Your kidneys are two bean-shaped organs that are found in the upper back part of the abdomen. They are responsible for ridding the body of waste from the blood, filtering water and producing urine. A kidney tumour (often known as renal tumour) in children develops from nephroblasts, the immature cells from which the kidneys develop. These cells often don’t mature normally, instead, they remain present in a child’s kidney after birth, and then they start to grow at a rapid and uncontrolled rate, becoming a large mass of tiny, immature cells. Other types of renal tumours include rhabdoid renal tumour, kidney sarcomas and renal carcinoma.