Lesson of the month 2: IgG4-related renal mass with spontaneous resolution.

Immunoglobulin G4-related disease (IgG4-RD) is a systemic inflammatory condition that may involve any organ in the body, including the kidneys. However, renal parenchymal lesions are not seen frequently and the treatment strategy remains unclear. We describe a case of IgG4-related renal mass, which resolved spontaneously. The patient presented with right loin pain, constitutional symptoms and raised inflammatory markers. Magnetic resonance imaging (MRI) showed a large infiltrative mass centered on the right renal hilum and biopsy demonstrated histological changes in keeping with IgG4-RD. A careful 'watch-and-wait' approach was taken and at six months following initial presentation, the patient's symptoms had fully resolved and inflammatory markers had normalised. Repeat MRI showed almost complete resolution of the mass. We propose that a careful 'watch-and-wait' approach could be considered as an alternative to immune suppression for IgG4-related renal masses, especially if they are not causing symptoms or organ compromise.