Malignant tumours in the kidney can originate from different tissues. By far the most common are renal cell carcinomas (also called renal carcinomas or adenocarcinomas of the kidney). They account for about 95 percent of all kidney tumours and mostly originate from the cells of the uriniferous tubules (tubule system). A special type of growth called an oncocytoma makes up about four percent of all kidney tumours. They look like a renal carcinoma in medical images, but do not metastasise.
Tumours of the renal pelvis are much rarer. Their fine tissue structure is similar to that of malignant tumours of the bladder and ureter, so they are treated differently to renal cell carcinomas.
The kidneys can also be affected by malignant tumours that do not originate from the kidney tissue, such as cancers of the lymphatic tissue (lymphomas) and the connective and supporting tissue (sarcomas). Like nephroblastomas, they are rare in adults. A nephroblastoma, also known as a Wilms tumour, is a malignant tumour of the kidney that occurs almost exclusively in children under the age of five.