Radio-frequency thermal ablation with NaCl solution injection: effect of electrical conductivity on tissue heating and coagulation-phantom and porcine liver study

RF tumor ablation with internally cooled electrodes and saline infusion: what is the optimal location of the saline infusion?

Background

Radiofrequency ablation (RFA) of tumors by means of internally cooled electrodes (ICE) combined with interstitial infusion of saline may improve clinical results. To date, infusion has been conducted through outlets placed on the surface of the cooled electrode. However, the effect of infusion at a distance from the electrode surface is unknown. Our aim was to assess the effect of perfusion distance (PD) on the coagulation geometry and deposited power during RFA using ICE.

Methods

Experiments were performed on excised bovine livers. Perfusion distance (PD) was defined as the shortest distance between the infusion outlet and the surface of the ICE. We considered three values of PD: 0, 2 and 4 mm. Two sets of experiments were considered: 1) 15 ablations of 10 minutes (n ≥ 4 for each PD), in order to evaluate the effect of PD on volume and diameters of coagulation; and 2) 20 additional ablations of 20 minutes. The effect of PD on deposited power and relative frequency of uncontrolled impedance rises (roll-off) was evaluated using the results from the two sets of experiments (n ≥ 7 for each PD). Comparisons between PD were performed by analysis of variance or Kruskal-Wallis test. Additionally, non-linear regression models were performed to elucidate the best PD in terms of coagulation volume and diameter, and the occurrence of uncontrolled impedance rises.

Results

The best-fit least square functions were always obtained with quadratic curves where volume and diameters of coagulation were maximum for a PD of 2 mm. A thirty per cent increase in volume coagulation was observed for this PD value compared to other values (P < 0.05). Likewise, the short coagulation diameter was nearly twenty five per cent larger for a 2 mm PD than for 0 mm. Regarding deposited power, the best-fit least square function was obtained by a quadratic curve with a 2 mm PD peak. This matched well with the higher relative frequency of uncontrolled impedance rises for PD of 0 and 4 mm.

Conclusion

Saline perfusion at around 2 mm from the electrode surface while using an ICE in RFA improves deposition of energy and enlarges coagulation volume.

Hepatic radiofrequency ablation--an overview from an engineering perspective

Radiofrequency (RF) ablation is receiving increasing attention as treatment for primary and metastatic liver cancer. RF ablation can be performed during open surgery, or minimally invasive through a small incision. An electrode is introduced into the liver tumor, and RF energy is applied. Tissue surrounding the electrode heats up, and is killed above approximately 50 degrees C, where tissue coagulation occurs. Ultrasound imaging is typically used to place the electrode, and monitor the ablation procedure; the exact dimension of the coagulation zone is not visible under ultrasound. Current devices can create coagulation zones between 4 and 7 cm diameter. For large tumors often multiple sequential applications are required, since current devices can only use a single electrode at a time. Current limitations include inadequate imaging modalities, uncontrolled shapes and size of coagulation zones, and inability to reach adequate temperatures close to large vessels. When future devices are available that improve upon these shortcomings, RF ablation may replace classical surgery as the standard treatment for liver cancer.

CT-guided radiofrequency ablation for lung cancer

Recently, percutaneous radiofrequency (RF) ablation has been increasingly performed as a local treatment for lung malignancies. In RF ablation, the application of radiofrequency agitates ions in the tissues surrounding the electrode, causing them to fluctuate at high speed, and this generates frictional heat. The generated heat coagulates the tissues. Puncture is carried out under computed tomography (CT) guidance in the same manner as that for needle biopsy. In animal studies, it was speculated that air functioned as an insulator and that the heat did not damage normal surrounding lung parenchyma to any great extent, because lung is filled with air. An experimental VX2 tumor in rabbits was well controlled by RF ablation. Since the clinical use of RF ablation for lung malignancies was first reported in 2000, many series have been published. The patients reported in these studies were not candidates for surgical treatment, either because of poor cardiopulmonary function and comorbidities, or because they refused surgery. With RF ablation, complete necrosis can be expected for tumors with a diameter of 3 cm or less. Palliative RF ablation may be indicated for large tumors. The most frequent complication associated with puncture was pneumothorax, with a frequency higher for RF ablation compared with that for needle biopsy. The initial results have been promising, but we await future reports for long-term results.

Esophageal temperature monitoring during radiofrequency catheter ablation: experimental study based on an agar phantom model

Although previous studies have established the feasibility of monitoring esophageal temperature during radiofrequency cardiac ablation using an esophageal temperature probe (ETP), some questions remain regarding its efficacy. The aims of this study were to study the effect of the location of the ETP on the temperature reached, and to test the characteristics of ETP as used in clinical practice. We constructed an agar phantom to model the thermal and electrical characteristics of the biological tissues (left atrium, esophagus and connective tissue). The ETP was positioned at 6.5 mm from an ablation electrode and at distances of 0, 5, 10, 15, 20 mm from the catheter axis. A thermocouple was located on the probe to measure the actual temperature of the external esophageal layer during the ablations (55 degrees C, 60 s). The mean temperatures reached at the thermocouple were significantly higher than those measured by the ETP (48.3 +/- 1.9 degrees C versus 39.6 +/- 1.1 degrees C). The temperature values measured with the ETP were significantly lower when the probe was located further from the catheter axis (up to 2.5 degrees C lower when the distance from the probe-catheter axis was 2 cm). The dynamic calibration of the ETP showed a mean value for the time constant of 8 s. In conclusion, the temperature measured by the ETP always underestimates the temperature reached in the thermocouple. This fact can be explained by the distance gap between the thermocouple and probe and by the dynamic response of the ETP. The longer the distance between the ETP and catheter axis, the higher is the temperature difference.

Ultrasound-guided percutaneous radiofrequency ablation in elderly breast cancer patients: preliminary institutional experience

Breast cancer is a major source of morbidity and mortality in the elderly population and the number of patients will increase by 30% in future decades. Surgery and endocrine therapy could be considered as the standard treatment in elderly breast cancer patients, but anaesthesia for surgery requires a specific approach taking into account physiological and psychological alterations secondary to ageing. In cases with major alterations of performance status, percutaneous radiofrequency ablation (RFA) could be substituted for the surgical treatment. The aim of the current study is to evaluate the efficacy and feasibility of this technique. Five tumours in four consecutive patients (aged 79-82 years) contraindicated for surgery with symptomatic cT1-2N0M0, positive oestrogen receptor status, low grade were treated by percutaneous radiofrequency-lump ablation under local analgesia, using percutaneous ultrasound guidance. Thermal lesions were produced with RF power 30 W, at a frequency of 500 kHz. Ultrasound-guided percutaneous biopsy of the RF treated breast was performed during the follow-up. We report a successful RFA lump ablation experience in the treatment of four tumours (4/5). One local recurrence occurred within 4 months after RFA. The other biopsies taken during the follow up showed all fat necrosis within the oil cyst and no malignant cells. One abscess occurred at 9 months within the treated area. After a mean follow up of 29.4 months, all the patients are still alive without any other signs of recurrence or metastases. Ultrasound-guided percutaneous RFA is safe and feasible in the management of breast cancer in elderly patients. Nevertheless, further large comparative studies are needed in order to validate such a minimally invasive procedure in current practice.

Investigation of the influence of blood flow rate on large vessel cooling in hepatic radiofrequency ablation

Radiofrequency (RF) ablation using high-frequency current has become an important treatment method for patients with non-resectable liver tumors. Tumor recurrence is associated with tissue cooling in the proximity of large blood vessels. This study investigated the influence of blood flow rate on tissue temperature and lesion size during monopolar RF ablation at a distance of 10 mm from single 4- and 6-mm vessels using two different approaches: 1) an ex vivo blood perfusion circuit including an artificial vessel inserted into porcine liver tissue was developed; and 2) a finite element method (FEM) model was created using a novel simplified modeling technique for large blood vessels. Blood temperatures at the inflow/outflow of the vessel and tissue temperatures at 10 and 20 mm from the electrode tip were measured in the ex vivo set-up. Tissue temperature, blood temperature and lesion size were analyzed under physiological, increased and reduced blood-flow conditions. The results show that changes in blood flow rate in large vessels do not significantly affect tissue temperature and lesion size far away from the vessel. Monopolar ablation could not produce lesions surrounding the vessel due to the strong heat-sink effect. Simulated tissue temperatures correlated well with ex vivo measurements, supporting the FEM model.

Experimental and Clinical Radiofrequency Ablation: Proposal for Standardized Description of Coagulation Size and Geometry

BACKGROUND

Radiofrequency (RF) ablation is used to obtain local control of unresectable tumors in liver, kidney, prostate, and other organs. Accurate data on expected size and geometry of coagulation zones are essential for physicians to prevent collateral damage and local tumor recurrence. The aim of this study was to develop a standardized terminology to describe the size and geometry of these zones for experimental and clinical RF.

METHODS

In a first step, the essential geometric parameters to accurately describe the coagulation zones and the spatial relationship between the coagulation zones and the electrodes were defined. In a second step, standard terms were assigned to each parameter.

RESULTS

The proposed terms for single-electrode RF ablation include axial diameter, front margin, coagulation center, maximal and minimal radius, maximal and minimal transverse diameter, ellipticity index, and regularity index. In addition a subjective description of the general shape and regularity is recommended.

