Benign fibromatous tumor (fibroma) of the kidney: a case report.

Benign fibromatous tumor (fibroma) of the urogenital tract is a distinctive pathologic entity occurring in the testis, paratesticular structures and renal peripelvis. We report a well-circumscribed tumor replacing the cortex and the medulla of more than half of the upper kidney, radiographically thought to be renal cell carcinoma. Histologically, the tumor was characterized by a variable cellularity and was composed of bland spindle-shaped cells loosely dispersed in a fibromyxoid to densely fibrous stroma in which calcifications and chronic inflammation were not observed. Immunocytochemistry showed that cells were positive for vimentin and, only focally, stained positive for desmin and alpha-smooth muscle actin. Differential diagnosis included a wide spectrum of benign and malignant spindle cell tumors. The clinicopathologic features were consistent with benign fibromatous tumor (fibroma) of the kidney. To our knowledge, this is the first case of benign fibromatous tumor (fibroma) of the kidney reported in the English literature.