Approximately 85% of renal cell cancers are adenocarcinomas, and most of those are of proximal tubular origin. Most of the remainder are transitional cell carcinomas of the renal pelvis. (Refer to the PDQ summary on Transitional Cell Cancer of the Renal Pelvis and Ureter Treatment ¹ for more information.) Adenocarcinomas may be separated into clear cell and granular cell carcinomas; however, the two cell types may occur together in some tumors. Some investigators have found that granular cell tumors have a worse prognosis, but this finding is not universal. Distinguishing between well-differentiated renal adenocarcinomas and renal adenomas can be difficult. The diagnosis is usually made arbitrarily on the basis of size of the mass, but size alone should not influence the treatment approach, since metastases can occur with lesions as small as 0.5 centimeter.