CONCLUSIONS

Adoption of the proposed standardized description method may help to fill in the many gaps in our current knowledge of the size and geometry of RF coagulation zones.

Lung cancer and radiofrequency ablation

Radiofrequency (RF) ablation is a recently developed technique for image-guided local destruction of selected tumors. Because the lung is a common site for cancer and usually has substantial functional reserve, RF ablation of lung cancers is an attractive option for minimally invasive treatment. The primary goal of the present review is to describe the natural history, staging systems, and conventional therapies for primary and secondary treatment of lung cancer, as well as the results of RF ablation in animal models and in humans for pulmonary applications, to clarify the appropriate role and limitations of this technology. The secondary goals are to review the principles of how RF works and to describe RF ablation techniques to familiarize interventionalists who may consider incorporating this technology into their practice and inform diagnostic radiologists of expected imaging findings and clinicians of their patients' anticipated courses and outcomes.

Influence of NaCl Concentrations on Coagulation, Temperature, and Electrical Conductivity Using a Perfusion Radiofrequency Ablation System: An Ex Vivo Experimental Study

PURPOSE

To determine, by means of an ex vivo study, the effect of different NaCl concentrations on the extent of coagulation obtained during radiofrequency (RF) ablation performed using a digitally controlled perfusion device.

METHOD

Twenty-eight RF ablations were performed with 40 W for 10 min using continuous NaCl infusion in fresh excised bovine liver. For perfusion, NaCl concentrations ranging from 0 (demineralized water) to 25% were used. Temperature, the amount of energy, and the dimensions of thermal-induced white coagulation were assessed for each ablation. These parameters were compared using the nonparametric Mann-Whitney test. Correlations were calculated according to the Spearman test.

RESULTS

RF ablation performed with 0.9% to 25% concentrations of NaCl produced a mean volume of coagulation of 30.7 +/- 3.8 cm(3), with a mean short-axis diameter of 3.6 +/- 0.2 cm. The mean amount of energy was 21,895 +/- 1,674 W and the mean temperature was 85.4 +/- 12.8 degrees C. Volume of coagulation, short-axis diameter, and amount of energy did not differ significantly among NaCl concentrations (p > 0.5). A correlation was found between the NaCl concentration and the short-axis diameter of coagulation (r = 0.64) and between the NaCl concentration and the mean temperature (r = 0.67), but not between the NaCl concentration and volume of coagulation.

CONCLUSION

In an ex vivo model, continuous perfusion with high NaCl concentrations does not significantly improve the volume of thermal-induced coagulation. This may be because the use of a low-power generator cannot sufficiently exploit the potential advantage of better tissue conductivity provided by NaCl perfusion.

Radio frequency ablation in the rabbit lung using wet electrodes: comparison of monopolar and dual bipolar electrode mode

Objective

To compare the effect of radio frequency ablation (RFA) on the dimensions of radio frequency coagulation necrosis in a rabbit lung using a wet electrode in monopolar mode with that in dual electrode bipolar mode at different infusion rates (15 mm/hr versus 30 ml/hr) and saline concentrations (0.9% normal versus 5.8% hypertonic saline).

Materials and Methods

Fifty ablation zones (one ablation zone in each rabbit) were produced in 50 rabbits using one or two 16-guage wet electrodes with a 1-cm active tip. The RFA system used in the monopolar and dual electrode wet bipolar RFA consisted of a 375-kHz generator (Elektrotom HiTT 106, Berchtold, Medizinelektronik, Germany). The power used was 30 watts and the exposure time was 5 minutes. The rabbits were assigned to one of five groups. Group A (n = 10) was infused with 0.9% NaCl used at a rate of 30 ml/hr in a monopolar mode. Groups B (n = 10) and C (n = 10) were infused with 0.9% NaCl at a rate of 15 and 30 ml/hr, respectively in dual electrode bipolar mode; groups D (n = 10) and E (n = 10) were infused with 5.8% NaCl at a rate of 15 and 30 ml/hr, respectively in a dual electrode bipolar mode. The dimensions of the ablation zones in the gross specimens from the groups were compared using one-way analysis of variance by means of the Scheffe test (post-hoc testing).

Results

The mean largest diameter of the ablation zones was larger in dual electrode bipolar mode (30.9±4.4 mm) than in monopolar mode (22.5±3.5 mm). The mean smallest diameter of the ablation zones was larger in dual electrode bipolar mode (22.3±2.5 mm) than in monopolar mode (19.5±3.5 mm). There were significant differences in the largest and smallest dimension between the monopolar (group A) and dual electrode wet bipolar mode (groups B-E). In dual electrode bipolar mode, the mean largest diameter of the ablation zones was larger at an infusion rate of 15 ml/hr (34.2±4.0 mm) than at 30 ml/hr (27.6±0.1 mm), and the mean smallest diameter of the ablation zones was larger at an infusion rate of 15 ml/hr (27.2±7.5 mm) than at an infusion rate of 30 ml/hr (24±2.9 mm).

Conclusion

Using a wet electrode, dual electrode bipolar RFA can create a larger ablation zone more efficiently than monopolar RFA.

Comparison of wet radiofrequency ablation with dry radiofrequency ablation and radiofrequency ablation using hypertonic saline preinjection: ex vivo bovine liver

OBJECTIVE

We wished to compare the in-vitro efficiency of wet radiofrequency(RF) ablation with the efficiency of dry RF ablation and RF ablation with preinjection of NaCl solutions using excised bovine liver.

MATERIALS AND METHODS

Radiofrequency was applied to excised bovine livers in a monopolar mode for 10 minutes using a 200 W generator and a perfused-cooled electrode with or without injection or slow infusion of NaCl solutions. After placing the perfused-cooled electrode in the explanted liver, 50 ablation zones were created with five different regimens: group A; standard dry RF ablation, group B; RF ablation with 11 mL of 5% NaCl solution preinjection, group C; RF ablation with infusion of 11 mL of 5% NaCl solution at a rate of 1 mL/min, group D; RFA with 6 mL of 36% NaCl solution preinjection, group E; RF ablation with infusion of 6 mL of 36% NaCl solution at a rate of 0.5 mL/min. In groups C and E, infusion of the NaCl solutions was started 1 min before RF ablation and then maintained during RF ablation (wet RF ablation). During RF ablation, we measured the tissue temperature at 15 mm from the electrode. The dimensions of the ablation zones and changes in impedance, current and liver temperature during RF ablation were then compared between the groups.

RESULTS

With injection or infusion of NaCl solutions, the mean initial tissue impedance prior to RF ablation was significantly less in groups B, C, D, and E (43-75 ohm) than for group A (80 ohm) (p < 0.05). During RF ablation, the tissue impedance was well controlled in groups C and E, but it was often rapidly increased to more than 200 ohm in groups A and B. In group D, the impedance was well controlled in six of ten trials but it was increased in four trials (40%) 7 min after starting RF ablation. As consequences, the mean current was higher for groups C, D, and E than for the other groups: 401+/-145 mA in group A, 287+/-32 mA in group B, 1907+/-96 mA in group C, 1649+/-514 mA in group D, and 1968+/-108 mA in group E (p< 0.05). In addition, the volumes of RF-induced coagulation necrosis were greater in groups C and E than in group D, which was greater than in groups A and B than in group E (p < 0.05); 14.3+/-3.0 cm(3) in group A; 12.4+/-3.8 cm(3) in group B; 80.9+/-9.9 cm(3) in group C; 45.3+/-11.3 cm(3) in group D and 81.6+/-8.6 cm(3) in group E. The tissue temperature measured at 15 mm from the electrode was higher in groups C, D and E than other groups (p < 0.05): 53+/-12 degrees C in group A, 42+/-2 degrees C in group B, 93+/-8 degrees C in group C; 79+/-12 degrees C in group D and 83+/-8 degrees C in group E.

CONCLUSION

Wet RF ablation with 5% or 36% NaCl solutions shows better efficiency in creating a large ablation zone than does dry RF ablation or RF ablation with preinjection of NaCl solutions.

Optimization of wet radiofrequency ablation using a perfused-cooled electrode: a comparative study in ex vivo bovine livers

Objective

To determine the optimized protocol for wet monopolar radiofrequency ablation (RFA) using a perfused-cooled electrode to induce coagulation necrosis in the ex vivo bovine liver.

Materials and Methods

Radiofrequency was applied to excised bovine livers in a monopolar mode using a 200W generator with an internally cooled electrode (groups A and B) or a perfused-cooled electrode (groups C, D, E, and F) at maximum power (150-200 W) for 10 minutes. A total of 60 ablation zones were created with six different regimens: group A - dry RFA using intra-electrode cooling; group B - dry RFA using intra-electrode cooling and a pulsing algorithm; group C - wet RFA using only interstitial hypertonic saline (HS) infusion; group D - wet RFA using interstitial HS infusion and a pulsing algorithm; group E - wet RFA using interstitial HS infusion and intra-electrode cooling; and group F - wet RFA using interstitial HS infusion, intra-electrode cooling and a pulsing algorithm. In groups C, D, E, and F, RFA was performed with the infusion of 6% HS through the perfused cooled electrode at a rate of 2 mL/minute. During RFA, we measured the tissue temperature at a distance of 15 mm from the electrode. The dimensions of the ablation zones and the changes in impedance, currents, and liver temperature during RFA were compared between these six groups.

