Extranodal Rosai-Dorfman disease with bilateral renal involvement: A diagnostic challenge.

Rosai-Dorfman disease (RDD) is a rare, benign, non-Langerhans cell histiocytosis disorder. Extranodal involvement is present in a significant number of cases; however, renal manifestations are infrequent, often resembling malignant conditions, leading to a diagnostic challenge. We present the case of a 51-year-old male with a six-month history of fatigue, generalized weakness, and cervical lymphadenopathy. Computed tomography (CT) revealed multiple pulmonary nodules, mild hepatomegaly, and bilateral renal masses encasing the renal vessels and proximal ureters. Positron emission tomography-computed tomography (PET-CT) showed metabolically active renal lesions along with hypermetabolic cervical and abdominal lymphadenopathy, which were initially suggestive of lymphoma. On renal biopsy, sheets of large histiocytes displaying emperipolesis, with immunohistochemical positivity for S100 and CD68, were found, thereby confirming RDD. This case highlights the significance of considering RDD in the differential diagnosis of bilateral renal masses accompanied by systemic lymphadenopathy. The definitive diagnosis depends on histopathological analysis, and prompt systemic therapy can lead to stabilization of the disease.