Polypoid urothelial tumor with inverted growth pattern in the renal pelvis: morphologic and molecular characteristics of a unique diagnostic entity.

We report 13 cases of unique polypoid urothelial tumors with inverted growth pattern (PUTIPs) occurring in the proximal ureter and renal pelvis. We describe their morphologic features and further characterize them in regard to TERT promoter mutation status and microsatellite instability. Thirteen cases were identified in our consult archives from 1994 to present. Patients ranged in age from 52 to 83 years at the time of diagnosis (mean, 68.4 years). Grossly, lesions were described variously as pink-tan to white exophytic and friable lesions that were polypoid or pedunculated, located in the renal pelvis or proximal ureter. The masses ranged in size from 0.5 to 3.2 cm (mean, 1.6 cm). PUTIP is a polypoid, plaque-like lesion with features of the following: (1) inverted papillary urothelial neoplasm of low malignant potential, lacking an exophytic papillary component; (2) florid von Brunn nest proliferation within and radiating outward from the polypoid lesion; and (3) in some cases, an inverted papilloma with densely hyalinized collagenous stroma. All 4 PUTIPs with formalin-fixed, paraffin-embedded material available were positive for the TERT promoter mutation 228G>A by polymerase chain reaction and were microsatellite stable. Given that PUTIP clinically forms a tumor and is typically treated by nephroureterectomy, it is best regarded as a unique benign urothelial neoplasm that exclusively occurs in the renal pelvis/proximal ureter.