Results

During RFA, the mean tissue impedances in groups A (243 ± 88 Ω) and C (252.5 ± 108 Ω) were significantly higher than those in groups B (85 ± 18.7 Ω), D (108.2 ± 85 Ω), E (70.0 ± 16.3 Ω), and F (66.5 ± 7 Ω) (p < 0.05). The mean currents in groups E and F were significantly higher than those in groups B and D, which were significantly higher than those in groups A and C (p < 0.05): 520 ± 425 mA in group A, 1163 ± 34 mA in group B, 652.5 ± 418 mA in group C, 842.5 ± 773 mA in group D, 1665 ± 295 mA in group E, and 1830 ± 109 mA in group F. The mean volumes of the ablation regions in groups E and F were significantly larger than those in the other groups (p < 0.05): 17.7 ± 5.6 cm³ in group A, 34.5 ± 3.0 cm³ in group B, 20.2 ± 15.6 cm³ in group C, 36.1 ± 19.5 cm³ in group D, 68.1 ± 12.4 cm³ in group E, and 79.5 ± 31 cm³ in group F. The final tissue temperatures at a distance of 15 mm from the electrode were higher in groups E and F than those in groups A, C, and D (p < 0.05): 50 ± 7.5℃ in group A, 66 ± 13.6℃ in group B, 60 ± 13.4℃ in group C, 61 ±12.7℃ in group D, 78 ± 14.2℃ in group E, and 79 ± 12.0℃ in group F.

Conclusion

Wet monopolar RFA, using intra-electrode cooling and interstitial saline infusion, showed better performance in creating a large ablation zone than either dry RFA or wet RFA without intra-electrode cooling.

Usefulness of Guiding Needles for Radiofrequency Ablative Treatment of Liver Tumors

PURPOSE

To evaluate the usefulness of a guiding needle for radiofrequency (RF) ablative treatment of liver tumors.

METHODS

Forty-two patients, 38-78 years old (57 +/- 17), with 42 liver tumors (18 HCC, 24 colon cancer metastases) underwent RF ablation using a 14-gauge guiding needle with an external insulated sheath in which any 18-gauge or smaller needle can be placed, including a specially designed 3.5 cm LeVeen RF electrode. One guiding needle was used in 20 tumors to provide biopsy and RF treatment in a single puncture. Three to five guiding needles were loaded in 22 tumors measuring 35 to 64 mm in their largest diameter before starting RF treatment requiring multiple overlapping RF applications.

RESULTS

In the 20 RF treatments combined with biopsy, the biopsy was always contributive. Because of pre-positioning of the sheath, postbiopsy modifications (bleeding or air artifacts) did not hinder subsequent RF treatment. The 22 large tumors received 5 to 12 RF applications (mean = 6.8) through the three to five preloaded guiding needles. The RF ablation zones measured 46 to 94 mm (mean = 55) in their largest dimension, with ablative margins in all cases. After 8 to 32 months (mean = 20), 14 of the 22 tumors are considered completely destroyed on computed tomography follow-up and one tumor seeding has been found.

CONCLUSION

The Leveen CoAccess needle allows precise tumor targeting when treating large tumors requiring multiple RF applications. It allows biopsies combined with RF ablation through a single tract.

Saline-Infused Bipolar Radiofrequency Ablation of High-Risk Spinal and Paraspinal Neoplasms

OBJECTIVE

We present three cases of symptomatic high-risk metastases involving the vertebral body treated using infused bipolar radiofrequency ablation either alone or in combination with vertebroplasty.

CONCLUSION

In our experience, the bipolar technique can reduce the risk of spinal cord damage in radiofrequency ablation of lesions within 1 cm of neural structures and involving paravertebral soft tissue.

Microwave Ablation: Results with a 2.45-GHz Applicator in ex Vivo Bovine and in Vivo Porcine Liver

PURPOSE

To characterize the relationship between applied power and treatment duration in their effect on extent of coagulation produced with a 2.45-GHz microwave applicator in both an ex vivo and a perfused in vivo liver model.

MATERIALS AND METHODS

All experimentation was approved by the Institute of Animal Care and Use Committee. Multiple tissue ablations were performed in ex vivo bovine liver (120 ablations) and in vivo porcine liver (45 ablations) with a 5.7-mm-diameter 2.45-GHz microwave applicator. The applied power was varied from 50 to 150 W (in 25-W increments ex vivo and 50-W increments in vivo), while treatment duration varied from 2 to 20 minutes (in eight time increments for ex vivo and five for in vivo liver). Three-dimensional contour maps of the resultant short- and long-axis coagulation diameters were constructed to identify the optimal parameters to achieve maximum coagulation in both ex vivo and in vivo models. Multivariate analysis was performed to characterize the relationship between applied power and treatment duration.

RESULTS

Power and treatment duration were both associated with coagulation diameter in a sigmoidal fashion (ex vivo, R(2) = 0.78; in vivo, R(2) = 0.74). For ex vivo liver, the maximum short-axis coagulation diameter (7.6 cm +/- 0.2 [standard deviation] by 12.3 cm +/- 0.8) was achieved at greatest power (150 W) and duration (20 minutes). In vivo studies revealed a sigmoidal relationship between duration and coagulation size, with a relative plateau in coagulation size achieved within 8 minutes duration at all power levels. After 8 minutes of treatment at 150 W, the mean short-axis coagulation diameter for in vivo liver was 5.7 cm +/- 0.2 by 6.5 cm +/- 1.7, which was significantly larger than the corresponding result for ex vivo liver (P < .05).

CONCLUSION

Large zones of ablation can be achieved with the 2.45-GHz microwave applicator used by the authors. For higher-power ablations, larger zones of coagulation were achieved for in vivo liver than for ex vivo liver with short energy applications, a finding previously not seen with other ablation devices, to the authors' knowledge.

Hepatic radiofrequency ablation using multiple probes: ex vivo and in vivo comparative studies of monopolar versus multipolar modes

OBJECTIVE

We wanted to compare the efficiency of multipolar radiofrequency ablation (RFA) using three perfused-cooled electrodes with multiple overlapping and simultaneous monopolar techniques for creating an ablation zone in ex vivo bovine livers and in in vivo porcine livers.

MATERIALS AND METHODS

In the ex vivo experiments, we used a 200 W generator (Valleylab, CC-3 model) and three perfused-cooled electrodes or internally cooled electrodes to create 30 coagulation zones by performing consecutive monopolar RFA (group A, n = 10), simultaneous monopolar RFA (group B, n = 10) or multipolar RFA (group C, n = 10) in explanted bovine livers. In the consecutive mode, three ablation spheres were created by sequentially applying 150 watts radiofrequency (RF) energy to the internally cooled electrodes for 12 minutes each for a total of 36 minutes. In the simultaneous monopolar and multipolar modes, RF energy was concurrently applied to the three perfused-cooled electrodes for 20 minutes at 150 watt with instillation of 6% hypertonic saline at 2 mL/min. During RFA, we measured the temperatures of the treated area at its center. The changes in impedance, the current and liver temperature during RFA, as well as the dimensions of the thermal ablation zones, were compared among the three groups. In the in vivo experiments, three coagulations were created by performing multipolar RFA in a pig via laparotomy with using same parameter as the ex vivo study.

RESULTS

In the ex vivo experiments, the impedance was gradually decreased during the RFA in groups B and C, but in group A, the impedance was increased during RFA and this induced activation by the pulsed RF technique. In groups A, B and C, the mean final-temperature values were 80+/-10 degrees C, 69+/-18 degrees C and 79+/-12 degrees C, respectively (p < 0.05). The multipolar mode created a larger volume of ablation than did the other modes: 37.6+/-4.0 cm3 (group A); 44.9+/-12.7 cm3 (group B); and 78.9+/-6.9 cm3 (group C) (p < 0.05). In the in vivo experiment, the pig well tolerated the RFA procedure and no major complications occurred during the 4 days of the follow-up period. The mean volume of coagulations produced by multipolar RFA in the pig liver was 60.5+/-17.9 cm3.

CONCLUSION

For the multiple probe RFA, the multipolar mode with hypertonic saline instillation was more efficient in generating larger areas of thermal ablation than either the consecutive or simultaneous monopolar modes.

Effects of Vascular Perfusion on Coagulation Size in Radiofrequency Ablation of Ex Vivo Perfused Bovine Livers

OBJECTIVES

A standardized perfused ex vivo bovine liver model was used to evaluate the effect of organ perfusion on coagulation size and energy deposition during radiofrequency ablation (RFA) procedures.

MATERIALS AND METHODS

Bovine livers were perfused in a tank after rinsing the prepared liver vessels with anticoagulants. Tyrode's solution, oxygenated and heated to 36.5 degrees C, was used as perfusion medium. A flow and pressure controlled pump regulated Portal vein circulation; a dialysis machine provided pulsatile arterial circulation. Impedance-guided radiofrequency ablations were performed with 4-cm LeVeen electrodes with and without underlying liver perfusion. Two-dimensional diameters (Dv, Dh) of each ablation area were measured after dissecting the livers.

