Two cases of primary cellular fibroblastic tumor of the ovary are described, which presented a challenging clinical and histopathological differential diagnosis of primary ovarian fibrosarcoma and mitotically active cellular fibroma. On sonographic evaluation they both appeared as solid lesions with irregular margins, and on histological analysis both showed a count of 5-7 mitotic figures per 10 high-power fields. Histopathological examination of the first case was characterized by the presence of hemorrhage, necrosis and severe nuclear atypia, with a metastatic peritoneal lesion, and the eventual diagnosis was primary ovarian fibrosarcoma. The second case, however, was diagnosed as a 'mitotically active cellular fibroma' owing to the lack of substantial severe cellular atypia, hemorrhage or necrosis. We speculate that ovarian fibrosarcoma at the preoperative examination might be considered in the differential diagnosis whenever a unilateral, heterogeneous, largely round solid ovarian lesion is detected.
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