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Bosniak/kidney mass/kidney cyst classification

  • benign simple cyst
    • hairline-thin wall of ≤2 mm
    • water density
    • no septa, calcifications, or solid components
    • no enhancement
    • work-up: none
    • percentage malignant: ~0%
  • benign cyst – “minimally complex”
    • few hairline thin <1 mm septa or thin calcifications (thickness not measurable)
    • perceived enhancement
    • non-enhancing high-attenuation (due to proteinaceous or hemorrhagic contents) renal lesions <3 cm
    • generally well marginated
    • work-up: none
    • percentage malignant: ~0%
  • minimally complex
    • 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 3
    • percentage malignant: ~5%
  •  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 ref
    • percentage malignant: ~55%
  • 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: ~100%

Bosniak Classification and Recommendations (multiphasic CT/MR):

Bosniak 1 – hairline-thin wall; no septa, calcifications, or solid components; water attenuation/signal intensity; no enhancement – no further imaging follow-up is required.

Bosniak 2 – (< 2 septations, < 2 mm thick) few hairline-thin septa with or without perceived (not measurable) enhancement; fine calcification or short segment of slightly thickened calcification in the wall or septa; homogeneously high-attenuating masses (≤ 3 cm) that are sharply marginated and do not enhance – no further imaging follow-up is required.

Bosniak 2F – (> 2 septations, > 2 mm thick) multiple hairline-thin septa with or without perceived (not measurable) enhancement, minimal smooth thickening of wall or septa that may show perceived (not measurable) enhancement, calcification may be thick and nodular but no measurable enhancement present; no enhancing soft tissue components; intrarenal non-enhancing high-attenuation renal masses (>3 cm) – refer or send to Urology (follow-up with appropriate imaging studies annually for 5 years).

Bosniak 3 – thickened irregular or smooth walls or septa, with measurable enhancement – refer to Urology for possible surgical excision.

Bosniak 4 – criteria of category 3, but also containing enhancing soft tissue components adjacent to or separate from the wall or septa – refer to Urology for possible surgical excision.

 

 

 

Imaging Management of Asymptomatic Renal Cyst/Mass:

Simple renal cyst or angiomyolipoma:

US findings – hairline-thin wall, no septa, no calcifications, no solid components, or well-demarcated homogeneously hyperechoic mass
CT/MRI findings – hairline-thin wall, no septa, no calcifications, no solid components, water attenuation/signal intensity, no enhancement, or solid mass with fat attenuation (and no calcification)
Recommendation – no further imaging follow-up is required.

Benign-appearing complicated renal cyst:
US findings – low level echoes or layering debris, 1-2 septations up to 2 mm thick, sub-centimeter hypoechoic lesions, growing cysts without increasing complexity
CT/MRI findings – 1-2 hairline-thin (up to 2 mm) septations with or without perceived (not measurable) enhancement, fine calcification or short segment of slightly thickened calcification in the wall or septa, homogeneously high-attenuating masses (< 3 cm that are sharply marginated and do not enhance) or >70 HU on non-contrast CT
Recommendation – no further imaging follow-up is required.

Indeterminate renal mass <1 cm:
US findings – lesions <1 cm with no worrisome features*
CT/MRI findings – lesions <1 cm benign-appearing, too small to characterize (including volume averaging effects)
Recommendation – no further imaging follow-up is required.

Indeterminate renal mass <1 cm:
US findings – lesions <1 cm with worrisome features*
CT/MRI findings – lesions < 1 cm with worrisome features*
Recommendation – may consider follow-up imaging in 1 to 2 years.  In select instances, the radiologist may recommend further evaluation with renal US instead.

Indeterminate renal mass >1 cm:
US findings – lesions >1 cm with worrisome features*
CT/MRI findings – lesions >1cm with worrisome features*
Recommendation – generally recommend CT 3-phase kidneys or MR kidneys with and without IV contrast.  In select instances, the radiologist may recommend further evaluation with renal US  instead.

*Worrisome features:  3 or more septations, septations >2 mm thick, thick or irregular wall, solid component, or increasing complexity

Suspicious renal mass >1 cm:
US findings – thick nodular multiple septa, thick nodular wall
CT/MRI findings – thick nodular multiple septa, septa or wall with measurable enhancement
Recommendation – refer to Urology for possible surgical excision.

Malignant renal mass:
US findings – irregular wall thickening, solid component
CT/MRI findings – irregular wall thickening, solid component, enhancing component, solid mass with coexisting fat and calcification
Recommendation – refer to Urology for possible surgical excision.

 

IMPRESSION – The lesion in the R/L kidney fits the radiologic definition of:

Bosniak Class I – No follow up is generally required since there are no identified radiologic criteria of malignancy.

Bosniak Class II – No follow up is generally required since there are no identified radiologic criteria of malignancy.

Bosniak Class II F – This lesion is likely benign, but follow up imaging is suggested.

Bosniak Class III – This lesion has malignant characteristics and urology consultation is suggested.

Bosniak Class IV – This lesion has malignant characteristics and urology consultation is suggested.

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