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The kidney is made up of many layers of cells. Kidney cancer can affect any one or all of these layers.
The main type of kidney cancer is called renal cell carcinoma (RCC). It begins in a layer of tissue called the renal tubular epithelium. More than 9 out of 10 malignant kidney tumors are this type. If you have this type of kidney cancer, you may have more than one tumor in one or both kidneys. These may be quite large by the time they are diagnosed. But most are found before they have spread, called metastasized, to other organs. If you have renal cell carcinoma, you will have one of five types. The pathologist identifies these types by looking at the cancer cells under a microscope. There appears to be a link between the type of renal cell carcinoma and how it behaves:
Clear cell. About 80 percent of people with RCC have this type, making it the most common type. The cancer cells appear pale or clear under the microscope.
Papillary. This is the second most common type of RCC, accounting for 10 to 15 percent of cases. The cells from papillary tumors appear pink from the dye used to prepare the tissue before examination. These cancers also form tiny fingerlike projections, which gives them a tubular appearance.
Chromophobe. This is a rare form of kidney cancer, making up about 5 percent of cases. Pale in color, these cells are larger than other types of renal cell carcinoma.
Collecting duct. This is an even rarer form of RCC. Collecting duct cancer cells form irregular tubes under the microscope.
Unclassified. This includes tumors that contain cells from multiple types of cancer or cells that do not fit into the other categories.
In some cases, the term sarcomatoid may be used to describe certain kinds of kidney cancer. Clear cell or papillary kidney cancer may be called sarcomatoid when the cancer cells resemble sarcoma cancer cells. Some experts believe that sarcomatoid kidney cancers are more difficult to treat.
There are other less common types of kidney tumors than RCC. These types include the following:
Transitional cell carcinoma. These account for about 5 to 10 percent of all kidney tumors. They are also known as urothelial carcinomas. They begin in the junction between the ureter and kidney, which is called the renal pelvis. They can act and look very much like bladder cancer.
Wilms’ tumor. This cancer nearly always occurs in children. It accounts for 5 to 6 percent of all kidney cancers.
Renal sarcoma. This is a very rare type of kidney cancer. It begins in the blood vessels and connective tissue surrounding the kidneys.
There are also several types of benign (noncancerous) kidney tumors, including renal cell adenomas, renal oncocytomas, and angiomyolipomas. These tumors may still affect kidney function and can cause pain and other symptoms, but usually they do not spread to other organs.