Cellular schwannoma is a variant that is highly cellular and shows increased mitotic activity. They are usually solitary and well-circumscribed but may be multinodular or plexiform. They are more commonly found in deep-seated locations such as the posterior mediastinum or retroperitoneum. The tumors predominantly show Antoni A areas composed of dense interlacing fascicles of spindled Schwann cells (as shown here). The hypocellular Antoni B foci are either completely absent or small when present (< 10% of the tumor). Mitotic activity is generally < 4 mitoses/10 HPF. Focal necrosis is present in about 10% of cases. The differential diagnosis of cellular schwannoma includes fibrosarcoma and leiomyosarcoma. Features favoring cellular schwannoma include: circumscribed tumor (capsule may or may not be present), high cellularity but disproportionately low mitotic activity, perivascular hyalinization, hypocellular Antoni B areas (when present), and diffuse strong immunoreactivity for S-100 protein. The biologic behavior of cellular schwannomas is benign. Recurrences are rare when completely excised and metastases have not been reported.