A large retroperitoneal malignant solitary fibrous tumor.

We report on a large, retroperitoneal, malignant, solitary fibrous tumor (SFT) with high proliferation activity. A 43-year-old man was admitted to our department complaining of a palpable mass. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed a large retroperitoneal tumor occupying the entire abdominal cavity. A laparotomy was performed for diagnosis and treatment, which revealed a tumor in the retroperitoneum but with no invasion to the surrounding organs, thereby allowing safe macroscopic excision. Histologically, the tumor was composed of spindle-shaped cells with patternless pattern and a hemangiopericytomatous appearance. Moreover, immunohistochemical staining was positive for CD34, vimentin, Bcl-2, and CD99 and negative for desmin, S-100p, and smooth muscle actin (AMA). The tumor exhibited high cellularity, moderate mitotic activity, pleomorphism, necrosis, and hemorrhagic changes. In addition, the Ki-67 labeling index was 37%. These findings confirmed the diagnosis of malignant SFT with high proliferation activity. Subsequently, adjuvant doxorubicin plus ifosfamide chemotherapy was performed. No signs of recurrence were observed 12 months after the surgery.