Clear cell renal cell carcinoma with smooth muscle stroma

A recent publication described 5 unusual clear cell renal tumors with prominent smooth muscle stroma that were characterized only by immunostaining. We report 3 additional tumors composed of clear cell renal cell carcinoma intimately admixed with abundant smooth muscle stroma. Epithelial differentiation of the malignant clear cell components and smooth muscle differentiation of the benign spindle cell stroma was confirmed by the immunostaining profiles and by electron microscopy. Fluorescence in situ hybridization analysis of chromosome 3 showed loss of the entire chromosome in 2 cases and loss of 3p in the third case. We therefore interpret these tumors as unique low-grade variants of clear cell renal cell carcinoma that have induced a prolific metaplastic stromal reaction. Extensive tissue sampling and immunostaining are recommended to distinguish cases with an extensive smooth muscle component from morphologically similar but benign lesions including angiomyolipoma, leiomyoma, or mixed epithelial and stromal tumor of the kidney.