Villous Adenoma of the Renal Pelvis: Clinicopathologic Spectrum and Diagnostic Considerations.

Villous adenoma (VA) of the renal pelvis is a rare intestinal epithelial neoplasm. It generally arises in elderly men and is frequently associated with chronic irritation, infection, or long-standing obstruction, most often due to nephrolithiasis or hydronephrosis. We report four previously unpublished patients diagnosed with VA of the renal pelvis. Each patient was evaluated clinicopathologically, radiologically, immunohistochemically to clarify morphologic diversity and biological behavior. We also review previously reported patients in the literature. All four patients were men, aged 45-70 years, each with common history of chronic infection or calculi. Radiological findings revealed advanced hydronephrosis and mucin-filled collecting systems (muconephrosis). Grossly, the lesions were mucinous and cystically dilated. Microscopically, the lesions from Patient 1 and 2 showed classic villiform mucinous epithelium without invasion; the lesion from Patient 3 exhibited serrated adenoma-like architecture with dysplastic nuclei; the lesion from Patient 4 demonstrated low-grade dysplasia with preserved crypt morphology. Immunohistochemically, all lesions expressed intestinal markers including KRT20, CDX2, SATB2, with variable KRT7 expression, wild-type p53 (TP53) expression. None of the lesions showed invasive growth or recurrence during follow-up. VA of the renal pelvis represents an intestinal-type lesion of upper urinary tract that develops in setting of chronic irritation and mucus accumulation and may be associated with intestinal metaplasia. Because its clinical and radiologic features often mimic inflammatory or cystic disease, histopathologic evaluation is essential for accurate diagnosis. Recognition of this rare entity is crucial to differentiate it from mucinous adenocarcinoma, as complete surgical excision is curative and long-term prognosis is excellent.