Wilms' tumor most commonly presents as an incidentally discovered, large, abdominal mass in infants. The overwhelming majority of lesions are in the kidney but there is occasional extrarenal occurrence. There is association with a variety of congenital syndromes and anomalies. Most nephroblastomas are solitary and well-circumscribed.
Size is widely variable.
The cut surface also varies, ranging from solid and grey to tan and lobulated, with areas of degeneration, hemorrhage, cystic change often present. Individual cases are often biphasic or monophasic. The stromal element occasionally shows rhabdomyosarcomatous differentiation. Anaplasia is a poor prognostic indicator in non-stage 1 tumors. Specific abnormal genetic loci, WT1 and WT2, on chromosome 11, have been identified.