A case of immunoglobulin G4-related inflammatory pseudotumor mimicking renal cell carcinoma.

In a 69-year-old woman with a history of Mikulicz's disease, a hypoechoic solitary renal mass was identified on routine ultrasound examination. Based on the findings of computed tomography (CT) and magnetic resonance imaging (MRI), renal cell carcinoma was a possible diagnosis. Subsequent partial nephrectomy revealed a mass characterized by an increased number of blood vessels, internal hemorrhage, and a thick fibrous capsule. Immunohistochemically, the mass comprised of tubulointerstitial nephritis with increased immunoglobulin (Ig)G4-positive plasma cells and fibrosis. Generally, diagnosis of IgG4-related kidney disease (IgG4-RKD) is not difficult when the kidney is involved together with other systemic involvements. However, diagnosis becomes harder when a solitary renal mass appears as a single-organ involvement. On precise review of our imaging findings, MRI signals were markedly affected by hemorrhage, so the mass showed hypointensity on both T1- and T2-weighted imaging, and the signal of in-phase images decreased. Dynamic MRI showed no apparent enhancement of the mass, while CT showed an apparent enhancement effect. Capsule formation was another key finding for IgG4-RKD and was recognized as a gradually enhancing boundary zone surrounding the mass on both CT and MRI. When a solitary renal mass is associated with hemorrhage and thick capsule formation, inflammatory pseudotumor should be considered as differential diagnosis.