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