Nephroblastoma, light micrograph. Nephroblastoma, or Wilms tumour, is the fourth most common type of childhood cancer and the most common type of kidney cancer in children. The peak incidence is seen between 2 and 5 years of age. It is only rarely seen as a congenital neoplasm. They are usually solitary, large, and sharply demarcated tumours. About 7% are multicentric and 5% of cases are bilateral. In its classic microscopic appearance, Wilms tumour is triphasic and consists of variable proportions of blastema, stroma, and epithelial cells. One component may dominate over others in a given case, resulting in biphasic and monophasic variants. The stromal component is usually fibroblastic or myxoid and may contain heterologous elements such as skeletal muscle, smooth muscle, bone, cartilage, adipose tissue (shown here), and neuroglial tissue. Besides the adipose tissue in the stroma, this image also shows nodular aggregates of blastema and epithelial component.