A case of pyelocalyceal diverticulum urothelial carcinoma that was difficult to distinguish from cystic renal cell carcinoma preoperatively

INTRODUCTION: Pyelocalyceal diverticulum is a rare disease sometimes difficult to distinguish from cysts. We report a case of urothelial carcinoma originating from a pyelocalyceal diverticulum, difficult to distinguish from cystic renal cell carcinoma preoperatively. CASE PRESENTATION: A 51‐year‐old Japanese man complained of gross hematuria. Computed tomography revealed a solid mass in one of the many cystic lesions in the left kidney. He was diagnosed with left cystic renal cell carcinoma and underwent retroperitoneal laparoscopic nephrectomy. Pathological examination revealed high‐grade invasive urothelial carcinoma arising within the renal pyelocalyceal diverticulum. The definitive diagnosis was high‐grade invasive urothelial carcinoma (pT3). In retrospect, the retrograde pyelography findings indicated the cyst and urinary tract connection. Residual ureterectomy and adjuvant chemotherapy were later performed. The patient has since been recurrence‐free. CONCLUSION: Whether cystic renal cell carcinoma is suspected on imaging, pyelocalyceal diverticulum should be considered a differential diagnosis, though unlikely to be encountered in daily practice.