[Renal solitary fibrous tumors: a clinicopathological analysis of five cases]

Objective: To investigate the clinicopathological features, diagnosis, and prognosis of renal solitary fibrous tumor (SFT). Methods: Five cases of renal SFT with unequivocal diagnoses at the Affiliated Hospital of Qingdao University between January 2011 and July 2025 were subject to analyses of their clinical, morphological, immunophenotypic, and molecular characteristics, accompanied by a literature review. Results: Two males and three females aged between 45 and 62 years were included, all of whom presented with the discovery of a renal mass during routine physical examinations. Gross examination showed that the five tumors were all confined in the kidney. The tumors were nodular with maximum diameters ranging from 2.5 cm to 11.0 cm (mean, 5.8 cm). Upon cross-sectioning, they exhibited gray-white or gray-yellow cut surface. Histologically, the tumor cells exhibited oval or short spindle shapes in four cases, presenting with varying densities and arranged in short bundles, woven patterns, and irregular formation. Various amounts of coarse collagen and scattered staghorn blood-vessels were found in the stroma. In one case (case 5), the tumor cells were long spindle-shaped, densely organized in bundles, and interwoven, exhibiting inconspicuous boundaries, moderate nuclear atypia, and at least 4 mitotic figures per 10 high-power fields. Irregular patchy collagen deposition was particularly prominent at the edges of the tumor tissue. In two cases (cases 3 and 5), scattered and various amounts of renal tubules were observed in the tumor. Two cases (cases 4 and 5) demonstrated focal invasion of the renal parenchyma, although no necrosis was noted. Immunohistochemical staining showed that the tumor cells were diffusely and strongly positive for vimentin and STAT6 in all 5 cases, and positive for CD34. Bcl-2 positivity was present in 4 of the 5 cases. All cases were negative for CKpan, EMA, PAX8, HMB45, Melan A, SMA, and S-100 protein. The p53 status was wild type, and the Ki-67 index ranged from 1% to 8%. Next-generation sequencing was conducted on one case (case 4), revealing the NAB2 (exon 3)::STAT6 (exon 18) gene fusion. The 5 patients were followed up for 1 to 158 months (mean, 56 months), and all were alive with no recurrence or metastasis. Conclusions: SFT of the kidney are rare and morphologically similar to extrarenal SFT. Key morphological features include short spindle-shaped tumor cells arranged in bundles, interwoven patterns or irregularly, accompanied by staghorn blood-vessels and scattered coarse hyaline collagen fibers. SFT with epithelial inclusions may represent a relatively common histological subtype in the kidney. Immunohistochemical staining that demonstrates diffuse and strong positivity for STAT6 and CD34 is instrumental in diagnosing this tumor. The pathogenesis is linked to the centromeric inversion of chromosome 12q, resulting in the fusion of the NAB2 and STAT6 genes. Most of these tumors exhibit favorable prognosis.