Renal cysts: Grading

Bosniak 1:
Simple cyst, anechoic on US and water density ( less than 10HU) on CT, imperceptable (hairline) wall, rounded, no calcification, no septation, no enhancement.
No further follow-up.
Malignant potential - nil

Bosniak 2:
Cystic or hyperdense, minimally complex, one or two thin ( less than 1mm, if more than 1 mm type 3) septations, thin calcification, no enhancement.
No further follow-up.
Malignant potential - nil

Bosniak 2F:
Cystic or hyperdense, minimally complex, well defined, round, thin (one or two) septations (less than 1mm), thick calcification, mild enhancement, less than 3cm diameter, at least 1/4 wall seen, solid component do not enhance.
Follow-up by US/CT.
Malignant potential is about 5%

Bosniak 3:
Indeterminate, thick or multiple septations, mural nodule, hyperdense on CT, shows enhancement, thick or irregular wall.
Need partial nephrectomy or RF ablation.
Malignant potential about 50%

Bosniak 4:
Clearly malignant, solid mass with cystic spaces, enhance.
Need partial / total nephrectomy.
Malignant rate nears 100%