Solitary fibrous tumor (SFT) is a fibroblastic neoplasm characterized by prominent, staghorn, or delicate "hemangiopericytoma (HPC)-like vasculature", NAB2::STAT6 gene fusion, and its surrogate STAT6 expression. SFT may exhibit an unpredictable clinical course, necessitating risk assessment based on tumor characteristics. Renal SFTs are rare and have not been well characterized. Forty-three primary kidney SFT cases are reviewed for clinical, morphological, and immunohistochemical (STAT6, BCL2, CD34, and PAX8) features. A four-variable risk stratification by Demicco was applied based on patient age, tumor size, mitotic activity, and tumor necrosis. The mean age was 49 years (range 11-83 years) with a slight female predominance (male:female = 20:23). The mean tumor size was 7.8 cm (1.6-32 cm). Tumors were mainly located at the hilus (22/32, 68%) and had well-demarcated borders (31/42, 74%). Morphologically, tumors were categorized as 1) Fibrous ("SFT-like", 19/43, 44%), characterized by hypocellularity and prominent fine-reticular or keloidal collagen; 2) Cellular ("HPC-like", 8/43, 19%), with hypercellularity, short spindle to small cell-like proliferation, and lacking a collagenous background; 3) mixed fibrous/cellular (16/43, 37%) displaying both components. Four cases featured a lipomatous component. BCL2 was positive in all tested cases (28/28), CD34 in all but 3 cases (93%), and STAT6 in all but one case (39/40, 97.5%). PAX8 was positive in 6/34 (18%) cases. Most cases (29/43, 68%) were classified as low-risk, followed by intermediate (12/43, 27%) and high-risk (2/43, 5%) groups. Five of 29 (18%) patients had metastatic disease, and two patients with high-risk and one with intermediate-risk tumors died from the disease, while 25 patients with low- or intermediate-risk tumors were alive for an average of about 36 months. We emphasize the usefulness of the risk stratification system in predicting prognosis. PAX8 expression in a subset of renal SFT represents a potential diagnostic pitfall.

Biopticka Laborator Plzen Czech Republic

Core Diagnostics Gurgaon India

Department of Diagnostic and Molecular Pathology Cumming School of Medicine University of Calgary Calgary AB Canada

Department of Pathology Adana Dr Turgut Noyan Application and Research Center Başkent University Faculty of Medicine Adana Türkiye

Department of Pathology Ankara Bilkent City Hospital Ankara Türkiye

Department of Pathology Brigham and Women's Hospital Boston MA USA

Department of Pathology Cruces U Hospital Biocruces Institute University of the Basque Country Barakaldo Spain

Department of Pathology Faculty of Medicine Charles University Plzen Czech Republic

Department of Pathology Hacettepe University School of Medicine Ankara Türkiye

Department of Pathology Johns Hopkins University Baltimore MD USA

Department of Pathology Koç University School of Medicine Istanbul Türkiye

Department of Pathology Kocaeli University School of Medicine Kocaeli Türkiye

Department of Pathology Memorial Hospital Istanbul Türkiye

Department of Pathology Montefiore Medical Center New York City NY USA

Department of Pathology Ondokuz Mayıs University School of Medicine Samsun Türkiye

Institute of Pathology University of Erlangen Nuremberg Erlangen Germany

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