Clinicopathologic characterization and assessment of prognostic factors for intraabdominal solitary fibrous tumors involving the gastrointestinal tract and liver.

Solitary fibrous tumors (SFT) are rare mesenchymal neoplasms with a potential to metastasize in 10-30% of cases. Several risk models have been designed to predict tumor behavior at the pleura and extrapleural sites. Intrabdominal SFTs primarily involving the gastrointestinal tract (SFTGI) and liver (SFTL) are rare. We analyzed a series of SFTGI and SFTL to describe the clinicopathologic features and evaluate prognostic factors. The cohort included 33 males and 25 females, with a median age of 58.5 years and a mean tumor size of 15.6 cm. Patients with SFTL were predominantly older females compared to patients with SFTGI. By univariate analysis, high mitotic count (> 4/10 HPF), tumor size, tumor necrosis, and nuclear pleomorphism were associated with both disease-specific survival (DSS) and metastasis-free survival (MFS) (p < 0.05). Tumor location (SFTL vs. SFTGI) also predicted MFS (p = 0.026). Only very high mitotic count (> 9/10 HPF) predicted local recurrence-free survival (LFS, p = 0.001). Further analysis showed that all adverse histologic parameters (necrosis, hypercellularity, pleomorphism) correlated with high mitotic grade (> 4/10 HPF) (p < 0.05). On multivariate analysis, only mitosis predicted DSS (p = 0.023), MFS (p = 0.01), and LFS (p = 0.017). Validation of the 3 risk models (mDemicco, Salas, Pasquali) showed variable associations with DSS, MFS, and LFS, while a simplied 3-tiered risk model based on mitosis (0-4, 5-9, > 9/10 HPF) performed well in predicting all risks. Our results suggest that prognostication of SFTs is mainly associated with mitotic activity, supporting the use of mitosis (> 4 and/or > 9/10 HPF) for tumor grading and risk stratification at specific locations.