Solitary fibrous tumor: a clinicopathological study of 110 cases and proposed risk assessment model.

Solitary fibrous tumor represents a spectrum of mesenchymal tumors, encompassing tumors previously termed hemangiopericytoma, which are classified as having intermediate biological potential (rarely metastasizing) in the 2002 World Health Organization classification scheme. Few series have reported on clinicopathological predictors with outcome data and formal statistical analysis in a large series of primary tumors as a single unified entity. Institutional pathology records were reviewed to identify primary solitary fibrous tumor cases, and histological sections and clinical records reviewed for canonical prognostic indicators, including patient age, tumor size, mitotic index, tumor cellularity, nuclear pleomorphism, and tumor necrosis. Patients (n=103) with resected primary solitary fibrous tumor were identified (excluding meningeal tumors). The most common sites of occurrence were abdomen and pleura; these tumors were larger than those occurring in the extremities, head and neck or trunk, but did not demonstrate significant outcome differences. Overall 5- and 10-year metastasis-free rates were 74 and 55%, respectively, while 5- and 10-year disease-specific survival rates were 89 and 73%. Patient age, tumor size, and mitotic index predicted both time to metastasis and disease-specific mortality, while necrosis predicted metastasis only. A risk stratification model based on age, size, and mitotic index clearly delineated patients at high risk for poor outcomes. While small tumors with low mitotic rates are highly unlikely to metastasize, large tumors ≥ 15 cm, which occur in patients ≥ 55 years, with mitotic figures ≥ 4/10 high-power fields require close follow-up and have a high risk of both metastasis and death.