MRI and CT Characteristics of Successfully Ablated Renal Masses: Imaging Surveillance After Radiofrequency Ablation

OBJECTIVE. The objective of our study was to evaluate the evolution of the appearances of successfully ablated renal masses on CT and MRI. MATERIALS AND METHODS. We conducted a retrospective review of 28 solid renal masses in 25 patients who underwent percutaneous radiofrequency ablation (RFA) between July 2003 and July 2006 in whom there was no evidence of residual tumor during at least 1 year of imaging follow-up and there was postablation biopsy proof of nonviable tissue within the ablation cavity. Three radiologists assessed the size, morphology, and CT or MRI characteristics of the initial tumor and of the ablated tumor or ablation cavity at imaging follow-up 1-2, 3-5, 6-11, and 12-24 months after RFA. RESULTS. The mean initial tumor volume was 5.5 cm(3) (range, 0.3-22.3 cm(3)). Within 1-2 months, the postablation beds of small masses (<= 3 cm(3)) were larger than the volume of the initial tumor. Large masses (> 3 cm(3)) did not show this increase in volume. At 12 months after RFA, the postablation beds had decreased in size but had not disappeared. On CT, the postablation beds did not show enhancement at any time. On MRI, the postablation beds often showed a thin rim of peripheral enhancement. Imaging follow-up often revealed local stranding in the perinephric fat adjacent to the ablation site. Exophytic tumors were more likely to separate from the renal parenchyma as they contracted toward their epicenter and were more likely to reveal a complete halo of soft-tissue attenuation in the adjacent perinephric fat, which became more apparent on the longer-term follow-up imaging studies. CONCLUSION. Successfully ablated tumors show predictable imaging features that can be used to guide interpretation of CT and MRI surveillance examinations.