Renal Solitary Fibrous Tumor With Müllerian Cysts: A Novel Differential Diagnosis for Biphasic Renal Neoplasms.

Solitary fibrous tumor is an uncommon mesenchymal neoplasm, initially described in the pleura and infrequently found in the kidney. It is characterized by haphazardly arranged spindle cells, staghorn vasculature, coexpression of CD34 and signal transducer and activator of transcription 6 (STAT6), and a NAB2::STAT6 gene fusion. We report a 64-year-old woman who presented with a 2.5 cm multilocular cystic renal mass. On microscopic examination, the tumor consisted of a spindle cell component closely intermingled with ciliated and hemorrhagic epithelial cysts. The spindle cell component was positive for CD34, B-cell lymphoma 2 (BCL2), and STAT6, confirming the diagnosis of a solitary fibrous tumor. The epithelial cysts expressed keratin 7, PAX8, BCL2, estrogen receptor 1, and progesterone receptor, indicative of Müllerian cysts. The occurrence of solitary fibrous tumor in the kidney is extremely rare, and its association with Müllerian cysts has not been previously reported in the kidney. This morphologic variant has a striking similarity with biphasic renal neoplasms, especially with mixed epithelial and stromal tumors and angiomyolipoma with epithelial cysts. This report describes a novel renal solitary fibrous tumor subtype within this differential and underscores clinical and pathological distinctions of biphasic renal neoplasms.