Multicystic congenital mesoblastic nephroma

This report describes an unusual example of congenital mesoblastic nephroma cellular variant that presented in a 1-week-old neonate as a multicystic tumor of the kidney. Extensive pseudocystic cavitation resulted from progressive accumulation of ground substance in a loosely myxoid tissue composed of stellate- and spindle-shaped cells that compressed and infiltrated renal tissue. The cells of the tumor were positive for vimentin and smooth muscle actin. The patient is alive and well 16 years after surgery. Differential diagnosis from segmental cystic dysplasia, cystic intralobar nephrogenic rest, cystic nephroma, cystic partially differentiated nephroblastoma, cystic nephroblastoma, and cystic clear cell sarcoma of the kidney, all of which may present at this age, is discussed.