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Radio-frequency Ablation of VX2 Rabbit Tumors: Assessment of Completeness of Treatment by Using Contrast-enhanced Harmonic Power Doppler US¹

¹ From the Institute of Diagnostic Radiology, University Hospital Zurich, Rämistrasse 100, CH-8091 Zurich, Switzerland (T.B.); Institute of Diagnostic and Interventional Radiology, Friedrich-Schiller-University Jena, Germany (A.M., J.R.R., M.F., W.A.K.); Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass (S.N.G., P.H., M.R.); Research Laboratories, Schering, Berlin, Germany (P.H., M.R.); Berchtold, Tuttlingen, Germany (W.M.); and Department of Biostatistics, Institute of Social and Preventive Medicine, University of Zurich, Switzerland (B.S.). Received September 20, 2001; revision requested November 23; revision received February 15, 2002; accepted April 15. Address correspondence to T.B. (e-mail: thomas_boehm@gmx.net).

PURPOSE: To assess contrast material–enhanced harmonic power Doppler and fundamental color Doppler ultrasonography (US) in the detection of residual viable tumor tissue after radio-frequency (RF) ablation in tumors embedded in fat.

MATERIALS AND METHODS: Twenty-eight VX2 tumors were implanted into the retroperitoneum of 14 rabbits. Tumors were examined with contrast-enhanced fundamental color Doppler US and harmonic power Doppler US before and 10 minutes after RF ablation. Saline-enhanced RF ablation (30 mL/h) was performed over 10 minutes with 28-W RF power. Follow-up included repeat US examinations. Necropsies and histopathologic assessment were performed after detection of residual untreated tumor at US or 3 weeks after ablation.

RESULTS: VX2 tumors reached a mean size of 21 mm ± 9 (SD) (size range, 6–43 mm) 25 days after implantation. All tumors larger than 31 mm showed signs of central necrosis at US. Before ablation, intense vascularity was detected in all tumors with both contrast-enhanced US modes. Histopathologic assessment at the end of the follow-up period revealed local relapses due to incomplete ablation in 14 (50%) of 28 cases. Detection of residual tumor was missed in all cases with contrast-enhanced color Doppler US. Contrast-enhanced harmonic power Doppler US depicted residual flow in 12 of the 14 cases (sensitivity, 86%) in which local relapses occurred. There was a significant (P < .005, McNemar test) improvement in detection of residual tumor when the harmonic power Doppler mode was used.

CONCLUSION: Contrast-enhanced harmonic power Doppler US has greater sensitivity than contrast-enhanced color Doppler US for detecting residual VX2 tumor following ablation. Therefore, contrast-enhanced harmonic power Doppler US may be a useful additional method for the detection of residual tumors after RF ablation.

© RSNA, 2002

Index terms: Animals • Radiofrequency (RF) ablation • Ultrasound (US), comparative studies • Ultrasound (US), experimental studies • Ultrasound (US), harmonic study

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