Several types of cancer can develop in the kidneys. Renal cell carcinoma known as RCC, is the most common form, which accounts for approximately 85% of all cases. In RCC, cancerous or malignant cells develop in the lining of the kidney's tubules and grow into a mass called a tumor. In most cases, a single tumor develops, although more than one tumor can develop within one or both kidneys. As with all cancers, early diagnosis of kidney cancer is important. The earlier the tumor is discovered, the better a patient's chances for survival. Tumors discovered at an early stage often respond well to treatment. Survival rates in such cases are high. Tumors that have grown large or spread or metastasized through the bloodstream or lymphatic system to other parts of the body are more difficult to treat and present an increased risk for mortality.