Congenital mesoblastic nephroma: relationship to other renal tumors of infancy.

Although nephroblastomas (Wilms tumor) do occur in the newborn most of the solid renal tumors in this age group constitute a distinctly different pathologic entity, congenital mesoblastic nephroma. While a more cellular histologic variant of congenital mesoblastic nephroma has been recognized recently, which occasionally may follow a malignant course, the tumor with conventional histology has never been reported to metastasize and follows a benign course when treated by adequate local excision alone. We report our 50-year experience with 11 cases of this tumor (9 cases with conventional histology and 2 with cellular variant histology). Typically, the tumor presented in the newborn as a large asymptomatic mass. Local invasion was never seen and the tumors were resected without difficulty. Grossly, the tumors usually resembled a uterine "fibroid" and, unlike nephroblastomas, rarely exhibited hemorrhage or necrosis. Microscopically, the conventional histologic congenital mesoblastic nephroma was composed of sheets of spindle-shaped cells. The cellular variant of congenital mesoblastic nephroma exhibited increased cellularity and a higher mitotic index. The relationship of congenital mesoblastic nephromas to other neonatal renal tumors is discussed and a unifying schema of neoplasia in infantile renal tumors is proposed.