A primary solitary tumor of the lesser omentum with immunohistochemical features of gastrointestinal stromal tumors.

We here present a primary solitary tumor of the lesser omentum that was found in a 71-yr-old woman. Differential diagnosis could not be made preoperatively; therefore, histopathological examination including immunohistochemical studies were performed to determine the nature of the tumor. The resected specimen, measuring 17 cm at the largest point, consisted of the outer solid part and the inner multiloculated cysts. Microscopically, the tumor was characterized by interlacing bundles of elongated spindle cells, with the nuclei focally showing a palisading pattern. However, skeinoid fibers were not observed anywhere. One to three mitoses per 50 high power fields were observed. Immunohistochemically, the tumor was negative for S-100 protein and smooth muscle-specific actin, but stained positive for CD34. The microscopic features were consistent with those of potentially malignant gastrointestinal stromal tumors. Stromal tumors that represent the differentiation toward neither typical leiomyomas or schwannomas rarely occur in the lesser omentum with only one such instance having been reported to date. Due to this rarity, it is difficult to make the differential diagnosis preoperatively, even with existing imaging techniques, and predicting the clinical behavior of such omental tumors is also often difficult. Therefore, complete resection should be performed when such tumors are encountered in daily practice.