Solitary fibrous tumor of kidney and renal hilus: a multi-institutional clinicopathologic analysis of 43 cases.

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.