MRI evaluation of complex renal cysts using the Bosniak classification: a comparison to CT

The purpose of this study was to compare the ability of magnetic resonance imaging (MRI) and computed tomography (CT) to discriminate between benign and malignant cystic renal lesions utilizing the Bosniak classification.

We retrospectively searched our Radiological Information System using renal/kidney cysts as entries. The search retrieved 2929 patients and 525 complex renal cysts. After exclusions, 42 complex cysts, from 37 patients, with CT and MRI, up to six months apart, were included. Surgery and pathology report and follow-up of at least 24 months were used as a standard of reference.

The mean age of patients was 51.4 years, ranging from 11 to 82 years old. Twenty-nine lesions were classified as Bosniak I, II or II-F by CT and/or MRI and 13 as Bosniak III or IV, by one of the methods. The interobserver agreement for Bosniak classification for CT was 0.87 and 0.93 for MRI. Fifteen lesions had higher Bosniak categories on MRI, included six with change in management. Only two lesions had a higher category on CT, one with change in management. The frequency of malignancy for Bosniak III was 50 % (2/4) for CT and 20% for MRI (1/5), as Bosniak upgrades by MRI resulted in surgery for benign lesions. Both methods had 100 % frequency of malignancy for category 4.

MRI led to category migration and management change of complex renal cysts in a significant proportion of cases, likely due to its superior soft tissue and contrast resolution. The impact of MRI on detection and outcomes of malignant complex renal cysts still requires further investigation.