Simple renal cyst is the most common focal lesion of the kidney, found in over 50% of the adult population; prevalence increases linearly with age (50+ at age >50, 70+ at age >70). Classified as Bosniak category I — characterized by thin imperceptible wall, clear serous fluid content, absence of internal structure (no septa, calcification, or solid component), and no enhancement. It originates from tubular epithelium and forms from obstruction and dilatation of renal tubules. Water density (0-20 HU), T1 hypointense, markedly T2 hyperintense, and shows no enhancement in any phase on non-contrast/contrast imaging. It is asymptomatic and incidentally detected; no follow-up or treatment is required. Uncomplicated simple cysts have no risk of malignant transformation. Rare complications include hemorrhage (hemorrhagic cyst → Bosniak II), infection, and rupture.
Age Range
30-90
Peak Age
60
Gender
Equal
Prevalence
Very Common
Simple renal cysts originate from renal tubular epithelium. The formation mechanism is tubular obstruction and progressive dilatation: during aging, tubules in nephrons develop focal obstruction (due to intratubular calcification, fibrosis, or epithelial hyperplasia) → glomerular filtrate accumulates proximal to obstruction → tubule progressively dilates → cystic cavity forms. The cyst wall is lined by a single layer of flattened tubular epithelium — this wall is so thin it is defined as 'imperceptible wall' on imaging. The cyst content is clear serous transudate fluid derived from glomerular filtrate — containing no protein or cellular elements. The water-like composition is reflected in imaging as water density (0-20 HU on CT), long T1 relaxation time on T1 (hypointense signal), and long T2 relaxation time on T2 (markedly hyperintense signal). No wall vascularization exists → does not retain contrast agent → shows no enhancement in any phase. This avascular, transudative, structureless content has no malignant transformation potential.
When all four Bosniak I criteria are met — water density (0-20 HU), imperceptible thin wall, absence of internal structure (septa, calcification, solid component), and absence of enhancement (<10 HU difference) — the diagnosis of simple cyst is definitive with 0% malignancy risk. Failure to meet any of these criteria upgrades the Bosniak class: visible thin septa or thin calcification → Bosniak II; enhancing septa/wall → Bosniak III; solid enhancing component → Bosniak IV.
Homogeneous water-density (0-20 HU) round/oval lesion on non-contrast CT. Wall is imperceptible. No internal structure — homogeneous, no septa/calcification/debris. Margins are sharp and regular. In small cysts (<1 cm), density may measure higher than actual value due to partial volume effect — this artifact should not raise malignancy suspicion. Cysts showing >20 HU density suggest proteinaceous or hemorrhagic content and are classified as Bosniak II.
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Water-density (__ HU) simple cyst with no internal structure and thin wall in the kidney; consistent with Bosniak category I, requiring no follow-up.
No cyst enhancement in portal venous phase of contrast-enhanced CT — <10 HU difference from pre-contrast density. While renal parenchyma enhances markedly (100-150 HU in nephrographic phase), the cyst remains at water density → cyst-parenchyma contrast is maximized and cyst becomes very clearly visible. <10 HU increase is attributed to 'pseudo-enhancement' artifact and is not considered true enhancement. >15-20 HU increase is considered measurable enhancement and requires further investigation for suspected solid component or vascularized wall.
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The cyst shows no enhancement in contrast phases with pre/post-contrast density difference <10 HU; absence of measurable enhancement supports the diagnosis of simple cyst (Bosniak I).
Markedly hyperintense signal on T2-weighted images — at the same signal intensity as CSF (cerebrospinal fluid). Homogeneous, sharply marginated, round/oval lesion. No internal structure (no septa, debris, or nodules). Signal intensity is homogeneous throughout the entire cyst — heterogeneity should raise suspicion for hemorrhagic or infected cyst. Remains markedly hyperintense on fat-sat T2 (signal unchanged as no fat content).
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Cystic lesion in the kidney with CSF-equivalent homogeneous hyperintense signal on T2-weighted images without internal structure, consistent with simple cyst.
Homogeneous hypointense signal on T1-weighted images — free water characteristically shows low T1 signal. Signal intensity is significantly lower than renal parenchyma and adjacent tissues. T1 hyperintensity is not seen in simple cyst — high T1 signal suggests proteinaceous (>6 g/dL protein) or hemorrhagic (methemoglobin) content and upgrades Bosniak class. No cyst enhancement on post-contrast T1.