RESULTS

In 4 bovine livers weighing 8.85 +/- 0.83 kg per organ (min, 7.7 kg; max, 9.7 kg) altogether 40 RF ablations were performed. A total of 20 ablations were generated with underlying liver perfusion (group 1) and 20 ablations with no liver perfusion (group 2). In group 1, Dv was 28.4 +/- 5.3 mm, Dh 38.6 +/- 7.8 mm, and energy deposition 36.9 +/- 18.0 kJ. The 20 ablation areas generated without liver perfusion displayed statistically significant differences, with Dv being 35.7 +/- 6.5 mm (P = 0.001), Dh 49.5 +/- 9.4 mm (P = 0.001), and energy deposition 25.5 +/- 13.0 kJ (P = 0.018).

CONCLUSION

The model reproduced the cooling effect of perfused tissue during RFA. The ablation areas produced under perfusion conditions had smaller diameters despite longer exposure times and higher energy deposition.

Radiofrequency ablation of the porcine liver in vivo: increased coagulation with an internally cooled perfusion electrode

RATIONALE AND OBJECTIVES

A major limitation of radiofrequency (RF) ablation is its inability to produce a large enough diameter of coagulation necrosis to encompass hepatic tumors with an appropriate ablative margin at a single RF application. We evaluated the in vivo efficiency of RF ablation (RFA) using an internally cooled perfusion (ICP) electrode with hypertonic saline infusion to induce coagulation necrosis compared with that of RFA using single needle electrode types.

MATERIALS AND METHODS

RF was applied to a porcine liver in monopolar mode using a 200 W generator and an internally cooled electrode (group A) or an ICP electrode (group B) at 200 W for 12 minutes or using a 60 W generator with a perfusion electrode at 40 W for 20 minutes (group C). In total, 36 (3 x 12) ablation zones were created using the three different regimens. In group B, 14.6% NaCl solution was infused at 1 mL/minute and in group C, 0.9% NaCl solution was infused at 1.5 mL/minute. The three groups were compared in terms of amount of delivered RF energy and dimensions and the coefficients of variation of the ablation zones.

RESULTS

The mean energies applied in the three groups were 52.3 +/- 10.3 kJ for group A, 115.4 +/- 10.5 kJ for group B, and 38.5 +/- 11.5 kJ for group C, respectively (P < .05). The mean ablation volumes in groups A, B and C were 13.1 +/- 4.7 cm3 in group A, 43.7 +/- 17.5 cm3 in group B, and 26.3 +/- 20.2 cm3 in group C, respectively (P < .05). In addition, the coefficients of variation of the volumes of the ablation zones in groups A, B, and C were 0.36, 0.4, and 0.78, respectively.

CONCLUSIONS

RFA using the ICP electrode showed better performance in terms of creating a larger ablation zone than RFA using an internally cooled or a perfusion electrode.

Prevention of biliary lesions that may occur during radiofrequency ablation of the liver: study on the pig

OBJECTIVE

To prevent bile duct injury by using a cold 5% glucose isotonic solution cooling in the bile ducts when radiofrequency (RF) is performed in a porcine model.

SUMMARY BACKGROUND DATA

Complications that may arise during liver RF ablation include biliary stenosis and abscesses.

METHODS

The RITA 1500 generator was used for the experiments. Two lesions were performed in the left liver. The pigs were killed 1 or 3 weeks after the procedure. An ex vivo cholangiogram was obtained by direct injection into the main bile duct. Samples of RF lesions, of liver parenchyma near and at a distance from the RF lesions, underwent pathologic studies. Two groups of 20 pigs each were treated: one without perfusion of the bile ducts and the other with perfusion of cold 5% glucose isotonic solution into the bile ducts. The Pringle maneuver was used in 50% of the RF procedures. Radiologic lesions were classified as biliary stenosis, complete interruption of the bile duct, or extravasation of the radiologic contrast liquid.

RESULTS

Histologic lesions of the bile ducts were observed near the ablated RF lesion site and at a distance from the RF lesions when a Pringle maneuver was performed. Radiologic and histologic lesions of the bile ducts were significantly reduced (P < 0.0001) when the bile ducts were cooled.

CONCLUSIONS

Cooling of the bile ducts with a cold 5% glucose isotonic solution significantly protects the intrahepatic bile ducts from damages caused by the heat generated by RF when performed close to the bile ducts.

Thermal tumour ablation: devices, clinical applications and future directions

Tumour ablation is clinically applied mainly for non-operable liver tumours, with increasing application to other organ sites like kidney, lung, adrenal gland and bone. Most current devices use radiofrequency (RF) current to heat tumour tissue surrounding the applicator, which is introduced into the tumour under imaging guidance. Tissue temperatures in excess of 100 degrees C are achieved, with cell death due to coagulative necrosis occurring above 50 degrees C. Limitations of current ablation devices include inadequate imaging, limited size of coagulation zone and reduced performance next to large vessels. This paper reviews current interstitial RF and microwave devices, clinical applications and future research directions in the field of high-temperature tumour ablation.

Electrical isolation during radiofrequency ablation: 5% dextrose in water provides better protection than saline

We quantify the ability of 0.9% NaCl (saline) and 5% dextrose in water (D5W) to protect tissues during RF ablation. Using computer simulations and phantom experiments, we determined that D5W provides significantly more electrical isolation than saline, which reduces unwanted heating of the adjacent tissue. Saline actually increased the amount of RF current in the adjacent tissue. Based on these results, we conclude that D5W is preferable to saline as a protective fluid.

A review of the general aspects of radiofrequency ablation

As an alternative to standard surgical resection for the treatment of malignant tumors, radiofrequency ablation (RFA) has rapidly evolved into the most popular minimally invasive therapy. To help readers gain the relevant background knowledge and to better understand the other reviews in this Feature Section on the clinical applications of RFA in different abdominal organs, the present report covers the general aspects of RFA. After an introduction, we present a simple definition of the energy applied during RFA, a brief historical review of its technical evolution, and an explanation of the mechanism of action of RFA. These basic discussions are substantiated with descriptions of RFA equipment including those commercially available and those under preclinical development. The size and geometry of induced lesions in relation to RFA efficacy and side effects are discussed. The unique pathophysiologic process of thermal tissue damage and the corresponding histomorphologic manifestations after RFA are detailed and cross-referenced with the findings in the current literature. The crucial role of imaging technology during and after RFA is also addressed, including some promising new developments. This report finishes with a summary of the key messages and a perspective on further technologic refinements and identifies some specific priorities.

CT mapping of the distribution of saline during radiofrequency ablation with perfusion electrodes

PURPOSE

During radiofrequency (RF) ablation, adjunctive saline increases the size of the ablation zone and therefore electrodes that simultaneously deliver current and saline have been developed, but the addition of saline also results in an irregular ablation zone. Our aim was to study the distribution of saline during RF ablation.

METHODS

Four patients were treated: 3 with liver metastases and 1 with hepatocellular carcinoma (HCC). Two different perfusion electrodes were used: a high-perfusion-rate, straight electrode (Berchtold, Germany) and a low-perfusion-rate, expandable electrode (RITA Medical Systems, USA). The saline perfusate was doped with non-ionic contrast medium to render it visible on CT and the electrical conductivity was measured. CT scans were obtained of each electrode position prior to ablation and repeated after ablation. Contrast-enhanced CT was performed 18-24 hr later to demonstrate the ablation zone. All treatments were carried out according to the manufacturer's recommended protocol.

RESULTS

The addition of a small quantity of non-ionic contrast did not alter the electrical conductivity of the saline. Contrast-doped saline extravasated beyond the tumor in all 3 patients with metastases but was limited in the patient with HCC. In some areas where saline had extravasated there was reduced enhancement on contrast-enhanced CT consistent with tissue ablation. One patient treated with the high-perfusion-rate system sustained a jejunal perforation requiring surgery.

CONCLUSION

Saline can extravasate beyond the tumor and with the high-perfusion-rate system this resulted in an undesirable extension of the ablation zone and a complication.

Combined Tumor Therapy by Using Radiofrequency Ablation and 5-FU–Laden Polymer Implants: Evaluation in Rats and Rabbits

PURPOSE

To evaluate the use of 5-fluorouracil (5-FU)-laden polymer implants as an adjunct to radiofrequency (RF) ablation for tumor treatment.

MATERIALS AND METHODS

All animal studies were performed in compliance with the Case Western Reserve University Institutional Animal Care and Use Committee guidelines. Three studies were performed to investigate (a) in vitro dissolution of 5-FU-laden polymer implants in saline and bovine serum, (b) tissue distribution of 5-FU and its metabolite, 5-fluorouridine (5-FUrd), in the ablated liver tissue of rats (n = 4), and (c) efficacy of combined approach (n = 4) compared with that of ablation alone (n = 6) for VX2 liver tumor model in rabbits. Characterization of 5-FU release in vitro and distribution of 5-FU in rat liver tissue were analyzed by using high performance liquid chromatography; in vivo efficacy was assessed by using computed tomography and pathologic examination.

