Doctors in southwestern China thought they were looking at a large but ordinary kidney cyst. Then, after they placed a thin tube to relieve a blocked kidney, the cyst began to shrink. Simple cysts are not supposed to do that, and the change exposed what had been hiding in plain sight: a giant pouch connected to the kidney's drainage system, sealed off by a stone.
The case, reported by urologists at the Fourth Affiliated Hospital of Dali University in Chuxiong, Yunnan province, was published online August 21 in the International Journal of Surgery Case Reports. It describes a 69-year-old man with a 9-centimeter giant calyceal diverticulum.
A Cyst That Did Something Cysts Can't
The man's left kidney lesion was found incidentally. A CT scan in November 2025 showed a cystic lesion measuring about 92 by 71 millimeters along with a 10-millimeter kidney stone. Contrast imaging showed no enhancement, and CT and MRI both suggested a simple renal cyst, a fluid-filled sac that is common with age and usually harmless.
Weeks later, he was admitted with renal colic, the sharp pain of a stone blocking urine flow. Doctors placed a ureteral stent, a thin tube that keeps urine moving from the kidney to the bladder. Afterward, the lesion unexpectedly got smaller and its wall thickened.
That shrinkage was the key clue, according to the authors. A simple cyst is a closed sac with no connection to the urinary system, so relieving pressure in the kidney should not deflate it. They called such a change impossible for a simple cyst.
A Stone Plugging a Hidden Pouch
A calyceal diverticulum is a cavity inside the kidney that connects through a narrow neck to a calyx, one of the cup-shaped chambers that collect urine. The authors cite an estimated incidence of 0.21% to 0.45%. Giant forms, larger than 8 centimeters, are easily mistaken for renal cysts.
Normally, contrast dye on delayed imaging flows through that narrow neck and fills the pouch, revealing the connection. The authors note that previous reports generally described necks that remained open on delayed images. In this man, the stone had completely blocked the neck. With no dye getting in, the scans produced a false-negative result and the pouch looked like a cyst.
Surgeons then performed retrograde flexible ureteroscopic laser lithotripsy, passing a thin scope up through the urinary tract and using a laser to break apart the stone lodged at the neck. Follow-up CT showed the lesion had collapsed, with gas and fluid inside, confirming that the cyst had really been a giant diverticulum that emptied once the stone was cleared.
An Infection, Then Recovery
The recovery was not entirely smooth. A CT scan on Feb. 1, 2026, taken before the stent was removed, showed the former cyst had been replaced by low-density changes with gas, suggesting an abscess or infection around the kidney. The authors' final diagnosis was a diverticulum stretched to an extreme size by its stone-blocked neck, which collapsed after the stone was cleared and then developed a secondary infection.
He received the antibiotic ceftazidime twice daily for three days, and the stent was removed on Feb. 5. He recovered well. At a three-month follow-up on April 5, the fluid collection had fully resolved, with no recurrence, no remaining stone, and normal kidney function.
Why the Right Label Matters
Mistaking a diverticulum for a cyst is more than a labeling error. The authors warn that treating a presumed cyst surgically without addressing the neck can lead to urine leakage or infection around the kidney.
Other reports show how that can play out. In a 2022 case in Frontiers in Surgery, a 53-year-old woman underwent laparoscopic surgery to unroof what appeared to be a simple kidney cyst and then developed persistent urine leakage. The lesion was a calyceal diverticulum. In a 2025 report from Kagoshima University in Japan, doctors drained a giant cystic kidney lesion in a pregnant woman at seven weeks of gestation, only to see fluid immediately build back up. They suspected a connection to the urinary system and, after she gave birth, confirmed a calyceal diverticulum and removed it with robot-assisted surgery.
Stones are frequent companions of these pouches because urine can stagnate inside them. One report notes that more than 40% of calyceal diverticula are associated with a stone. Kidney stones themselves are common, and the National Institute of Diabetes and Digestive and Kidney Diseases offers guidance on their symptoms and treatment.
The Chinese team's takeaway is aimed at radiologists and urologists. When a cystic lesion sits next to the kidney's collecting chambers and a stone is present on the same side, a diverticulum should be suspected even if contrast imaging looks negative. Watching how the lesion changes after a stent or stone treatment can help clarify the diagnosis and prevent unnecessary surgery.
As a single case report, the findings cannot say how often large kidney cysts are actually hidden diverticula. People told they have a kidney cyst that is large, changing, or accompanied by stones can ask their urologist whether further evaluation is needed.
Key Questions Answered
What is a calyceal diverticulum?
It is a rare pouch inside the kidney that connects to the urine-collecting system through a narrow channel.
Why did doctors think it was a cyst?
A stone had completely blocked the pouch's opening, so contrast dye could not enter, and CT and MRI made it look like a simple cyst.
What revealed the true diagnosis?
The lesion shrank after a ureteral stent was placed, something a simple cyst should not do. Breaking up the stone then caused the pouch to collapse.
How was the patient treated?
Surgeons used a laser passed through a thin scope to break up the stone. A later infection was treated with antibiotics, and he recovered.
Is this condition common?
No. The authors cite an incidence of about 0.21% to 0.45%, and giant diverticula larger than 8 centimeters are rarer still.
Published by Medicaldaily.com
