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Malignancy in Solitary Fibrous Tumor

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Biologic Behavior of Solitary Fibrous Tumor: The behavior of solitary fibrous tumor is difficult to predict from histologic features. The majority of solitary fibrous tumors (about 70%) lack pleomorphism, necrosis, and hemorrhage and have low mitotic activity (<4/10 hpf). They behave in a benign fashion. However, some histologically benign SFT's have metastasized.

The criteria of malignancy proposed by Enzinger and Smith include: high cellularity with overlapping nuclei, pleomorphism, large size (> 5cm), increased mitotic activity (> 4/10 hpf), and foci of hemorrhage and necrosis. Other authors have suggested less stringent criteria. The term low malignant potential has been used for tumors with any degree of increased cellularity, cytologic atypia, and slightly increased mitotic activity (1-3/10 hpf).

Mitotic activity and necrosis are the two most useful criteria in separating benign from malignant solitary fibrous tumors.

The tumor shown in this image had high cellularity, cytologic atypia, and >5 mitotic figures/10 hpf. The tumor originated in the deep soft tissues of the retroperitoneum and metastasized to the lung.

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