Abstract
Mesenchymal tumors of the kidney, although infrequently encountered, constitute a wide spectrum of lesions. The relative rarity of these tumors means that in some instances criteria to differentiate between benign and malignancy are currently incompletely defined. More recently a variety of novel stromal tumors have been characterized, with hemangioblastoma and myopericytoma being notable examples. The identification of a subset of spindle cell tumors as synovial sarcoma, on the basis of the presence of a characteristic genetic translocation, has facilitated the correct classification of a number of tumors previously labeled as fibrosarcoma, malignant fibrous histiocytoma, or more recently cystic embryonal sarcoma. In this review, we have detailed the spectrum of both benign and malignant stromal tumors of the adult kidney, described the gross and microscopic features, with an emphasis on immunoexpression and the differential diagnosis of each tumor type.