Renal Cyst

Fluid-filled renal lesion

Bosniak classification

• Imaging-based classification of renal cystic lesions

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    Class I: Benign cysts

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    Class II: Minimally complicated cysts; benign

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    Class IIF: Requires CT/MR imaging follow-up

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    Class III: More complicated cysts; usually managed surgically (biopsy controversial)

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    Class IV: Malignant lesions; require surgery

Simple cyst

• US: Simple, uncomplicated cyst; spherical or ovoid, anechoic content, sharply defined, imperceptible wall, and acoustic enhancement

• NECT: Sharply marginated, round, smooth, homogeneous, hypodense (< 20 HU) mass

• MR: ↓ signal intensity on T1WI and ↑ signal intensity on T2WI

• No enhancement

Complicated, hemorrhagic cyst

• Ultrasound: Internal echoes (clot); thick calcified wall ± multiloculated (chronic)

• NECT: ↑ density

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    > 70 HU on NECT = hyperdense, benign renal cyst requiring no follow-up or treatment

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    20-70 HU on NECT: Requires contrast administration to exclude enhancing components; alternatively, US or MR imaging (T1WI, T2WI, DWI) can be used if patient cannot tolerate IV contrast administration

• MR: Usually ↑ signal intensity on T1WI and ↓ signal intensity on T2WI (signal changes according to hemorrhage stage)

• No enhancement

Neoplastic cystic masses: Enhancing soft tissue component

Imaging recommendations

• CT: NECT + CECT nephrographic phase (100 seconds after IV contrast administration), section thickness ≤ 5 mm

• Consider US or MR for cystic lesions with equivocal enhancement at CECT

Renal cell carcinoma

Multilocular cystic nephroma

Renal abscess

Renal metastases and lymphoma

Autosomal dominant polycystic kidney disease

Uremic cystic disease

Asymptomatic or palpable mass and flank pain

Present in 20-30% of middle-aged adults, incidence increases with age

Image evaluation and classification of cystic masses are key to management

Fluid-filled renal lesion

Best diagnostic clue

• Water density, nonenhancing, spherical lesion with no visible wall on CT

• Anechoic lesion with through transmission and no visible wall on US

Bosniak classification

• Imaging-based classification of renal cystic lesions

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    Used with CT and MR imaging

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      Lesion classification may be upgraded on MR images because of ↑ contrast resolution

• 5 categories

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    Class I: Benign cysts

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      Well-defined, rounded, homogeneous, lucent (0- to 20-HU, near-water-density) mass with thin or imperceptible, nonenhancing wall and contents; no septa or calcifications

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    Class II: Minimally complicated cysts; benign

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      Thin (< 1 mm) septations, smooth, ± perceived (not measurable) enhancement

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      Calcification of short segment of wall or septa

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      Hyperdense cyst (> 20 HU), no enhancement, spherical, partially exophytic, usually ≤ 3 cm in diameter

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    Class IIF: Requires CT/MR imaging follow-up

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      Multiple thin septations ± perceived (not measurable) enhancement

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      Minimal wall or septal thickening ± perceived (not measurable) enhancement

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      Thick calcifications

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      Hyperdense, intraparenchymal cysts > 3 cm

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      No enhancing nodules

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    Class III: More complicated cysts; usually managed surgically (biopsy controversial)

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      Irregular and thickened septa &/or wall + measurable enhancement

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      ± thickened and irregular calcification

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      Benign lesions: Hemorrhagic cysts, renal abscess, mixed epithelial and stromal tumor, complicated benign septated cysts

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      Malignant lesions: Cystic nephroma, multiloculated cystic renal cell carcinoma

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    Class IV: Malignant lesions; require surgery

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      Enhancing component, irregular wall thickening

Grayscale ultrasound

• Simple, uncomplicated cyst: Spherical or ovoid, anechoic content, sharply defined, imperceptible wall, and acoustical enhancement

• Hemorrhagic cyst: Internal echoes (clot); thick calcified wall ± multiloculated (chronic)

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    Cyst complexity may be ↑ on US than on CT

• Infected cyst: Thick wall with scattered internal echoes ± debris-fluid level

Color Doppler

• Neoplastic cystic lesion

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    Blood-flow within solid components, mural nodules