IMAGING

Follow-up after percutaneous radiofrequency ablation of renal cell carcinoma: contrast-enhanced sonography versus contrast-enhanced CT or MRI
Meloni MF, Bertolotto M, Alberzoni C, Lazzaroni S, Filice C, Livraghi T, Ferraioli G
Radiology Department, Vimercate General Hospital, Milan, Italy
AJR Am J Roentgenol. 2008; 191: 1233-8

• Objective: The purpose of this study was to assess, with contrast-enhanced CT or MRI as the reference imaging technique, the diagnostic performance of low-mechanical-index contrast-enhanced sonography in detecting local tumor progression after percutaneous radiofrequency ablation of renal tumors.
• Materials and Methods: Twenty-nine patients with 30 renal tumors (18 men, 11 women; mean age, 73 years; range, 53-83 years) underwent percutaneous radiofrequency ablation at a single center between March 1998 and January 2007. The imaging follow-up schedule was both contrast-enhanced sonography and CT or MRI 4 months after completion of treatment and every 4 months for the first year. Thereafter, the follow-up schedule was contrast-enhanced sonography every 4 months with CT or MRI every 8 months. The chi-square test with Yates correction was used to evaluate positive and negative predictive values and accuracy.
• Results: One patient was scheduled to undergo surgical resection, and another patient was lost to follow-up. Twenty-seven patients with 28 renal tumors participated in follow-up. The concordance between contrast-enhanced sonographic and CT or MRI findings was 100% for 27 of 28 tumors (96.4%) that had a hypervascular pattern before treatment. In the case of the tumor that was hypovascular at imaging performed before percutaneous radiofrequency ablation, local tumor progression was missed at contrast-enhanced sonography. The sensitivity, specificity, positive predictive value, negative predictive value, and overall accuracy of contrast-enhanced sonography were 96.6%, 100%, 100%, 95.8%, and 98.1%.
• Conclusion: Contrast-enhanced sonography is an effective alternative to CT and MRI in the follow-up of renal tumors managed with percutaneous radiofrequency ablation.
  
  
• Editorial Comment
  Percutaneous radiofrequency (RF) ablation and cryoablation are increasingly being used as minimally invasive treatments for renal tumors in patients whose condition is inadequate for surgery. Accurate imaging evaluation of ablated tumors is essential in order to detect the adequacy of treatment and to guide patient management. In comparison with normal renal parenchyma, renal tumors treated with RF ablation usually appear as low-attenuation regions at computed tomography (CT). On conventional magnetic resonance imaging (MRI) these treated lesions appears as areas with iso- to hyperintensity at T1-weighted imaging and area of hypointensity at T2-weighted imaging. After intravenous injection of contrast material, successfully treated renal tumors appear in either one method, as focal masses that demonstrate no evidence of contrast enhancement. These focal masses continue to decrease in size during the follow-up examinations. Residual or recurrent tumor is characterized by the presence of abnormal areas of contrast enhancement.
  The authors of this manuscript compared the findings at real-time low-mechanical-index contrast-enhanced sonography with those at CT or MRI in the follow-up of patients with renal cell carcinoma treated with RF ablation. They showed that in patients presenting hypervascular tumors before treatment, contrast-enhanced sonography has similar accuracy to that of CT or MRI for the detection of local tumor progression. Hypovascular renal tumors however were not adequately assessed by this technique. Since the evaluation of perfusion patterns with contrast-enhanced ultrasonography using contrast-pulse sequence imaging is useful in the follow-up of cryoablated renal tumors, it seems that this method is an effective alternative to CT and MRI in the follow-up of renal tumors in patients in whom the use of iodinated or paramagnetic contrast agent should be avoided and in those with any other clinical condition that precludes the use of CT or MRI evaluation.

Dr. Adilson Prando
Chief, Department of Radiology and
Diagnostic Imaging, Vera Cruz Hospital
Campinas, São Paulo, Brazil
E-mail: adilson.prando@gmail.com