Bosniak Classification

Bosniak Classification of Renal Cysts

Bosniak I: Benign Simple Cyst

• Hairline-thin wall of ≤2 mm

• Water density (-10 to 20 HU)

• No septa, calcifications, or solid components

• No enhancement

• Percentage malignant: ~0%

Bosniak II: Benign Cyst - Minimally Complex

• Few hairlines thin <1 mm septa or thin calcifications (thickness not measurable)

• Perceived (not measurable) enhancement

• Non-enhancing high-attenuation (due to proteinaceous or hemorrhagic contents) renal lesions <3 cm

• Generally well marginated

• Percentage malignant: ~0-6%

Bosniak IIF: Minimally Complex Cyst

• Multiple hairline thin septa or minimally smooth thickened walls or septa

• Perceived but no measurable enhancement of wall or septa

• Calcification can be present and may be thick and nodular

• Generally well marginated

• High-attenuation lesion >3 cm diameter, totally intrarenal (<25% of wall visible); no enhancement

• Requiring follow-up (F for follow-up): needs ultrasound/CT/MRI follow up - no strict rules on the time frame but reasonable at 6 months, 12 months, then annually for 5 years

• Percentage malignant: ~5-26%

Bosniak III: Indeterminate Cystic Mass

• Thickened irregular or smooth walls or septa with measurable enhancement

• Treatment/work-up: partial nephrectomy or radiofrequency ablation in poor surgical candidates

• Percentage malignant: ~55-72%

Bosniak IV: Clearly Malignant Cystic Mass

• Bosniak III criteria + enhancing soft tissue components adjacent to but independent of wall or septum

• Treatment: partial or total nephrectomy

• Percentage malignant: ~91-100%