First case report of pre-mortem biopsy-proven kidney involvement by necrobiotic xanthogranuloma with monoclonal gammopathy.

Necrobiotic xanthogranuloma (NXG) is a rare non-Langerhans cell histiocytosis with characteristic cutaneous features and rare visceral involvement. More than 80% of individuals with this disease have a detectable paraprotein but the precise pathogenesis remains obscure. A 68-year-old man with known cutaneous necrobiotic xanthogranuloma presented with acute kidney injury and imaging suggestive of bilateral perinephric infiltration. Renal biopsy showed a prominent histiocytic infiltration of renal capsule and cortex with necrobiosis and characteristic 'Touton-type' giant cells suggestive of necrobiotic xanthogranuloma involvement. Kidney function returned to normal and cutaneous lesions improved with a combination of corticosteroid, chlorambucil and rituximab. This case represents only the second reported incidence of kidney involvement by necrobiotic xanthogranuloma and the first with acute kidney injury and pre-mortem histopathology. This report adds to a small body of literature on the diagnosis and management of visceral involvement by this rare disease.