RESULTS

Results of the in vitro dissolution study showed that a 75% release of 5-FU occurred in 2 days when exposed to bovine serum and in 9 days when exposed to phosphate-buffered saline. In the ablated rat liver, the 5-FU level was higher at the center and lower at the periphery of the tissue both at 24 hours (41.0 mg per kilogram tissue vs 15.0 mg per kilogram tissue, respectively) and at 48 hours (8.0 mg per kilogram tissue vs 2.0 mg per kilogram tissue, respectively). The 5-FUrd concentration was twofold higher peripherally than centrally and was higher at 48 hours than at 24 hours. In rabbits, local delivery of 5-FU immediately after RF ablation provided a significant (P < .05) reduction in tumor size compared with ablation alone (1.80 cm3 +/- 0.28 [standard error] vs 3.53 cm3 +/- 0.52, respectively; P = .034) and a more than 20-fold reduction in tumor size compared with the control (1.80 cm3 +/- 0.28 vs 41.95 cm3 +/- 11.58, respectively; P = .018).

CONCLUSION

Combined treatment by using 5-FU polymer implants and RF ablation shows uniform sustained release of 5-FU for 48 hours at least 8 mm from the edge of the ablation zone and appears to be successful at controlling the growth of an experimental tumor in rabbits appreciably better than does ablation alone.

Microwave ablation of hepatic tumors using dual-loop probes: results of a phase I clinical trial

Hepatic tumors are a common cause of death worldwide. However, few patients are candidates for resection at the time of presentation. Microwave ablation is a viable alternative available for these patients. To date, only straight antennas are used for microwave ablation. Recently, a prototype loop-shaped microwave antenna was developed that, in animal studies, more effectively kills tumors. For this study, the dual-probe lesions were created by placing the probes in both tumors and normal livers. Lesions were created with 60 watts applied power for 5-7 minutes. The livers were sectioned and stained for viability. The average ablation volume was 63.9 +/- 8.7 cm(3). Microwave ablation with the loop probes results in complete tumor kill at the ablation/tumor interface, and adjacent to surrounding blood vessels. In addition, vessels within the ablation/tumor interface failed to show viable cells. The shape of the lesions was not distorted by proximity to blood vessels. The advantages of this configuration over conventional straight probes include the ability to encircle a tumor, deliver large amounts of precisely targeted microwave energy to the tumor, and spare normal liver outside the loop.

Image-guided tumor ablation: standardization of terminology and reporting criteria

The field of interventional oncology with use of image-guided tumor ablation requires standardization of terminology and reporting criteria to facilitate effective communication of ideas and appropriate comparison between treatments that use different technologies, such as chemical (ethanol or acetic acid) ablation, and thermal therapies, such as radiofrequency (RF), laser, microwave, ultrasound, and cryoablation. This document provides a framework that will hopefully facilitate the clearest communication between investigators and will provide the greatest flexibility in comparison between the many new, exciting, and emerging technologies. An appropriate vehicle for reporting the various aspects of image-guided ablation therapy, including classification of therapies and procedure terms, appropriate descriptors of imaging guidance, and terminology to define imaging and pathologic findings, are outlined. Methods for standardizing the reporting of follow-up findings and complications and other important aspects that require attention when reporting clinical results are addressed. It is the group's intention that adherence to the recommendations will facilitate achievement of the group's main objective: improved precision and communication in this field that lead to more accurate comparison of technologies and results and, ultimately, to improved patient outcomes. The intent of this standardization of terminology is to provide an appropriate vehicle for reporting the various aspects of image-guided ablation therapy.

Multipolar radiofrequency ablation of hepatic tumors: initial experience

Institutional review board approval and patient informed consent were obtained. Use of a multipolar radiofrequency (RF) ablation device in patients with hepatic malignancy was prospectively evaluated with regard to feasibility, achieved ablation zone size and shape, technical effectiveness, and complications. Nineteen malignant liver tumors were treated with the multipolar resistance-controlled RF ablation system, with which up to three internally cooled bipolar coagulation electrodes can be operated simultaneously. Postinterventional imaging was performed with dynamic contrast material-enhanced magnetic resonance (MR) imaging and MR imaging-based three-dimensional planimetry. Complete tumor destruction was achieved in 18 of 19 tumors. Mean ablation zone volume was 52 mL +/- 45 (standard deviation). Thirteen patients were treated with a percutaneous approach; six, with an intraoperative approach. Maximum ablation size was 91 x 62 x 79 mm with the percutaneous and 73 x 98 x 74 mm with the intraoperative approach. Of the 18 completely evaluable ablation zones, 13 were concentric, two were moderately eccentric, two were eccentric, and one was wedge-shaped. The multipolar RF ablation device achieves large ablation zones and has high technical effectiveness in treating hepatic tumors.

Minimally Invasive Treatment of Renal Cell Carcinoma: Comparison of 4 Different Monopolar Radiofrequency Devices

OBJECTIVES

Radiofrequency Ablation is an investigational treatment option for RCC. The aim of our study was to test the ablation algorithms of four different RF systems in a standardized ex vivo perfused porcine kidney model.

MATERIALS AND METHODS

A multiline monopolar dry electrode (impedance-based system), a multiline monopolar dry electrode (temperature-based system), a single monopolar wet electrode (impedance-based system) and a single monopolar dry, internally-cooled electrode (impedance-based system) were selected. RF energy was applied at different treatment parameters (power with and without control, tissue temperature, saline enhancement) for 1, 3, 5 and 9 minutes in healthy perfused ex vivo porcine tissue. Each treatment parameter was repeated 5 times. Maximum vertical, long-axis and short-axis diameters of the macroscopic lesion were measured and lesion volumes/ shapes were calculated.

RESULTS

Lesion volumes increased significantly with the pre-selected tissue temperature and saline enhancement. Saline enhancement created larger, but irregular shaped lesions. The impedance-based system created lesion volumes that were predictable by treatment time and generator power. Lesions were unpredictable when uncontrolled generator power was applied. The created lesion shape was dependent on the selected electrode configuration.

CONCLUSIONS

The currently available monopolar RFA systems offer different specific technical features to control tissue ablation. Detailed knowledge of the specific characteristics of each RF system is necessary to provide a higher chance of successful clinical outcome by complete and reliable ablation.

Radiofrequency Ablation with a New Perfused-Cooled Electrode Using a Single Pump: An Experimental Study in Ex Vivo Bovine Liver

The purpose of this study was to assess the efficacy of a new perfused-cooled electrode that uses a single pump for creating a large ablation zone in explanted bovine liver. This was done by comparing with the radiofrequency (RF) ablation zones that were created with a monopolar cooled electrode to the RF ablation zones that were created by the new perfused-cooled electrode. We developed a new perfused-cooled electrode that uses a single pump by modifying a 17-gauge cooled electrode (Radionics) with a 2.5-cm outer metallic sheath (15-gauge) in order to allow use of the internal cooling water (5.85 % hypertonic saline) for the infused saline. Thirty ablation zones were created in explanted bovine livers (12-min ablation cycle; pulsed technique; 2000 mA, maximum) with three different regimens: group A, RF ablation with the 17-gauge cooled electrode; group B, RF ablation with the 15-gauge cooled electrode; group C, RF ablation with the perfused-cooled electrode. T2-weighted magnetic resonance (MR) imaging was obtained immediately after RF ablation for calculating volumes of the ablation zone. Following MR imaging, the ablation zones were excised and measured for transverse diameters and vertical diameters. The transverse diameter, vertical diameter, and the calculated volumes of the ablation zones on MR imaging were compared among the groups. Ablation zones created with the perfused-cooled electrode (group C) were significantly larger than those created with the 17-gauge cooled electrode (group A) and the 15-gauge cooled electrode (group B) according to the transverse diameter and vertical diameter on the gross specimens (p < 0.05): 3.6 +/- 0.38 cm and 4.4 +/- 0.20 cm in group A, 3.7 +/- 0.08 cm and 4.6 +/- 0.16 cm in group B, and 5.4 +/- 0.65 cm and 6.0 +/- 0.56 cm in group C, respectively. On the MR imaging, the calculated volumes of the ablation zones in group C were significantly larger than those in groups A and B (p < 0.05): 23.1 +/- 8.7 cm3 in group A, 28.9 +/- 5.7 cm3 in group B, and 80.0 +/- 34 cm3 in group C, respectively. A new perfused-cooled electrode using a single pump could efficiently increase the size of the ablation zone in liver compared with a monopolar cooled electrode, and this was due to its simultaneous use of internal cooling and saline infusion.

Radiofrequency Thermal Ablation: Increase in Lesion Diameter with Continuous Acetic Acid Infusion

PURPOSE

To evaluate the influence of continuous infusion of acetic acid 50% during radiofrequency ablation (RFA) on the size of the thermal lesion produced.

METHODS

Radiofrequency (RF) was applied to excised bovine liver by using an expandable needle electrode with 10 retractable tines (LeVeen Needle Electrode, RadioTherapeutics, Sunnyvale, CA) connected to a commercially available RF generator (RF 2000, RadioTherapeutics, Sunnyvale, CA). Experiments were performed using three different treatment modalities: RF only (n = 15), RF with continuous saline 0.9% infusion (n = 15), and RF with continuous acetic acid 50% infusion (n = 15). RF duration, power output, tissue impedance, and time to a rapid rise in impedance were recorded. The ablated lesions were evaluated both macroscopically and histologically.

