Cystic renal lesions

Main types of non-infectious cysts

Several common types of non-inflammatory cystic changes are distinguished:

• Hemorrhagic cysts (Haemorrhagic Cysts): on unenhanced CT, they show high attenuation (higher than simple cysts) and appear smooth and rounded. Upon contrast administration, the attenuation does not change—there is no true enhancement. Without evaluating unenhanced scans, they can easily be mistaken for a solid tumor.
• Peripelvic cysts: are of lymphatic origin and located in the renal sinus. Extending along the infundibula of the calyces, they can mimic hydronephrosis. However, CT in the pyelographic phase shows that the cysts do not communicate with the dilated renal pelvis.
• Parapelvic cysts: are serous cysts originating from the parenchyma protruding into the sinus. They are easily assessed on ultrasound.
• Multicystic renal dysplasia (Multicystic Renal Dysplasia): a non-hereditary unilateral condition, a common cause of a palpable abdominal mass in newborns.
• Localized cystic disease (Localised Cystic Disease): multiple cysts within a single kidney, with normal intervening cortical tissue. The contralateral kidney is typically normal.

Bosniak classification

To determine the risk of malignancy in cystic lesions, contrast-enhanced CT (or MRI) and the Bosniak classification are used:

1. Category IIF: cysts with prominent calcification. Even without contrast enhancement, they appear too atypical for simple benign cysts (Category II) and require follow-up imaging.
2. Category III: large lesions with thick walls and minimal wall enhancement. Ultrasound may reveal a complex, heterogeneous structure. These are frequently cystic carcinomas, but histology (e.g., following nephrectomy) may be required to differentiate them from cystic nephroma or hemorrhagic cysts.
3. Category IV: cystic renal cell carcinoma. CT and MRI show a large multiseptated cyst with a solid nodule that actively enhances with contrast.

Important: MRI demonstrates septa better, but assessment of contrast enhancement is the primary factor in determining malignancy.

Autosomal dominant polycystic kidney disease (ADPKD)

Autosomal dominant polycystic kidney disease (Adult Polycystic Kidney Disease) is a hereditary disorder with 100% penetrance.

Symptoms usually appear in adulthood. The kidneys are enlarged and contain numerous cysts of varying size. The pathology frequently affects other organs: cysts may be detected in the liver, pancreas, and spleen (though liver failure is rare). Concomitant aneurysms of the circle of Willis are characteristic.

Complications include intracystic hemorrhage (hyperdense lesions) and infection. CT and MRI detect cysts better than ultrasound and are necessary to rule out tumors in the setting of polycystic disease. FDG-PET is effective for detecting infected cysts.

Inflammatory and parasitic lesions

This group includes:

• Renal abscess (Renal Abscess): most commonly the result of an ascending infection (E. coli) or hematogenous seeding (Staphylococcus in infected stones). On CT, it appears as a fluid-attenuation lesion with no central enhancement, but with a thick, irregular enhancing wall. The presence of gas within the lesion is pathognomonic, though rare.
• Acute focal bacterial nephritis: focal inflammation without purulent liquefaction. On CT, it presents as a wedge-shaped or rounded lesion with heterogeneous contrast enhancement, frequently accompanied by perinephric fat stranding.
• Malacoplakia (Malacoplakia): a rare granulomatous inflammation, often mimicking an abscess or carcinoma. It occurs in women and immunocompromised individuals.
• Hydatid cysts (Hydatid Cysts): rare thick-walled intrarenal lesions (occasionally with calcifications). Upon rupture into the collecting system, they cause acute pain and passage of scolices in the urine.

Class and exam questions

What else to study on this topic

• Cystic nephroma: clinical presentation and imaging features
• Management strategies for patients with Bosniak category IIF and III renal cysts
• Renal hematomas: evolution of ultrasound and CT appearances according to the stage
• Renal abscess: differential diagnosis with lymphoma and infarction
• Rare vascular lesions (aneurysms and arteriovenous malformations)

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.