It is a mesenchymal tumour. Macroscopically the tumour is a solid un-encapsulated mass which often occurs near the renal hilum. It tends to invade the surrounding structures and renal parenchyma. Haemorrhage and necrosis are infrequent. Histologically, it is typically composed of connective tissue growing between nephrons, usually replacing most of the renal parenchyma.
The classic cytological description of the lesion is that of cellular clusters of spindle cells, mild nuclear pleomorphism, mitotic activity and no blastema.