Ultrasound Overestimates the Area of Necrosis During Radiofrequency Ablation in Porcine Livers 117 ULTRASOUND OVERESTIMATES THE AREA OF NECROSIS DURING RADIOFREQUENCY ABLATION IN PORCINE LIVERS Esperanza Grace Santi, M.D., 1 Sandy Lu, M.D., 2 Mario Milo, M.D., 3 Redigo Aguilar, M.D., 4 Hendra Nurjadin, M.D., 5 Susanne Gosalvez-Pe 6 and Stephen Wong, M.D. 7 117 Phil. J. Internal Medicine, 47: 117-120, May-June, 2009 Original Articles 1 Fellow, Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, Manila, Philippines, Tel. (02) 731-3001 loc 2282 2 Resident, Department of Radiology, University of Santo Tomas Hospital, Manila, Philippines 3 Consultant, Department of Radiology, University of Santo Tomas Hospital, Manila, Philippines 4 Fellow, Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, Manila, Philippines 5 Fellow, Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, Manila, Philippines 6 Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, Manila, Philippines 7 Consultant, Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, Manila, Philippines Reprint request to: Esperanza Grace R. Santi, M.D., Section of Gastroenterology, Department of Internal Medicine, University of Santo Tomas Hospital, España, Manila, Philippines, Tel. No. 731- 3001 local 2282 or 740-3178 ABSTRACT Background: Ultrasound is the most common imaging modality used to guide electrode insertion and estimate the necrotic zone during radiofrequency ablation (RFA) of hepatic tumors. However, most investigators agree that ultrasound inaccurately predicts the necrotic zone after RFA despite the scarcity of experimental and clinical evidence. Objective: We aimed to compare the necrotic diameter estimated from ultrasound during RFA with the actual gross diameter in ex vivo porcine livers. Methods: An internally-cooled electrode with a 3 cm uninsulated tip (Cooltip Radiofrequency System, Valleylab) was used to perform RFA for 6 and 12 minutes in ex vivo porcine livers under ultrasound guidance. Maximum horizontal and longitudinal diameters of the gross ablation zones were compared with the diameters of the hyperechoic zone created on ultrasound during RFA. Results: There was a trend for increased horizontal diameter for both ultrasound (RFA 6 mins=3.8+0.5cm vs. RFA 12 mins = 5.1+1.0cm; p = 0.079) and grossly (RFA 6mins = 2.7+0.27cm vs. RFA 12mins = 3.1+0.24cm; p = 0.068) with increased RFA duration from 6 to 12 minutes, which was not observed for the longitudinal diameter. Ultrasound consistently overestimated the actual necrotic diameter by 1.3+0.8cm (range = 0.4 to 3.6cm) (p = 0.01) whether RFA was performed for 6 (1.0+0.3cm) or 12 (1.8+1.2cm) minutes. Conclusion: Ultrasound led to overestimation of the true size of ablation zones regardless of the duration of ablation. The slight increae in the discrepancy with increased RFA duration suggests that an increased hyperechoic zone with prolonged ablation will not necessarily mean a proportionate increase in actual necrotic diameter. Keywords: Hepatocellular carcinoma, Radio frequency ablation (RFA), Ultrasound, porcine liver, liver treatment, hepatoma, HCC INTRODUCTION Radiofrequency ablation (RFA) is the most frequent local ablation therapy used in patients with primary and metastatic hepatic tumors not amenable to surgical resection. Selective tumor ablation is facilitated by the precise insertion of a radiofrequency probe into the target lesion under image guidance. Multiple imaging techniques (sonography, computed tomography, and magnetic resonance) can be used to guide the percutaneous placement of thermal energy applicators into the selected target. However, the most common imaging technique used for guidance during RFA is sonography. 1 This is largely due to the benefits of sonography which include the real time visualization of applicator placement, portability of the technology, nearly universal availability and low cost. However, sonography may result in occasional poor lesion visualization because of a lack of innate tissue conspicuity, or overlying bone or gas containing structures. 2 The production of microbubbles in and surrounding the tumor being ablated during the course of conventional ultrasound-guided RFA of liver tumors has traditionally been taken as a rough estimate of the extent of tissue necrosis. However, despite one clinical study that showed that the extent of microbubble formation 5 minutes after RFA on conventional ultrasound correlated closely with the area of necrosis as depicted by contrast-enhanced ultrasound done 3-5 days after RFA,3 the correlation between microbubble formation and actual histologic necrosis has been less reliable with very few studies available in the literature. 2,4 We therefore aimed to compare the necrotic diameters during ultrasound- guided RFA as depicted by the outermost extet of microbubble formation with actual gross necrotic dimensions in ex vivo procine livers. MATERIALS AND METHODS RF Ablation System An internally cooled RFA electrode (Cooltip Radiofrequency system, Valleylab. CO, USA) was