RESULTS

The ablated lesions appeared as spherical or ellipsoid, well-demarcated pale areas with a surrounding brown rim with both RF only and RF plus saline 0.9% infusion. In contrast, thermolesions generated with RF in combination with acetic acid 50% infusion were irregular in shape and the central portion was jelly-like. Mean diameter of the coagulation necrosis was 22.3 +/- 2.1 mm (RF only), 29.2 +/- 4.8 mm (RF + saline 0.9%) and 30.7 +/- 5.7 mm (RF + acetic acid 50%), with a significant increase in the RF plus saline 0.9% and RF plus acetic acid 50% groups compared with RF alone. Time to a rapid rise in impedance was significantly prolonged in the RF plus saline 0.9% and RF plus acetic acid 50% groups compared with RF alone.

CONCLUSIONS

A combination of RF plus acetic acid 50% infusion is able to generate larger thermolesions than RF only or RF combined with saline 0.9% infusion.

Image guided tumour ablation

Several different technologies have been employed for the local ablation of tissue by thermal techniques. At the present time the most widely favoured technique is radiofrequency ablation (RFA) but developments in other techniques, e.g. microwave may change this. In many countries RFA or percutaneous ethanol injection (PEI) are accepted therapies for patients with Childs Pugh Class A or B cirrhosis and early hepatocellular carcinoma (HCC). Results for RFA in large series of patients with liver metastases from colon cancer are very promising. Five-year survival rates of 26% from the time of first ablation and 30% from the diagnosis of liver metastases for patients with limited (<6, <5 cm) liver disease who are not surgical candidates compares well with post resection series where 5-year survival rates vary between 29% and 39% in operable candidates. Sufficient experience has now been gained in lung and renal ablation to show that these are minimally invasive techniques which can produce effective tumour destruction with a limited morbidity. More novel areas for ablation such as adrenal or pelvic recurrence are being explored.

Radiofrequency ablation: importance of background tissue electrical conductivity--an agar phantom and computer modeling study

PURPOSE

To determine whether radiofrequency (RF)-induced heating can be correlated with background electrical conductivity in a controlled experimental phantom environment mimicking different background tissue electrical conductivities and to determine the potential electrical and physical basis for such a correlation by using computer modeling.

MATERIALS AND METHODS

The effect of background tissue electrical conductivity on RF-induced heating was studied in a controlled system of 80 two-compartment agar phantoms (with inner wells of 0.3%, 1.0%, or 36.0% NaCl) with background conductivity that varied from 0.6% to 5.0% NaCl. Mathematical modeling of the relationship between electrical conductivity and temperatures 2 cm from the electrode (T2cm) was performed. Next, computer simulation of RF heating by using two-dimensional finite-element analysis (ETherm) was performed with parameters selected to approximate the agar phantoms. Resultant heating, in terms of both the T2cm and the distance of defined thermal isotherms from the electrode surface, was calculated and compared with the phantom data. Additionally, electrical and thermal profiles were determined by using the computer modeling data and correlated by using linear regression analysis.

RESULTS

For each inner compartment NaCl concentration, a negative exponential relationship was established between increased background NaCl concentration and the T2cm (R2= 0.64-0.78). Similar negative exponential relationships (r2 > 0.97%) were observed for the computer modeling. Correlation values (R2) between the computer and experimental data were 0.9, 0.9, and 0.55 for the 0.3%, 1.0%, and 36.0% inner NaCl concentrations, respectively. Plotting of the electrical field generated around the RF electrode identified the potential for a dramatic local change in electrical field distribution (ie, a second electrical peak ["E-peak"]) occurring at the interface between the two compartments of varied electrical background conductivity. Linear correlations between the E-peak and heating at T2cm (R2= 0.98-1.00) and the 50 degrees C isotherm (R2= 0.99-1.00) were established.

CONCLUSION

These results demonstrate the strong relationship between background tissue conductivity and RF heating and further explain electrical phenomena that occur in a two-compartment system.

Local therapeutic treatments for focal liver disease

Patients diagnosed with primary hepatic malignancies or metastases to the liver remain a difficult population to treat. A small percentage of these people can undergo surgical resection or transplantation. The remaining nonsurgical aggregate does not often benefit from conventional radiation and chemotherapy; minimally invasive means either to cure or palliate these patients are a requirement for complete cancer care. This article discusses image-guided local therapies used to treat this difficult patient population, focusing predominantly on radiofrequency ablation.

Radiofrequency versus microwave ablation in a hepatic porcine model

PURPOSE

To compare microwave (MW) and radiofrequency (RF) ablation in a hepatic porcine model.

MATERIALS AND METHODS

Institutional animal research committee approval was obtained. Nineteen pigs were divided into groups based on time of sacrifice (group A, immediate; group B, 2 days; group C, 28 days; group D, 28 days). Groups A, B, and C each underwent a combination of RF and MW ablation. Group D underwent either four MW or four RF ablations. Ablation was performed with a prototype MW device (915 Mhz, 40 W, 10 minutes) and a commercial RF system (150 W, 10 minutes, 3-cm deployment). Computed tomography (CT) was performed in groups B and C at 2 days and in group C at 28 days. Group D underwent serial laboratory testing. Specimens were serially sectioned, and short-axis diameter and length of each were measured. The percentage deflection caused by local blood vessels (heat-sink effect) was also measured in group A. Likelihood ratio tests and unpaired t tests were used for statistical analyses as appropriate.

RESULTS

MW ablation zones were longer at days 0, 2, and 28 (P < .05), but short-axis diameter was not different from that with RF ablation at any time point (P > .05). Local blood vessels caused 3.5% +/- 5.3 (standard deviation) deflection at MW ablation compared with 26.2% +/- 27.9 at RF ablation (P < .05). MW and RF ablation zones were indistinguishable at CT or pathologic evaluation. Laboratory test results were similar between RF ablation-only animals and MW ablation-only animals, with the exception of a slightly higher alkaline phosphatase levels at day 2 in RF ablation-only animals (P < .02).

CONCLUSION

MW and RF ablation zones are similar in pathologic appearance and imaging characteristics. Increased length with MW ablation is likely caused by the length of the radiating segment of the antenna. MW ablation may be less affected by the heat-sink effect that is thought to contribute to local recurrence after RF ablation.

An ex-vivo experimental study on optimization of bipolar radiofrequency liver ablation using perfusion-cooled electrodes

PURPOSE

To determine optimal parameters for bipolar radiofrequency ablation (RFA) using perfusion-cooled electrodes to create a large ablation volume in ex vivo bovine liver.

MATERIAL AND METHODS

Three sets of RF experiments were performed using a 200-Watt generator and two 15-gauge perfusion-cooled or internally cooled electrodes in ex vivo bovine livers. In the first set of experiments, to find the ideal inter-electrode distance for creating large coagulation necrosis, 30 ablation lesions were created by bipolar RFAs at inter-electrode spacings of 3 cm, 4 cm, and 5 cm. In the second set of experiments, to explore the ideal duration of RF application, bipolar RFAs were performed for 10 min and 20 min. In the first and second experiments, 10 lesions were made for each condition with infusion of 6% hypertonic saline (HS) at 2 ml/min. In the third set of experiments, 10 ablation lesions were created by bipolar RFAs using internally cooled electrodes without HS infusion. The mean volume of those ablation lesions was then compared to that of the lesions created by bipolar RFA using perfusion-cooled electrodes in the second experiments. Tissue impedance, dimension, and shape of the ablated areas were compared in each condition.

RESULTS

In the first set of experiments, bipolar RFA created a homogeneous oval or spherical-shaped ablation area between the electrodes at 3-5 cm spacing, but showed a more spherical-shaped lesion at 3 cm inter-electrode spacing than at 4 cm and 5 cm spacing. In the second set of experiments, RF energy delivered for 20 min created a larger dimension of coagulation necrosis than energy delivered for 10 min: 107.6 +/- 34 cm3 versus 59.5 +/- 27 cm3 (P<0.05). In addition, the mean volume of ablation regions obtained with bipolar RFA using the internally cooled electrode was 47.5+/- 17 cm3, which was significantly less than that with bipolar RFA using perfusion-cooled electrodes (P <0.05).

CONCLUSION

Bipolar RFA using perfusion-cooled electrodes achieves homogeneous areas of coagulation necrosis between two electrodes, preferably at 3 or 4 cm inter-electrode distance for 20 min, and is better in creating large coagulation necrosis than bipolar RFA using internally cooled electrodes.

Radiofrequency Thermal Ablation: The Role of MR Imaging in Guiding and Monitoring Tumor Therapy

Performing RFA procedures under MR imaging involves two distinct processes: interactive guidance of the RF electrode into the targeted tumor and monitoring the effect of therapy. The justification for using MR imaging for electrode guidance is quite similar to its use to guide biopsy and aspiration procedures, where MR imaging offers advantages related to superior soft tissue contrast, multiplanar capabilities, and high vascular conspicuity that facilitate safe and accurate guidance in selected lesions. The major contribution of MR imaging to thermal ablation procedures is its ability to monitor tissue changes associated with the heating process instantaneously, an attribute that is not paralleled by any other currently available imaging modality. Such ability facilitates a controlled approach to ablation by helping to detect inadequately treated tumor foci for subsequent interactive repositioning of the RF electrode during therapy. As such, MR imaging guidance and monitoring enable treatment of the entire tumor on a single-visit basis while avoiding undue overtreatment and preserving often critically needed organ function. Although knowledge of interventional MR imaging concepts and familiarity with its technology and with the related safety issues are indispensable for interventional radiologists attempting thermal ablation procedures in the MR imaging environment, understanding the tissue basis of necrosis imaging is becoming an essential part of the knowledge base for the larger sector of general radiologists who are required to interpret the follow-up MR imaging scans of the increasing number of thermal ablation patients.

