Bosniak classification of renal cyst

Bosniak I lesions follow the imaging characteristics of simple fluid with imperceptibly thin walls and no septations, calcifications or enhancement. On CT, simple fluid possesses an attenuation at or around 0 HU. On sonography, simple fluid is anechoic with increased through transmission.

Bosniak II lesions contain hair-thin septations, faint calcifications, and/or non-simple fluid. Again, no contrast enhancement should be present. CT demonstrates a faint calcification not seen on MR (blue arrow). Ultrasound clearly depicts the septum.

Bosniak III lesions may demonstrate wall or septal thickening, bulky calcifications, or septal enhancement. Unenhanced CT demonstrates a lobulated cystic lesion, with an irregularly calcified septum and the hint of other septa.

The hallmark of Bosniak IV lesions is enhancement. Any enhancement other than that of thin walls or septa places a lesion in this category. CT and MRI correlate well to demonstrate an enhancing cystic and solid right renal lesion, pathologically proven to be a clear cell renal cell carcinoma.