Kidney cancer results from the malignant transformation of kidney cells that grow uncontrollably and eventually invade healthy cells. It can affect one or both kidneys. Sometimes the cancer cells cause kidney structures to become misshapen and interfere with their ability to function properly.  Over time, these tumors can spread (metastasize) to local and distant tissues and organs, complicating the condition and its treatment.

Cancer that begins in the kidney, or primary kidney cancer,  is rare, representing only 2 percent of cancers in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). Cancer can spread to the kidneys from another area of the body, such as the prostate or liver.

The kidneys are two bean–shaped organs that are located in the upper back portion of the abdomen, on each side of the spine. The kidney’s main function is to eliminate toxins and waste from the blood and concentrate it into urine. From the kidneys, urine travels through tubes called ureters to the bladder, where it is stored until urination. 

Other kidney functions affect many other body systems. The kidneys are also responsible for:

• Maintaining appropriate levels of water and salt in the body
  
  
• Producing the hormone renin, which monitors blood pressure
  
  
• Producing the hormone erythropoietin, which regulates production of the body’s red blood cells

Kidney tumors can be benign (noncancerous) or malignant (cancerous). While there are many types of kidney cancers, renal cell carcinoma is by far the most common. It accounts for more than 90 percent of kidney cancers, according to the American Cancer Society (ACS).

Most kidney cancer patients are diagnosed between 50 and 70 years of age. It is rare for children and young adults to develop this disease. Additionally, kidney cancer is more prevalent among men than women. Two-thirds of cases affect men, and almost twice as many men die from the disease than women according to the ACS.

The ACS estimates that there will be 38,890 new cases of kidney cancer in the United States in 2006 and the disease will result in 12,840 deaths. The five-year survival rate for kidney cancer is over 90 percent when the disease is detected in the earliest stage.