Magnetic resonance imaging characteristics of renal tumors after radiofrequency ablation

Objectives. The interpretation of radiographic findings in renal tumors treated with minimally invasive modalities, such as radiofrequency ablation (RFA), is critical for assessing treatment adequacy. Magnetic resonance imaging (MRI) is commonly used for patients with renal insufficiency or contrast allergy. Because the MRI experience with renal RFA is limited, we reviewed our experience and report the unique MRI characteristics of RFA-treated renal tumors. Methods. A single-institution database of renal RFA was reviewed to identify patients followed up with MRI. A radiologist and urologist retrospectively reviewed all MRI scans to identify the characteristic lesion findings after RFA. Results. Eleven patients (12 tumors) treated by RFA were followed up with MRI examinations. The mean follow-up was 10 months. The mean tumor size at ablation was 2.4 cm, with minimal reduction over time. Ablated lesions were characterized by high signal intensity on T₁-weighted images, low signal intensity on T₂-weighted images, and an absence of contrast enhancement. Of the patients treated by percutaneous RFA, 71% developed a characteristic peritumor halo, seen as a rim of low signal intensity on T₁-weighted and T₂-weighted images, surrounding a zone of perinephric fat just external to the ablated area. Two lesions demonstrated enhancement on the initial postablation MRI scan, indicating incomplete ablation. Conclusions. The MRI characteristics of successfully ablated renal tumors include the absence of enhancement, minimal size reduction over time, and high and low signal intensity on T₁-weighted and T ₂-weighted imaging, respectively. Knowledge of these findings can aid radiologists and urologists in correctly assessing the success of RFA for kidney lesions.