Solitary Fibrous Tumor With Extensive Epithelial Inclusions Clinicopathologic and Molecular Study of Three Cases of an Underrecognized Variant in Abdominopelvic Sites

Objectives: Solitary fibrous tumor (SFT) harboring extensive epithelial inclusions is rare and can stimulate a biphasic neoplasm composed of epithelial and stromal elements. Methods: Three cases of SFT with extensive epithelial inclusions were retrieved. H&E stain, immunohistochemical stain, and targeted next-generation sequencing were performed. Results: There were two male patients and one female patient aged 54, 32, and 68 years. All tumors were located in abdominopelvic sites involving the kidney (case 1), omentum (case 2), and prostate (case 3), respectively. Microscopically, all tumors were circumscribed and composed of a background of SFT admixed with randomly embedded glands or cysts, organizing sometimes in a phyllodes-like architecture. The covered epithelium displayed a range of morphologies from simple cystic to stratified and to complex papillary proliferation. Immunohistochemically, both STAT6 and CD34 were expressed in the spindle cells but not in the epithelial inclusions. RNA sequencing revealed fusions involving NAB2-STAT6 in all cases. DNA sequencing demonstrated TERT c.-124C>T mutation in case 1. Prognostic stratification scores were intermediate in case 1 and low in cases 2 and 3. Conclusions: SFT with extensive epithelial inclusions represents a rare but potentially underrecognized variant of SFT and shows compatible molecular features with conventional SFT.