Renal cancer arises from the outer portion of the kidney called the parenchyma (as opposed to the renal pelvis, which briefly collects the urine, which is then carried by a tube, the ureter, to the bladder for storage before it is expelled). The parenchyma is made up of many single units called nephrons that filter water and toxic products from the blood.

Types  More than 85 percent of renal cancers develop in glandular tissue (adenocarcinoma). The pure clear cell variety is seen in 25 percent of cases, the granular in 12 percent, the clear cell mixed with other more aggressive components such as granular in 50 percent and sarcomatous elements in about 10 percent. Studies using a test called flow cytometry suggest that the number of chromosomes in the tumor cells may help predict the chance for recurrence and disease-free survival. The normal two chromosomes (diploid) is favorable, while any other number (aneuploid) is less favorable. For example, two of every three low-grade (Grade 1) and low-stage tumors are diploid with less than 20 percent developing metastases and more than 80 percent surviving 5 years.
    Another renal cancer, Wilms' tumor, is found exclusively in children. ( See "Childhood Cancers" )

How It Spreads  Kidney cancer cells frequently spread through the bloodstream by penetrating blood vessels within the tumor, then entering the renal vein and/or the inferior vena cava  , the largest abdominal vein. Once cells have entered the bloodstream, they can lodge in the lungs, liver, brain or bone.