Hidden giant calyceal diverticulum: a rare case report caused by kidney stone obstruction of the calyceal neck.

INTRODUCTION AND IMPORTANCE: A calyceal diverticulum is a rare cystic cavity that communicates with the renal calyx. Giant forms (>8 cm) are easily misdiagnosed as renal cysts. Unlike prior reports with patent diverticular necks on delayed imaging, this case features complete stone obstruction leading to false-negative conventional imaging, with the diagnosis revealed only after ureteral stenting and stone treatment. CASE PRESENTATION: A 69-year-old man was incidentally found to have a 9 cm left renal cystic lesion. Initial CT/MRI suggested a simple cyst. After ureteral stent placement for an obstructing stone, the cyst shrank and its wall thickened. Retrograde flexible ureteroscopic laser lithotripsy fragmented an obstructing stone at the calyceal neck. Postoperative CT showed collapse of the cyst with gas and fluid accumulation, confirming a giant calyceal diverticulum that had decompressed after stone removal. CLINICAL DISCUSSION: The key diagnostic clue was dynamic reduction in size following ureteral stent placement - something impossible for a simple cyst. Complete obstruction of the diverticular neck by a stone caused false-negative findings on contrast studies. Treatment focused on stone removal, leading to diverticular collapse. CONCLUSION: For cystic lesions near the calyces with ipsilateral calculi, a calyceal diverticulum should be suspected even when contrast imaging is negative. Dynamic changes after ureteral stenting or stone intervention can aid in diagnosis and prevent unnecessary surgery.