Type 2 calyceal diverticulum with an unusual appearance in the lower pole of the kidney

A 45-year-old woman presented to our clinic with intermittent left flank pain. The family physician referred her for renal cystic mass with a calcified appearance. The non-contrast spiral abdominal computed tomographic (CT) scan demonstrated the mass-like cystic lesion with a densely calcified lesion in the lower pole of the kidney. A detailed history revealed that she underwent shock wave lithotripsy (SWL) for the lower pole renal stone one year ago. After SWL, the stone fragments migrated to the dependent diverticulum region and produced the misleading appearance of a Bosniak type III lesion. Contrast-enhanced computed tomography (CT) scan was done for further evaluation, and finally, the diagnosis of the calyceal diverticulum was confirmed in the lower pole of the kidney. Calyceal diverticula are the outpouching of the pyelocalyceal system lined by non-secretory transitional epithelium. It is a rare condition that occurs in less than 0.5% of the population. Most patients are asymptomatic and have been discovered incidentally in routine imaging modalities. As most of the patients are asymptomatic, many do not need intervention. However, in some instances, patients present with flank pain, hematuria, urinary tract infection, and stone formation in the diverticulum. They are in the differential diagnosis of renal cystic lesions such as simple renal cyst, renal cortical abscess, and parapelvic cyst. In renal cystic lesion besides of simple renal cyst or renal cystic mass, we should keep the differential diagnosis of the calyceal diverticulum type 2, especially in patients that underwent SWL for renal stones; the fragmented residual stone may have migrated to this dilated region and produce the deceptive appearance of a Bosniak type III lesion.