Congenital and polycystic diseases
Conditions associated with genetic mutations or tissue dysplasia:
- Autosomal recessive polycystic kidney disease (ARPKD): Manifests in newborns and young adults. Characterized by enlarged kidneys with bilateral multiple microcysts that are too small for detailed visualization.
- Autosomal dominant polycystic kidney disease (ADPKD): Usually manifests around 30 years of age. Kidneys are enlarged, containing bilateral multiple cysts—both simple and complex, sometimes with areas of calcification.
- Multicystic dysplastic kidney (MCDK): Unilateral lesion detected in infants (or later as an incidental finding). Cysts are separated by non-functional tissue, while the ureter may be absent or atretic.
- Obstructive renal dysplasia (ORD): Detected in the neonatal period. A unilateral condition with parenchymal cysts, dilation of the collecting system, and a thinned cortex without differentiation.
Tumor-like lesions
This group includes benign masses that are difficult to differentiate from malignant cystic renal cell carcinoma (RCC) on diagnostic imaging.
- Cystic nephroma: Has a bimodal distribution (boys in the first decade of life and women in the 5th–6th decade). A unilateral, multilocular mass with a capsule and variable enhancement of the septa.
- Mixed epithelial and stromal tumor: Occurs in postmenopausal women. A unilateral multilocular cyst with a solid component, delayed enhancement of the septa, and nodules or calcifications in the walls.
Cysts associated with syndromes and external factors
Renal involvement can be a consequence of systemic diseases or toxic exposure:
- Von Hippel-Lindau (vHL) syndrome: Bilateral simple and complex cysts are detected in 75% of patients. High risk of malignancy (up to a 70% lifetime probability of developing RCC).
- Tuberous sclerosis (TS): Unilateral small cysts in limited numbers are observed in 14–47% of patients. The risk of cancer is elevated due to epithelial atypia.
- Lithium-induced disease: Patients on long-term lithium therapy develop bilateral microcysts (1–2 mm) in the cortex and medulla, sometimes with calcifications.
- Acquired cystic disease: Associated with end-stage kidney disease (ESKD). Characterized by the presence of three or more small cysts in each of the shrunken (atrophic) kidneys bilaterally.
Briefly: answers to topical questions
To which category in the Bosniak classification do simple renal cysts belong?
Simple renal cysts belong to Category I (Type I) according to the Bosniak classification.
- Category I (Type I) is a simple benign cyst (simple benign cyst), which is a homogeneous lesion with fluid density and a thin, smooth wall.
Such cysts lack septa (septa), calcifications, and solid components, and show no contrast enhancement after intravenous administration. The risk of malignancy is 0%, so imaging follow-up is not recommended.
What extrarenal manifestations are characteristic of autosomal dominant polycystic kidney disease (ADPKD)?
Autosomal dominant polycystic kidney disease is characterized by involvement of multiple organs besides the kidneys. The main extrarenal manifestations include:
- Liver cysts — found in 80% of patients, while hepatic failure rarely develops even with massive infiltration.
- Pancreatic cystic changes — detected in 10% of patients.
- Splenic cysts — are also a typical manifestation of the disease.
- Aneurysms of the arteries of the circle of Willis — a concomitant pathology occurring in 10–16% of patients according to autopsy data.
What diagnostic imaging methods are used to detect and differentially diagnose renal cysts?
Ultrasound, CT, and MRI are used for the detection and differential diagnosis of renal cysts; FDG-PET is used to detect infected cysts.
- Ultrasonography (US) — used to evaluate cysts, but internal echoes can give a cyst a solid appearance and lead to an overestimation of the complexity category.
- Computed tomography (CT) — used for differential diagnosis, including hyperdense cysts, and to assess contrast enhancement; multiphase CT is used for renal masses.
- Magnetic resonance imaging (MRI) — better demonstrates the internal architecture of cystic masses, including septa, and is suitable for monitoring patients at genetic risk for renal malignancies.
- Fluorodeoxyglucose PET (FDG PET) — effective for detecting infected cysts.
What renal neoplasms, besides cysts, are typical of tuberous sclerosis?
Besides cysts, angiomyolipomas and renal cell carcinoma are typical of tuberous sclerosis.
- Angiomyolipoma (angiomyolipoma) — a benign mass composed of adipose tissue, smooth muscle, and abnormal blood vessels. This is the most common renal lesion in this disease, often multiple and occurring at a young age.
- Renal cell carcinoma (renal cell carcinoma, RCC) — a malignant tumor whose risk of development is increased due to the presence of atypical cells lining the renal cysts. In tuberous sclerosis, this cancer occurs at a younger age and is more frequently bilateral.
Which genetic mutations cause autosomal dominant polycystic kidney disease (ADPKD) and at what age does it manifest?
Autosomal dominant polycystic kidney disease is caused by mutations in the PKD1 and PKD2 genes, and predominantly manifests around 30 years of age.
- PKD1 gene — the mutation is localized on the short arm of chromosome 16 and causes the disease in 90% of cases.
- PKD2 gene — mutation on chromosome 4, found in 10% of cases.
About 50% of cases are due to spontaneous mutations. The disease most commonly manifests in the third decade of life, but due to high phenotypic variability, it can be detected antenatally or in childhood.
Class and exam questions
What is the main difference in the imaging presentation of autosomal recessive and autosomal dominant polycystic kidney disease?
In ARPKD, cysts are so small that they are visualized as microcysts, whereas in ADPKD, large simple and complex cysts are visible, sometimes with calcification.
Which benign cysts carry a high risk of developing cancer?
Primarily for cysts in von Hippel-Lindau syndrome (70% lifetime risk) and in tuberous sclerosis.
What does acquired cystic kidney disease look like on imaging?
It is detected in patients with end-stage renal failure as 3 or more small cysts against the background of bilateral renal atrophy.
What else to study on this topic
- Composition and types of renal calculi (oxalates, struvites, urates)
- Causes of the formation of various stone types
- Features of diagnostic imaging in urolithiasis (non-contrast CT)
- Dual-energy CT: physical principles of the method
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