Light micrograph section through human kidney tissue showing Wilms' tumour. This is a rare kidney cancer that primarily affects children. Also known as nephroblastoma, it's the most common cancer of the kidneys in children. Wilms' tumour most often affects children 3 to 4 years age group, but becomes less common after the age of 5. Most often this tumour affects just one kidney, but sometimes it can be found in both kidneys simultaneously.