Artificially induced pneumothorax for percutaneous transthoracic radiofrequency ablation of tumors in the hepatic dome: initial experience

After institutional review board approval, informed consent was obtained from six patients (four men and two women, aged 47-74 years) with a total of six tumors of the liver dome. These patients were treated by means of radiofrequency (RF) ablation with computed tomographic (CT) guidance and a transthoracic approach. With use of general anesthesia, a right pneumothorax was induced by means of manual injection of air until the route allowing access to the tumor was cleared of all lung parenchyma. Then RF ablation was performed with transthoracic extrapulmonary transdiaphragmatic access. After retrieval of the RF electrode, the pneumothorax was fully aspirated. All procedures were successfully performed without complications. Artificially induced pneumothorax appears useful and safe for CT-guided RF ablation of liver dome tumors, although this experience was minimal, with only six patients treated.

Combination radiofrequency ablation with intratumoral liposomal doxorubicin: effect on drug accumulation and coagulation in multiple tissues and tumor types in animals

PURPOSE

To determine whether use of radiofrequency (RF) ablation combined with intravenously (IV) administered liposomal doxorubicin, as compared with use of RF ablation or doxorubicin alone, facilitates increased tissue coagulation and interstitial drug accumulation in animal models.

MATERIALS AND METHODS

The institutional animal care and use committee approved this study. In experiment 1, multiple canine sarcomas were implanted in seven mildly immunosuppressed dogs and grown to a mean diameter of 4.8 cm. Tumors were assigned to three treatment groups: internally cooled RF ablation (12 minutes, 2000-mA pulsed technique) followed by IV liposomal doxorubicin (10 mg per animal) (n = 6), RF ablation alone (n = 6), and liposomal doxorubicin alone (n = 4). In experiment 2, the livers and kidneys of 10 rabbits and the thigh muscles of 10 rats were randomly assigned to one of two treatment groups: conventional RF ablation (90 degrees C +/- 2, 5 minutes) followed by IV liposomal doxorubicin (5 mg per rabbit, 1 mg per rat) or RF ablation alone (n = 5, each). Coagulation diameter and interstitial doxorubicin concentration (tissues were homogenized in acid alcohol, with doxorubicin extracted for 24 hours at 5 degrees C and quantified with fluorimetry) were measured 48 hours after treatment and compared. Multivariate analysis of variance and subsequent pairwise t tests (alpha = .05, two-tailed test) were performed.

RESULTS

Data are means +/- standard errors of the mean. A larger diameter of tumor destruction was observed in canine sarcomas treated with RF ablation-liposomal doxorubicin (3.7 cm +/- 0.6) compared with that in tumors treated with RF ablation (2.3 cm +/- 0.1) or liposomal doxorubicin (0.0 cm +/- 0.0) alone (P < .01). A new finding was a completely necrotic red zone (1.6 cm +/- 0.7) surrounding the central RF ablation-induced white coagulation zone. Greater but nonuniform drug uptake was observed particularly in this red zone (77.0 ng/g +/- 18.2) compared with uptake in the central zone (15.1 ng/g +/- 3.2), peripheral area of untreated tumor (38.9 ng/g +/- 8.0), and tumors treated with liposomal doxorubicin alone (43.9 ng/g +/- 6.7 for all regions) (P < .01 for all individual comparisons). In experiment 2, use of combined therapy led to increased coagulation in all tissues (liver: 17.6 mm +/- 3.1, P = .03; kidney: 11.0 mm +/- 3.1, P = .03; muscle: 13.1 mm +/- 1.3, P < .01) compared with use of RF ablation alone (liver, 13.4 mm +/- 1.5; kidney, 7.9 mm +/- 0.7; muscle, 8.6 mm +/- 0.5). Combined therapy, as compared with liposomal doxorubicin therapy alone, was also associated with increased doxorubicin accumulation in liver, kidney, and muscle (1.56 microg/g +/- 0.34, 4.36 microg/g +/- 1.78, and 3.63 microg/g +/- 1.43, respectively, vs 1.00 microg/g +/- 0.18, 1.23 microg/g +/- 0.32, and 0.87 microg/g +/- 0.53, respectively) (P < or = .01 for all individual comparisons).

CONCLUSION

Use of RF ablation combined with liposomal doxorubicin facilitates increased tissue coagulation and interstitial doxorubicin accumulation in multiple tissues and tumor types and may be useful for treatment of large tumors and achieving an ablative margin within the untreated tissue surrounding RF ablation-treated tumors.

Image-guided tumor ablation: standardization of terminology and reporting criteria

The field of interventional oncology with use of image-guided tumor ablation requires standardization of terminology and reporting criteria to facilitate effective communication of ideas and appropriate comparison between treatments that use different technologies, such as chemical (ethanol or acetic acid) ablation, and thermal therapies, such as radiofrequency, laser, microwave, ultrasound, and cryoablation. This document provides a framework that will hopefully facilitate the clearest communication between investigators and will provide the greatest flexibility in comparison between the many new, exciting, and emerging technologies. An appropriate vehicle for reporting the various aspects of image-guided ablation therapy, including classification of therapies and procedure terms, appropriate descriptors of imaging guidance, and terminology to define imaging and pathologic findings, are outlined. Methods for standardizing the reporting of follow-up findings and complications and other important aspects that require attention when reporting clinical results are addressed. It is the group's intention that adherence to the recommendations will facilitate achievement of the group's main objective: improved precision and communication in this field that lead to more accurate comparison of technologies and results and, ultimately, to improved patient outcomes.

Comparison of renal ablation with monopolar radiofrequency and hypertonic-saline-augmented bipolar radiofrequency: in vitro and in vivo experimental studies

OBJECTIVE

We sought to determine whether hypertonic-saline (HS)-augmented bipolar radiofrequency ablation has advantages over monopolar radiofrequency ablation for creating larger areas of coagulation necrosis in the kidney.

MATERIALS AND METHODS

Using a 200-W generator and bipolar perfused-cooled electrodes or a monopolar cooled-tip electrode, we performed 14 radiofrequency ablations in explanted bovine kidneys. Radiofrequency was applied in standard monopolar (n = 7) or bipolar (n = 7) modes at 100 W for 10 min. In the bipolar mode, the perfused-cooled electrodes were placed at interelectrode distances of 3 cm, and a 6% sodium chloride solution was instilled into tissue at a rate of 2 mL/min through the electrodes. For in vivo experiments, either monopolar (n = 7) or HS-augmented bipolar (n = 7) radiofrequency ablation was performed in the lower pole of canine kidneys. Three days after the procedure, contrast-enhanced CT scans were obtained to evaluate the volumes of the ablation regions, and the kidneys were harvested for gross measurements. Technical parameters such as changes in impedance and current during radiofrequency ablation and dimensions of the thermal ablation zones were compared between the two groups.

RESULTS

In ex vivo and in vivo experiments, the frequency of the pulsed radiofrequency application caused by rises in impedance was higher in the monopolar mode than in the bipolar mode during the application of radiofrequency energy. The in vivo study showed that the bipolar radiofrequency ablation allowed larger mean current flows than the monopolar radiofrequency ablation (i.e., mean +/- SD, 1,654 +/- 144 mA vs 967 +/- 597 mA) (p < 0.05). Ex vivo studies revealed that the volumes of bipolar radiofrequency-induced ablation regions were substantially larger than those of monopolar radiofrequency-induced ablation regions (26.1 +/- 10.5 cm(3) vs 10.2 +/- 4.2 cm(3)). In vivo studies showed bipolar radiofrequency ablation achieved larger coagulation necrosis than monopolar radiofrequency (3.2 +/- 0.3 cm vs 2.4 +/-0.4 cm) (p < 0.05). This was confirmed by the measured volume of nonenhancing area on contrast-enhanced CT (20.4 +/- 6.4 cm(3) vs 13.5 +/- 6.0 cm(3)).

CONCLUSION

HS-augmented bipolar radiofrequency ablation using perfused-cooled electrodes shows better performance in creating coagulation necrosis than monopolar radiofrequency ablation in the kidney of an animal model.

Radiofrequency ablation in the liver using two cooled-wet electrodes in the bipolar mode

The purpose of this study was to demonstrate the efficacy of bipolar radiofrequency ablation (RFA) using cooled-wet electrodes inducing coagulation in ex vivo bovine livers and in in vivo canine livers. In ex vivo experiments, 20 coagulations were created by monopolar (group A), and bipolar RFA (group B) using a 200 W generator (Valleylab) and one or two cooled-wet electrodes. In in vivo experiments, one coagulation was created by bipolar RFA in each of eight dogs via laparotomy. In ex vivo and in vivo experiments, RF was applied to one or two electrodes at 100 W for 10 min. The dimensions of the coagulations were compared in the two groups. In ex vivo experiments, the mean volumes of the coagulations produced in group B (54.0+/-16.5 cm3) were greater than those produced in group A (33.9+/-12.7 cm3) (P=0.007). In in vivo experiments, bipolar RFA produced a coagulation of 39.4+/-15.6 cm3 without a major complication. The present study showed that a RF electrode system using two cooled-wet electrodes in the bipolar mode created larger coagulation volumes than the monopolar mode, and this system can be used to create large coagulation without major complications.

