A malignant solitary fibrous tumor in the retroperitoneum.

Solitary fibrous tumor in the retroperitoneum is rare. We report a case of a malignant solitary fibrous tumor in the retroperitoneum, with a lymph node metastasis, in a 48-year-old-woman. Computed tomography and magnetic resonance imaging indicated a mass in the retroperitoneal space beside the left kidney, with a swollen paraaortic lymph node. Surgical findings revealed a circumscribed tumor in the retroperitoneum. A pathohistological examination revealed two separate components: a benign hypocellular area rich in collagen fibers and a malignant hypercellular area with nuclear atypia. The histological finding for the lymph node was the same as that in the hypercellular area. Because of these findings, the mass was diagnosed as malignant solitary fibrous tumor. Only surgical treatment was performed, and the patient is alive without recurrence 2.5 years after the surgery. Immunohistochemical staining for p53 was positive in the malignant component but negative in the benign area, indicating that the expression of p53 may play a critical role in the malignant transformation of solitary fibrous tumor.