The renal pelvis arises from the ureteric bud (mesonephric duct), whereas renal parenchyma develops from the nephrogenic cap. This difference in derivation may account, at least in part, for the marked difference in behavior of tumors arising in the two sites. Tumors of the renal pelvis develop from transitional cell epithelium and vary from relatively benign papillomas to highly malignant, infiltrating, sessile tumors. Squamous cell carcinoma may develop as further metaplasia of transitional cell carcinoma occurs, or separately, as the result of reaction to long-continued irritation. A very rare type of pelvic tumor is the mucoid adenocarcinoma.
The transitional epithelium of the renal pelvis shares the property of its counterpart, throughout the urinary collecting system, in its response to carcinogenic agents excreted in the urine, to irritation, or most often, to unknown agents.Although still relatively uncommon, it is believed by recent authors46 that the incidence is rising.