Dedifferentiation and progression of an intracranial solitary fibrous tumor: autopsy case of a Japanese woman with a history of radiation therapy of the head during infancy.

Solitary fibrous tumor (SFT) is usually an indolent neoplasm with a low rate of local recurrence and metastasis. Although dedifferentiation of low-grade sarcoma is well documented, the concept of dedifferentiated SFT was not recognized until recently. A case of intracranial SFT with seven recurrences within 5 years, showing progression and dedifferentiation during the course of disease, is reported here. A 51-year-old woman with a history of irradiation during infancy presented with a SFT in the right posterior fossa. Because of the close proximity to the brain stem, the tumor could not be removed completely. The tumor recurred 12, 16, and 28 months after the initial operation. With the repeated recurrences, cellularity, mitotic count, and Ki-67 (MIB-1) index increased gradually. The histology suddenly changed at the fourth recurrence, which occurred 16 months after postoperative radiation therapy for the third recurrence. The tumor revealed a fibrosarcoma-like appearance with necrosis and markedly increased mitotic activity. The tumor further recurred 50, 52, and 55 months after the initial operation with the same fibrosarcoma-like histology. The patient died of uncontrolled tumor 58 months after the initial operation. In this case radiation may have played some role in the tumorigenesis, progression, and dedifferentiation of the SFT.