Clinicopathologic spectrum of renal-pelvic fibroepithelial polyps: A series of five patients with emphasis on mimicry of malignancy.

Renal-pelvic fibroepithelial polyps (FEPs) are rare benign mesodermal lesions that can radiologically and clinically mimic urothelial carcinoma, sometimes prompting radical surgery. We reviewed five consultation cases of renal pelvis or proximal-ureter FEPs, integrating histology, immunohistochemistry, and targeted molecular testing. Patients were all females, tumors measured 3.3-9.0 cm (mean 5.2 cm), and four arose in the renal pelvis with variable ureteral extension. Three patients underwent nephroureterectomy for presumed malignancy. Histologically lesions shared a polypoid architecture with fibrovascular cores lined by benign -appearing urothelium; stromal cellularity separated lesions into hypercellular (patients 1-2) and hypocellular stromal patterns. Hypocellular lesions displayed fibrocollagenous stroma with bland spindle cells and were lined by benign urothelium. Hypercellular examples showed epithelioid smooth muscle type or primitiveappearing-stromal proliferations with occasional mitoses and focal Ki-67 elevation but lacked significant atypia, necrosis, infiltrative growth, or lymphovascular invasion. Immunoprofiles confirmed urothelial epithelial differentiation (AE1/AE3, GATA3, PAX8) with areas of anastomosing cords and pyelitis cystica and variable stromal labeling for desmin, CD10, ER, and PR; myogenin/MyoD1 were negative. A fusion/cancer gene- NGS panel (including GLI1 and NCOA1/2) was negative in the tested case. No recurrences were observed with mean followup of 24 months. We emphasize distinguishing features from papillary urothelial carcinoma, mixed -epithelialstromal tumor/smooth muscle adenoma-like renal tumor (SMART), rhabdomyosarcoma, and -translocation associated mesenchymal neoplasms, noting that focal -fibroadenoma like- stromal areas can occur in FEPs. Recognition of the confined polypoid growth beneath intact urothelium, bland cytology, supportive immunohistochemistry, and selective molecular testing when stromal atypia raises concern, can avoid misclassification and unnecessary radical surgery.