[A case of IgG4-related kidney disease mimicking a renal cell carcinoma].

A 56-year-old man who had a 28mm renal mass on computed tomography (CT) pointed out at another hospital visited our department. The tumor was buried and located in the renal hilus, which was slightly enhanced in the early phase and washed out in the delayed phase. We diagnosed it as papillary renal cell carcinoma and conducted partial nephrectomy. The tumor was ocher and a solid nodule without capsule formation. Although there was no malignancy in histopathological examination, plasma cell infiltration was found and the IgG4/IgG-positive cell ratio was over 40%. Additionally, the serum IgG4 level was elevated postoperatively and the patient was diagnosed as having IgG4-related kidney disease. IgG4-related kidney disease is often found as multiple low-density lesions in CT. Because of the solitary nodule-like formation, it was difficult to distinguish from hypovascular renal cell carcinoma such as the papillary type. When a buried and solitary hypovascular tumor is detected, we must consider IgG4-related kidney disease as a differential diagnosis. Measuring the serum immunoglobulin and complement levels may be helpful for avoiding unnecessary surgery.