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The cyst shows homogeneous hypointense signal on T1-weighted images consistent with pure fluid content; absence of T1 hyperintensity excludes proteinaceous or hemorrhagic content.
Anechoic (completely echo-free), round/oval, thin-walled lesion on B-mode ultrasound. Prominent posterior acoustic enhancement (through-transmission) — confirmation of fluid-filled cavity. Wall is thin and regular. No internal structure — no echoes, septa, or debris. Refraction shadows ('edge artifact') may be seen at lateral margins — this is a benign cyst artifact, not a malignancy marker. No vascularity in cavity or wall on Doppler.
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Anechoic, thin-walled simple cyst with posterior acoustic enhancement in the kidney on US without internal structure or vascularity; consistent with Bosniak I.
Simple cyst shows no diffusion restriction on DWI. At low b-values (b=0-50) the cyst may appear hyperintense (T2 shine-through), but at high b-values (b=800-1000) signal is lost. The cyst shows high ADC value on ADC map (>2.5 × 10⁻³ mm²/s) — free water diffusion. This finding distinguishes it from infected cyst (low ADC) and cystic RCC (low ADC in solid component).
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No diffusion restriction is observed in the cyst on DWI with high ADC value; this finding is consistent with free water diffusion and reduces the likelihood of infected cyst or cystic neoplasm.
Criteria
Located in renal cortex, growing toward parenchymal surface. Most common type.
Distinct Features
Easily visualized on US. May extend to perinephric space when exophytic. Partial volume artifact less problematic.
Criteria
Located in renal sinus/hilar region. May be of lymphatic origin.
Distinct Features
May mimic hydronephrosis on US (disruption of sinus echogenicity). Differentiated by CT/MRI — parapelvic cyst shows no communication with collecting system, no opacification on excretory phase.
Criteria
Size >10 cm. Mass effect or compression symptoms may develop.
Distinct Features
No malignancy risk if simple cyst criteria are maintained (water density, no enhancement). Sclerotherapy or laparoscopic decortication may be applied if symptomatic. Spontaneous rupture is a rare complication.
Distinguishing Feature
Bosniak II cyst may have few thin septa (≤2, <2 mm), thin calcification, or hyperdense content (>20 HU); simple cyst lacks these features. Neither shows measurable enhancement and malignancy risk is <5%.
Distinguishing Feature
Cystic RCC shows thick irregular septa, solid enhancing component, and wall nodules; simple cyst has no internal structure or enhancement. Cystic RCC is classified as Bosniak III-IV. DWI restriction present in solid component.
Distinguishing Feature
Renal abscess shows thick enhancing wall, peripheral edema, and marked DWI restriction (low ADC); simple cyst has no enhancement, edema, or DWI restriction. Clinical: fever+leukocytosis in abscess, asymptomatic in simple cyst.
Distinguishing Feature
Multilocular cystic nephroma characterized by multiple septations, enhancing septa, and herniation into collecting system; simple cyst is unilocular, non-septated, non-enhancing. Cystic nephroma classified as Bosniak III-IV.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upSimple renal cyst (Bosniak I) is definitively benign — malignancy risk is 0%. No follow-up or treatment is required. Reporting: small cysts (<1 cm) may not be reported; cysts >1 cm are described with a 'no follow-up required' note. For symptomatic giant cysts (compression, pain), percutaneous aspiration+sclerotherapy or laparoscopic decortication may be applied. Rare complications: hemorrhage → hemorrhagic cyst (Bosniak II, T1 hyperintense), infection → infected cyst (DWI restriction, rim enhancement), spontaneous rupture. ADPKD exclusion: bilateral multiple cysts + family history + increased kidney size should raise ADPKD suspicion. In the 2019 Bosniak update, hyperdense cyst >70 HU on non-contrast CT (benign hemorrhagic cyst) meeting 'too small to characterize' criteria requires no follow-up as Bosniak II.
Simple renal cysts are benign and require no treatment. Very large cysts may become symptomatic and aspiration or sclerotherapy may be applied. No risk of malignant transformation. Follow-up is unnecessary.