justifying their designation as stromal tumors. The present report describes nine cases of small intestinal stromal tumors with eosinophilic stromal globules composed of tangles of curved fibers with crossbands simulating an appearance of skeins, designated as skeinoid fibers. Patients' ages ranged from 28 to 87 years; and four were male. The tumors presented as well-delineated mural nodules ranging from 1.8 to 13 cm in size, causing intestinal obstruction or hemorrhage. Four were in the duodenum, three in the jejunum, and two unspecified. Microscopically, seven were benign; one, to the largest, was definitely malignant and metastasized to the liver. Another, the second largest (7.5 cm), showed moderate atypia with two mitoses per 10 high-power fields. The light microscopic appearance, including immunohistochemistry, were typical for small intestinal stromal tumors. Skeinoid fibers were strongly periodate-Schiff's procedure-positive and stained blue with the trichrome stain. They appeared as a few micra-sized specks to large globules reaching a few millimeters. Skeinoid fibers were also found in three neurogenic spindle cell tumors (an acoustic neuroma, a neurofibroma, and a plexosarcoma of the mesentery), suggesting that such fibers are possible ultrastructural markers for neurogenic tumors and thus small intestinal stromal tumors with skeinoid fibers are neurogenic in origin.