Focal xanthogranulomatous inflammation of the kidney cyst without pyelitis: a rare presentation mimicking kidney cancer.

Xanthogranulomatous pyelonephritis (XGP) is one of the most common granulomatous diseases of the kidney. XGP is subdivided into diffuse, segmental, and focal subtypes. Preoperative diagnosis of focal XGP on radiological imaging can be challenging, especially without involvement of the renal pelvis. Here, we report the case of a 61 year-old male with focal xanthogranulomatous inflammation of the kidney without pyelitis. The lesion presented as a mural nodule on the left renal cortical cyst, and kidney cancer was suspected on the preoperative image. The patient underwent a successful partial nephrectomy. Pathological examination revealed that the mural nodule displayed xanthogranulomatous inflammation and was clearly distinct from the renal pelvis. Thus, it should be considered that a mural nodule in a renal cortical cyst could be xanthogranulomatous inflammation.