Isolated Renal/Peripelvic Rosai-Dorfman Disease Mimicking Renal Cell Carcinoma: A Case Report

Rosai-Dorfman disease (RDD) is a rare non-Langerhans cell histiocytosis that usually presents with massive cervical lymphadenopathy. Isolated renal or peripelvic involvement is exceedingly rare and may radiographically mimic renal cell carcinoma (RCC), creating a major diagnostic and management challenge.

A 72-year-old woman with hypertension and a known left staghorn calculus was incidentally found to have a 2.6 × 3.0-cm enhancing left renal mass after a mechanical fall. Computed tomography demonstrated enhancement from 51 Hounsfield units on noncontrast imaging to 110 Hounsfield units during the arterial phase, raising concern for RCC; no significant lymphadenopathy was present. She underwent robotic-assisted partial nephrectomy with concurrent nephrolithotomy and ureteral stent placement. Histopathology showed large histiocytes with emperipolesis. The histiocytes were positive for S100, CD68, CD163, OCT2, and cyclin D1 and negative for CD1a, ALK1, SOX10, BRAF V600E, synaptophysin, and CD34, confirming RDD without malignancy. IgG4-related disease was excluded by morphology, absence of significant IgG4-positive plasma cells, and an IgG4:IgG ratio below 40%. Postoperative PET/CT demonstrated no hypermetabolic or nodal disease. Given completely resected, unifocal, asymptomatic disease, observation was selected in accordance with NCCN guidance.

Isolated renal/peripelvic RDD can closely mimic RCC on imaging. Recognition requires histopathologic identification of emperipolesis supported by the characteristic immunophenotype. When technically feasible, partial nephrectomy can establish the diagnosis, provide definitive local treatment, and avoid unnecessary radical nephrectomy. RDD should be considered in the differential diagnosis of atypical enhancing renal masses.