Benign Renal Cystic Disease

CHAPTER 36 Benign Renal Cystic Disease

John Chandler Williams, MD, MS

Describe the radiographic characteristics of a simple renal cortical cyst on intravenous pyelography and CT scan images.

On intravenous pyelography, a simple cyst is a well-circumscribed recent mass that may distort the contour of the kidney and may show a “beak sign” with normal parenchyma. Computed tomography demonstrates a mass with water density (–10 and +20 HU) with an imperceptible wall, sharp margins with the renal parenchyma, and no enhancement after intravenous contrast administration.

Describe the radiographic characteristics of a simple renal cortical cyst on MRI scans and ultrasound.

On T1-weighted MRI, the mass is homogenous with low signal intensity (less than liver or renal cortex), and on T2-weighted images the lesion is homogenously hyperdense. There is no enhancement following intravenous administration of gadolinium-DTPA. On ultrasound, cysts are well demarcated with no internal echoes. They are usually spherical in shape and demonstrate an increase in through transmission with acoustic enhancement within the cyst.

Describe the radiographic characteristics of a complicated cyst.

A complicated cyst has an unusual contour, internal septations, calcifications, and/or evidence of enhancement on a contrasted CT scan.

What are the Bosniak criteria for cyst classification?

Morton A. Bosniak, MD, originally described his classification in 1986 and has subsequently updated it many times. The classification delineates cysts into 5 categories of renal cysts. The categorizations have management implications associated with the potential risk of malignancy.

What is Bosniak I?

Bosniak I cysts are solitary and fit the sonographic and CT criteria for simple cysts. Cysts would be sharp, thin, smooth walled, spherical, or ovoid water density lesions (–10 to + 20 HU), with no internal echoes and no enhancement after intravenous contrast administration on CT. There is less than a 2% chance of being associated with a malignancy.

What is Bosniak II?

Bosniak II cysts have thin central septations, thin peripheral calcifications, or they can be hyperdense nonenhancing lesions with 50 to 100 HU. They are less than 3 cm in size. There is a 10% to 13% chance of being associated with a malignancy.

What is Bosniak IIF?

These cysts can have several thin central septations, thickened peripheral calcifications, or they can be hyperdense minimally enhancing lesions. They are greater than or equal to 3 cm in size. They may be totally intrarenal. There is a 14% to 25% chance of being associated with a malignancy. “F” is for follow-up. These cysts need follow-up examinations to demonstrate stability. About 10% of llF cysts will progress. Radiographic changes in the cyst warrant excision.

What is Bosniak III?

These are complicated cysts with extensive calcifications, thickened walls (greater than or equal to 1 mm thick), and irregular borders. They have irregularly enhancing walls. There should be a high level of suspicion for malignancy in these lesions. There is approximately a 50% to 54% chance of being associated with a malignancy. Bosniak III cysts warrant excision.

What is Bosniak IV?

Bosniak IV describes a complex renal cystic mass with thick septa and/or thick walls, thickened calcifications that can be central or peripheral, and/or an enhancing component on contrasted images. These lesions can be found in association with a solid tumor. There is a 90% to 92% chance of being associated with a malignancy. Bosniak IV cysts require excision.

In the Bosniak renal cyst criteria, which is more ominous: tissue calcification or tissue enhancement with contrast?

Tissue enhancement with contrast is more ominous and more likely to be associated with malignancy.

What are other factors to consider in regard to renal cyst imaging and chance of malignancy?

Bosniak IIF and III cysts have a higher malignancy rate in patients with a history of primary renal malignancy or either a coexisting Bosniak IV lesion and/or a solid renal neoplasm.

Should a complex cystic renal mass be biopsied?

Percutaneous needle biopsy plays a limited role in the evaluation of cystic renal masses. Differences of opinion can arise regarding which category a particular case should be placed (whether Bosniak category II, IIF, or III). Follow-up imaging is advocated in these situations to determine progression. Some centers advocate needle biopsy of Bosniak III lesions.

What would be the expected positive biopsy rate of a Bosniak III lesion?

Centers that advocate biopsy of Bosniak III lesions report 39% nonmalignancy rate in their biopsy experience, thus saving patients from unnecessary surgery. In addition, biopsy as part of a percutaneous ablation of the renal lesion has also been advocated. With current laparoscopic and robot-assisted laparoscopic techniques, excisional biopsy of the entire lesion or partial nephrectomy is possible with low morbidity to the patient.

What findings would suggest an inflammatory etiology, what should be done to confirm this, and how should it be treated if inflammation is confirmed?

If the urologist suspects the cystic mass is inflammatory (pyuria, etc) or there is radiological evidence suggesting inflammation (such as perinephric fat stranding), puncture is acceptable. When the outcome is infection, the lesion can be treated and followed.

What are the risks of percutaneous needle biopsy of a cystic renal mass?

Risks of percutaneous renal cyst biopsy include bleeding, infection, and possible tumor track spread. Although considered rare, Dr Bosniak considered needle track spread of tumor an underestimated and underreported risk of cyst biopsy. A core biopsy of the wall of a cystic lesion (benign or malignant) can cause it to rupture and spill its contents into the surrounding tissues. Furthermore, a negative biopsy result does not absolutely rule out malignancy, particularly in cystic lesions that have less bulk of tissue to sample.

What are the clinical or historical correlates to a complicated renal cystic mass found on imaging study?