Malignant metastatising solitary fibrous tumor of the ovary with additional dedifferentiation and osteoid deposition: an unusual presentation, with a brief description of a diagnostic alghoritm

Summary Solitary fibrous tumor (SFT) is a rare spindle cell neoplasm showing fibroblastic differentiation, initially observed in the pleura, but now currently recognized to develop in any extrapleuric location 1,2. In the female genital tract, SFTs are extremely rare and have a predilection for the vulva, vagina and cervix 2,3. There are very few cases of ovarian SFTs having been reported in the literature. Malignant SFTs of the ovary are exceedingly rare neoplasms characterized by their mesenchymal origin and distinctive histopathological features. First identified as a separate entity in soft tissues, SFTs of the ovary represent a diagnostic and therapeutic challenge due to their rarity and overlapping characteristics with other ovarian neoplasms. These tumors are generally considered benign, but their malignant variants can exhibit aggressive behavior, including metastasis and recurrence. The pathogenesis of SFTs is associated with molecular abnormalities, particularly NAB2-STAT6 gene fusions 4, which play a crucial role in diagnosis and may have prognostic implications. Our case of ovarian malignant SFT showed an unusual pattern of dedifferentiation. The conventional SFT component displays a pattern less architecture, uniform fibroblastic morphology, prominent branching vessels, and is diffusely positive for CD34 and STAT6. However, there is an abrupt transition to a pleomorphic, high mitotic rate component with fascicular spindle cell morphology resembling a smooth muscle neoplasm. This dedifferentiated area is positive for SMA and desmin but negative for CD34 and STAT6 and includes focal ossification. Stains for MDM2, CDK4, and caldesmon are negative. The case is notable for its atypical progression, as most dedifferentiated SFTs transition directly from a benign-appearing SFT to a high-grade component without signs of malignancy in the conventional SFT region.