The kidneys' main job is to filter the blood and rid the body of excess water, salt, and waste products. The filtered waste products are concentrated into urine. Urine leaves the kidneys through long slender tubes called ureters that connect to the bladder. Urine flows down the ureters into the bladder where it is stored until urination.

Although our kidneys are important, we actually need less than one complete kidney to do all of the important functions discussed above. Tens of thousands of people in the United States are living normal healthy lives with just one kidney. Some people may not have any working kidneys at all, and survive with the help of a medical procedure called dialysis. Dialysis uses a specially designed machine that acts like a real kidney to filter the blood.

Renal Cell Carcinoma

Renal cell carcinoma is the most common type of kidney cancer. It accounts for more than 90% of malignant kidney tumors.

Like all cancers, renal cell carcinoma begins small and grows larger over time. Although renal cell carcinoma usually grows as a single mass within the kidney, a kidney may contain more than one tumor. Sometimes tumors may be found in both kidneys at the same time. Some renal cell carcinomas are noticed only after they have become quite large; most are found before they metastasize (spread) to other organs through the bloodstream or lymph vessels. Like most cancers, renal cell carcinoma is difficult to treat once it has metastasized.

There are 5 main types of renal cell carcinoma that are identified by examining the tumor under a microscope: clear cell, papillary, chromophobe, collecting duct, and "unclassified."

When viewed under a microscope, clear cell renal cell carcinoma appears very pale or clear. This is the most common form of renal cell carcinoma. About 70% to 80% of people with renal cell carcinoma have this kind of cancer.

Papillary renal cell carcinoma is the second most common type—about 10% to 15% of people have this kind. These cancers form little finger-like projections papillae in some if not most of the tumor. Some doctors call these cancers chromophilic because the cells take up certain dyes used in preparing the tissue to be viewed under the microscope, causing them to appear pink.

Chromophobe renal carcinoma is the third most common type—accounting for about 5% of cases. The cells of these cancers are also pale, like the clear cells, but are much larger and have certain other features that can be recognized.

The fourth type, collecting duct renal carcinoma, is very rare. The major feature is that the cancer cells can form irregular tubes. About 5% of renal cancers are unclassified because their appearance doesn’t fit into any of the other categories.

Another aspect of a renal cell carcinoma is its grade. This refers to how closely the cancer cells’ nuclei (part of a cell in which DNA is stored) look like normal kidney cells’ nuclei.

Renal cell cancers are usually graded on a scale of 1 through 4. Grade 1 renal cell cancers have cell nuclei that differ very little from normal kidney cell nuclei. These cancers usually grow and spread slowly and tend to have a good outlook (prognosis.) At the other extreme, grade 4 renal cell cancer nuclei look quite different from normal kidney cell nuclei and have a worse prognosis.

Although the cell type and grade are sometimes helpful in predicting a prognosis, the cancer's stage is by far the best predictor of survival. The stage describes the cancer's size and how far it has spread beyond the kidney. Staging is explained in the section How Is Kidney Cancer (Renal Cell Carcinoma) Staged?

Other Cancerous Kidney Tumors

Renal cell carcinoma accounts for about 90% of malignant kidney tumors. Less common types of cancerous tumors include transitional cell carcinomas, Wilms tumors, and renal sarcomas.

Transitional cell carcinoma: About 5% to 10% of all kidney tumors are transitional cell carcinomas, also known as urothelial carcinomas. Transitional cell carcinomas begin in the renal pelvis (the junction of ureter and kidney). Under the microscope, they look like bladder cancer cells and act very much like bladder cancer. Studies have shown that, like bladder cancer, these cancers are linked to cigarette smoking and occupational exposures to certain cancer-causing chemicals.

If you have transitional cell carcinoma, you may have the same signs and symptoms as patients with renal cell cancer—blood in the urine and, sometimes, back pain.

These cancers are usually treated by surgically removing the whole kidney and the ureter, as well as that portion of the bladder where the ureter attaches. Chemotherapy and radiation therapy are often used in addition to surgery, depending on how much cancer is found. If you have early transitional cell carcinoma, you have several treatment options available. There are different ways to surgically treat early disease. Newer surgical techniques are also being studied. You should talk with your surgeon and be aware of your options and the benefits and risks of those options.

About 90% of transitional cell carcinomas of the kidney are curable if they are found early enough. The chances for cure drop dramatically if the tumor has grown into the ureter wall or kidney or if it has a more aggressive (high-grade) appearance when viewed under the microscope.

After surgery, follow-up visits to your doctor for monitoring with x-rays and cystoscopies (looking into the bladder) are extremely important because transitional cell carcinoma can come back in the bladder, as well as other places in the body.

Wilms’ tumor: About 5% to 6% of all kidney cancers are Wilms' tumors. This type of cancer is almost always found in children and is extremely rare among adults. To learn more, see the separate American Cancer Society document on "Wilms' Tumor".

Renal sarcoma: Renal sarcomas are a rare type of kidney cancer (less than 1% of all kidney tumors) that begins within the kidney's connective tissue. More information on sarcomas is available in the American Cancer Society document “Sarcoma – Adult Soft Tissue Cancer.”

Benign (Noncancerous) Kidney Tumors

Some types of kidney tumors do not usually spread (metastasize) to other parts of the body, although they can still grow and cause problems. These include renal cell adenomas, renal oncocytomas, and angiomyolipomas.

Renal adenoma: Renal adenomas are very small, slow growing, benign tumors that, under a microscope, look a lot like low-grade renal cell carcinomas. In rare cases, tumors first thought to be renal adenomas may turn out to be small renal cell carcinomas.

Oncocytoma: Oncocytomas are a type of benign kidney tumor that can sometimes grow quite large. Because oncocytomas do not normally metastasize to other organs, removing the kidney can often produce a cure.

Angiomyolipoma: Angiomyolipomas are another rare benign kidney tumor. They often develop in people with tuberous sclerosis.