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Bosniak Classification For Renal Cysts

The Bosniak classification for renal cysts is helpful in the assessment of the malignant risk of the cystic renal lesions.

It is also useful in determining which cysts require follow-up and treatment.

The classification mainly describes the CT features but these features can be identified on MRI and ultrasound as well.

Bosniak Classification:

The Bosniak classification has divided the renal cysts into five categories.

Bosniak Classification
Bosniak Classification
  1. Type I – Simple Cyst:

  • In Bosniak I the cyst has thin, well-defined wall
  • It contains homogeneous water attenuation (0-20 HU) non-enhancing content.
  • It has no septa, calcifications or soft tissue component.
  • It does not require any further workup.
  • It has 0% chances of being malignant.
Bosniak Classification-Radiology Definition
Bosniak I: CT Abdomen (Axial View)
  1. Type II – Minimally Complicated Cyst:

  • In Bosniak II there is homogeneous high attenuation (20-40 HU, having proteinaceous content) renal lesions measuring less than 3cm.
  • These lesions are well marginated.
  • There are few thin septa that measure less than 1mm.
  • These cysts may show thin peripheral calcifications.
  • No work up and follow up is required.
  • These lesions are benign with 0% chance of malignancy.
Bosniak classification
Bosniak II: CT Abdomen (Saggital view)
  1. Type IIF – Follow up lesion:

  • In Bosniak IIF the lesions are more than 3cm in size and are intrarenal.
  • These are thick walled, high-density lesions. Wall may show enhancement.
  • There are increased number of thin septa which also shows enhancement.
  • Thick nodular calcifications are also present.
  • These lesions need ultrasound/CT follow-up.
  • There are 5% chances of malignancy.
Bosniak classification
Bosniak IIF: CT Abdomen (Sagittal View)
  1. Type III – Complicated (Surgical) Lesion:

  • Bosniak III lesions are multiloculated lesions with irregular margins and having uniform wall thickening
  • There are multiple irregular, thick, enhancing septa.
  • Multiple coarse irregular calcifications are present.
  • The non-enhancing mural nodule is seen.
  • These lesions are 50% malignant.
  • These lesions require partial nephrectomy and biopsy or radiofrequency ablation in the elderly patients with poor surgical risk.
Bosniak III CT Abdomen Axial view C+
Bosniak III CT Abdomen Axial view C+
Bosniak classification
Bosnial III: CT Abdomen (Axial view)
  1. Type IV – Malignant Cystic Lesion:

  • Bosniak IV lesions are the lesions with large cystic or necrotic component.
  • The lesions have irregular wall thickening.
  • These lesions have the solid enhancing element.
  • They are 100% malignant.
  • Treatment required is partial or total nephrectomy.
Bosniak classification
Bosniak IV: CT Abdomen (Axial View)
Types Walls Septa Calcifications Soft Tissue Component Chances of Malignancy Treatment
I Thin, well defined No No No 0% No
II Thin, well marginated Few, thin<1mm Thin, peripheral No 0% No
IIF Thick, enhancing Incresed no of thin, enhancing Septa Thick, nodular No 5% Follow up with CT/ultrasound
III Thick, irregular Multiple, thick, enhancing Multiple, coarse, irregular Non-enhancing mural nodule 50% Partial nephrectomy & biopsy
IV Thick, irregular Multiple, thick, enhancing Multiple, coarse, irregular Enhancing solid element 100% Total or partial nephrectomy

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