Wilms' tumor is a cancerous tumor of the kidney that usually occurs in young children.

When an unborn baby is developing, the kidneys are formed from primitive cells. Over time, these primitive cells become more specialized. The cells mature and organize into the normal kidney structures. Sometimes, clumps of these cells remain in their original, primitive form. If these cells begin to multiply after birth, they may ultimately form a large mass of abnormal cells. This is known as a Wilms' tumor.

Wilms' tumor is a type of malignant tumor. This means that it is made up of cells that are significantly immature and abnormal. These cells are also capable of invading nearby structures within the kidney and traveling out of the kidney into other structures. Malignant cells can even travel through the body to invade other organ systems, most commonly the lungs and brain. These...