Bosniak classification version 2019 updates the 1986/2005 system for stratifying malignancy risk in cystic renal masses on CT/MRI. Key changes include MRI incorporation, precise enhancement definitions, expanded Bosniak II criteria, and reduced Bosniak III category to minimize unnecessary surgeries.

Simple cysts (Bosniak I) in 20-40% adults >50 years. Complex cystic masses (IIF-IV) comprise 1-4% renal masses. Updated malignancy risk stratification: * I/II: 0% * IIF: ~7% * III: ~60% * IV: >90%

Incidental finding on imaging for unrelated conditions (90%). Symptomatic: flank pain, hematuria, palpable mass (large/advanced lesions).

Bosniak I/II: benign simple epithelial cysts Bosniak IIF: mostly benign, low-risk complex cysts Bosniak III: indeterminate (multilocular cystic RCC, hemorrhagic cysts) Bosniak IV: cystic renal cell carcinoma (90%+ malignant)

• Bosniak I (benign, 0% risk)
• Thin wall (≤2 mm, may enhance)
• Simple fluid content (-9 to 20 HU)
• No septa, calcifications, or solid components

• Bosniak II (benign, 0% risk) - EXPANDED
• Six subtypes:
  • Thin septa (≤2 mm, 1-3, may enhance ± calcification)
  • Mildly thickened wall/septa (~3 mm perceived thickness)
  • Homogeneously hyperattenuating (70+ HU, non-enhancing)
  • Homogeneous >20 HU cysts (non-enhancing)
  • Too small to characterize (<15 mm, homogeneous)
  • Complete cortical rim of fat

• Bosniak IIF (follow-up, ~7% risk)
• Wall/septa >3 mm and <4 mm (smooth, measurable enhancement)
• ≥4 septa (≤2 mm thick, may enhance)

• Bosniak III (surgery, ~60% risk) - REDUCED
• Thick (≥4 mm) or irregular enhancing walls/septa
• No non-enhancing thick/irregular walls (now II/IIF)

• Bosniak IV (malignant, >90% risk)
• Enhancing soft-tissue nodule (≥4 mm)
• Any size nodule with acute margins

• Enhancement definition: ≥20 HU (CT) or visually perceived/measurable (MRI)

Screening only. Cannot assess enhancement or precise measurements. Simple cysts: anechoic, thin walls, posterior enhancement.

* I/II: no follow-up * IIF: surveillance CT/MRI (6, 12, 24 months) * III: surgical resection * IV: nephron-sparing surgery/ablation Reduces unnecessary surgeries by 20-30% vs original classification.

• Abscess: clinical fever, wall enhancement
• Hydatid cyst: daughter cysts, eccentric wall calcification
• Hemorrhagic cyst: evolves on follow-up
• Lymphangioma: multiple septations
• Metastases: multiple lesions

* Radiology 2019: Bosniak v2019 * RadioGraphics 2021: Pictorial Guide * Radiology Assistant: Bosniak 2019

Medical Disclaimer: Portions of this content were AI-generated to facilitate rapid knowledge synthesis. Radiologists and clinicians must independently verify all information against peer-reviewed literature, institutional protocols, and patient-specific factors before clinical application.