Retroperitoneal cellular schwannoma: report of a case diagnosed by the presence of S-100 protein.

A case of cellular schwannoma originating from the retroperitoneum is reported. The histological features of the tumor resembled those of a smooth muscle tumor. However, the tumor cells were positive for S-100 protein, which is almost exclusively identified within nerve sheath tumor cells. This finding confirmed the diagnosis of cellular schwannoma. This type of tumor is generally considered to be benign but is sometimes mistaken for a malignant tumor. These findings indicate that it is important to prove whether or not S-100 protein is present in the cells of a tumor of unknown origin especially arising in the mediastinum or retroperitoneum.