A Case of IgG4-Related Retroperitoneal Fibrosis from the Renal Pelvis Mimicking Bilateral Hydronephrosis.

We describe a 49-year-old woman who presented with continuous bilateral lumbago. As the patient's ultrasonography manifestations were very similar to those of bilateral hydronephrosis, we performed retrograde pyelography and ureteroscopy. However, apart from slight left ureteropelvic junction obstruction, there was no hydronephrosis. Since malignant disease could not be completely denied, computed tomography-guided biopsy was performed. However, the tissue did not show evidence of malignancy. As the patient continued to have lumbago, we measured serum IgG4 levels because of suspicion of retroperitoneal fibrosis secondary to IgG4-related disease, which proved to be high. Further, immunostaining of the renal pelvic biopsy samples showed IgG4-positive cells. Therefore, diagnosing IgG4-related retroperitoneal fibrosis, we administered corticosteroids. The patient responded favorably to the drug, with gradual regression of the lesion.