What is an Interpolar Region Cyst?
An interpolar region cyst refers to a cyst located in the middle portion of the kidney, between the upper and lower poles. This is an anatomical descriptor indicating the cyst's position within the renal parenchyma, rather than a specific disease entity.
Anatomical Context
The kidney is divided into three regions:
- Upper pole (superior portion)
- Interpolar region (middle portion)
- Lower pole (inferior portion)
When a cyst is described as "interpolar," it simply means the cyst is situated in the middle third of the kidney, between the upper and lower poles. This location descriptor is commonly used in radiology reports to precisely document where renal cysts are found.
Clinical Significance
The interpolar location itself does not determine the clinical significance or management of a renal cyst. What matters is:
- Cyst characteristics: Simple vs. complex features on imaging
- Size: Larger cysts (>15 cm are considered "giant") may cause symptoms 1
- Relationship to renal structures: Proximity to collecting system, vessels, or parenchyma
- Enhancement pattern: Pseudoenhancement is more likely in smaller cysts and those adjacent to highly enhancing parenchyma 2
Types of Cysts That Can Occur in the Interpolar Region
Simple Renal Cysts
Most interpolar cysts are simple renal cysts—benign, fluid-filled lesions that:
- Increase in incidence with age
- Are typically asymptomatic
- Require no treatment or follow-up if truly simple 3
- Appear as thin-walled structures with fluid isodense to cerebrospinal fluid on imaging
Parapelvic Cysts
Parapelvic cysts can occur in the interpolar region when they arise adjacent to the renal sinus 4. These are:
- Generally single and larger than typical simple cysts
- Incompletely surrounded by renal parenchyma
- Important to recognize as they may indicate underlying conditions like Fabry disease, autosomal dominant polycystic kidney disease, or tuberous sclerosis 4
Key Clinical Pitfall
Do not confuse anatomical location (interpolar) with pathological diagnosis. The term "interpolar cyst" only tells you where the cyst is located, not what type of cyst it is. The critical determination is whether the cyst is:
- Simple (benign, no follow-up needed)
- Complex (requires further evaluation)
- Part of a systemic disease process
Imaging Evaluation
When an interpolar cyst is identified:
- CT characteristics: Measure attenuation, assess for enhancement, evaluate wall thickness
- Size matters for pseudoenhancement: Smaller cysts (<3 cm) adjacent to highly enhancing parenchyma may show false enhancement >10 HU 2
- MRI with subtraction sequences: Can definitively distinguish true enhancement from pseudoenhancement
Bottom line: "Interpolar region cyst" is purely a location descriptor—the cyst's imaging characteristics and clinical context determine its significance and management, not its anatomical position within the kidney.