We presented a case which was strongly suspected of an extrapleural malignant solitary fibrous tumor within the muscle. A 74-year-old man presented a painless mass in his left thigh in May 1999. The lesion was depicted as a mass in his vastus lateralis muscle on MRI. The histological features of the tumor at open biopsy were spindle cell proliferation with myxoid and fibrocollagenous background and positive immunoreactivity for CD34. It was compatible for solitary fibrous tumor. The tumor was resected as a wide resection, and the patient received radiation therapy in the operation region. A lesser part of the resection specimen showed a malignant pattern. It was therefore suspected as a very rare malignant solitary fibrous tumor, only one reported to our knowledge.
