An S100 negative granular cell tumor with malignant potential: report of a case.

Malignant granular cell tumors are rare, but exhibit typical histologic, immunohistochemical, and electron microscopic features that also characterize their benign counterparts. Although most granular cell tumors are S100 protein positive, we report an S100 negative granular cell tumor with histopathologic evidence supporting malignancy. This tumor is best categorized as a malignant granular cell tumor rather than a granular cell variant of a malignant histiocytic, mesenchymal, or epithelial neoplasm. A multidisciplinary approach is essential to diagnosis.