Combination radiofrequency thermal ablation and adjuvant IV liposomal doxorubicin increases tissue coagulation and intratumoural drug accumulation

There has been marked interest in minimally-invasive, image-guided radiofrequency (RF) tumour ablation (i.e. coagulating tumour using short duration heating ( < 15 min) by directly applying temperatures > 50 degrees C via needle electrodes) to treat focal liver, renal, breast, bone and lung tumours. In spite of advances in RF technology and improved understanding of tumour biophysiology that now enable experimental treatment of tumours up to 5cm, investigators have been unable to achieve complete ablation in many cases, particularly at the tumour margins and adjacent to blood vessels. One strategy for overcoming these limitations has been to take advantage of complementary interactions between RF thermal ablation and chemotherapy, particularly liposomal doxorubicin preparations, to attempt more complete tumour destruction. This paper will review published laboratory investigations demonstrating that this combined treatment paradigm has the unique potential both to potentiate preferential delivery of cytotoxic agents in liposome vehicles and to maximize the completeness of ablation of a treated tumour. New confirmatory data describing increased tumour destruction with RF ablation combined with different liposome preparations, documenting increased lipid peroxidation and expanding on previously published tumour growth studies is presented. Additionally, early clinical data including a randomized, pilot clinical study on 10 patients with primary and metastatic liver tumours, in which a non-optimized combination of RF ablation and IV liposomal doxorubicin (Doxil) increased the volume of tumour destruction 25-30% compared to RF alone, will also be described in detail.

Radiofrequency Tumor Ablation: Insight into Improved Efficacy Using Computer Modeling

OBJECTIVE

To use computer modeling of the Bio-Heat equation to demonstrate factors influencing RF ablation tissue heating.

CONCLUSION

Computer modeling demonstrates the importance of energy deposition, tumor and background tissue electrical and thermal conductivity, and perfusion on RF ablation outcomes.

Ex Vivo Experiment of Saline-Enhanced Hepatic Bipolar Radiofrequency Ablation with a Perfused Needle Electrode: Comparison with Conventional Monopolar and Simultaneous Monopolar Modes

The purpose of this study was to validate the saline-enhanced bipolar radiofrequency ablation (RFA) technique using a perfused electrode to increase RF-created coagulation necrosis, to compare that technique with monopolar RFAs and to find appropriate concentrations and volumes of perfused NaCl solution for the bipolar RFA. A total of 90 ablations were performed in explanted bovine livers. In the initial experiments to determine appropriate conditions for bipolar RFA, we created five thermal ablation zones in each condition, with instillations of varied concentrations (0.9-36%) or injection rates (30 mL/hr-120 mL/hr) of NaCl solution. After placement of one or two 16-gauge open-perfused electrodes into bovine livers, the NaCl solution was instilled into the tissue through the electrode. In the second part of the study, 10 ablation zones were created using one or two perfused electrodes for each of five groups under different conditions: a conventional monopolar mode with 0.9% NaCl solution (group A) or with 6% NaCl solution (group B), a simultaneous monopolar mode with 6% NaCl solution (group C) and a bipolar mode with 6% NaCl solution (groups D and E). RF was applied to each electrode for 20 min in groups A, B, C, and E, or for 10 min in group D. During RFA, we measured the tissue temperature 15 mm from the electrode. The temperature changes during the RFA and the dimensions of the ablation zones were compared among the groups. Bipolar RFA created larger short-axis diameters of coagulation necrosis with 6% NaCl solution (35.8 +/- 15 mm) than with 0.9% NaCl solution (17 +/- 9.7 mm) (P < 0.05). However, concentrations of NaCl solution above 6% did not further increase the extent of coagulation necrosis. In addition, bipolar RFA with 6% NaCl solution instillation at a rate of 1.0 mL/min (37.9 +/- 5.4 mm) or 2.0 mL/min (35.6 +/- 9.3 mm) produced larger diameters at the mid-point between the electrodes of the ablated lesion than did 0.5 mL/min (25.8 +/- 9.3 mm) (P < 0.05). The bipolar mode showed a more rapid increase in temperature at the mid-point between the two probes, up to 60 degrees C, than did the monopolar modes (P < 0.05). In addition, the bipolar RFA (group E) treated for 20 min showed a larger value of the short-axis diameter than did the conventional or simultaneous monopolar modes (P < 0.05), and bipolar RFA (group D) treated for 10 min, showed similar results with conventional monopolar modes treated for 20 min (P > 0.05): 31.0 +/- 5.4 mm (group A); 28.8 +/- 3.8 mm (group B); 25.5 +/- 6.4 mm (group C); 32.6 +/- 4.2 mm (group D); 49.4 +/- 5.0 mm (group E). Bipolar RFA with instillation of 6% NaCl solution through an open perfusion system demonstrates better efficacy in creating a larger ablation zone than does conventional or simultaneous monopolar modes at the various times examined. Therefore, hypertonic saline-enhanced bipolar RFA seems to be a promising approach for treating larger liver tumors.

Effect of vessel diameter on the creation of ovine lung radiofrequency lesions in vivo: Preliminary results

OBJECTIVE

We sought to evaluate the effect of radiofrequency ablation (RFA) on pulmonary vessels with respect to potential of injury of these structures, to assess perfusion-mediated "heat sink" effect, and to consider acute and chronic complications.

MATERIAL AND METHODS

RFAs targeted to perihilar, middle third, and peripheral lung regions were created in vivo in the lung of 10 crossbred sheep. The RITA generator and the Starburst XLi electrode with deployable hooks were used. The approach was open, performed under general anesthesia. Lesions 4 cm in diameter at a target temperature of 80 degrees C were created. Acute (immediate postinterventional euthanasia), subacute (96 h), and chronic (28 days) lesions were evaluated macroscopically, and histologic analysis of the vessels was performed. Patency of the vessels, both arteries and veins, was macroscopically assessed by presence or absence of thrombus and the degree of vascular injury and the viability of perivascular pneumocytes as well as endobronchial injury were histologically assessed.

RESULTS

In the acute, subacute, and chronic setting, heat sink effect, indicated by invagination of the tissue between vessel and ablated region, was only observed in vessels greater than 3 mm in diameter. Thrombus was seen in 20% of the vessels smaller than 3 mm. On histopathology, vessels smaller than 3 mm showed at least partial vessel wall injury, characterized by endothelial cell necrosis and luminal thrombus. In the vessels greater than 3 mm the extent of vessel wall injury decreased with increasing vessel diameter. No acute complications were noted. For the chronic complications a bronchopleural fistula and a lung abscess were found.

CONCLUSION

There seems to be a narrow transition zone for pulmonary vessels around 3 mm, beyond which the heat sink effect was seen consistently and substantial vascular injury was rare.

Percutaneous radiofrequency ablation of hepatic colorectal metastases: technique, indications, results, and new promises

Surgical resection is the standard of care for colorectal metastases isolated to the liver. However, only 10-25% of the patients are eligible for resection because of extent and location of the disease in the liver or concurrent medical conditions. Image-guided radiofrequency (RF) ablation is a minimally invasive technique that is emerging as a viable alternate treatment of nonsurgical patients with limited hepatic metastatic disease. Several series have shown that RF ablation can result in complete tumor eradication in properly selected candidates and have provided indirect evidence that the treatment improves survival. In a recent multicenter trial including 423 patients, overall survival of RF-ablation treated patients reached 47% at 3 years and 24% at 5 years. RF ablation technology is undergoing continuous improvement, and its clinical application has been successfully expanded to the treatment of colorectal metastases to the lung. Randomized trials comparing RF ablation with either surgical resection or chemotherapy protocols, however, are still missing. In this article, we review technique, indications, clinical results, and future prospects of RF ablation in the therapeutic management of metastatic colorectal cancer patients.

Hybrid radiofrequency and cryoablation device: preliminary results in an animal model

PURPOSE

To determine whether the simultaneous application of combined bipolar radiofrequency (RF) ablation and cryoablation in a hybrid system produces larger ablation zones than RF or cryoablation alone.

MATERIALS AND METHODS

Multiple 15-minute ablations were performed in ex vivo bovine liver (n = 167) with a hybrid applicator system with RF ablation alone (0.3-0.7 A), cryoablation alone (3,500 psi, two freeze/thaw cycles), and combined RF/cryoablation (0.4-0.7 A, 1,000-3,500 psi) with use of a novel applicator consisting of two 2.5-cm active bipolar RF poles located on the same 18-gauge needle separated by two embedded cryoablation nozzles. Resultant coagulation diameters were compared with use of analysis of variance for more than three groups or Student t tests for two groups. Confirmation of the optimal parameters of combination RF/cryoablation was performed by reassessing a range of argon pressure (1,000-3,500 psi) and RF current (0.4-0.7 A) in in vivo porcine liver (n = 36). Arrays of two to four RF/cryoablation applicators were also assessed in ex vivo (n = 54) and in vivo (n = 12) liver.

RESULTS

In ex vivo liver, simultaneous RF/cryoablation (0.6 A, 3,000 psi) produced 3.6 cm +/- 0.4 of short-axis coagulation. This was significantly larger than that achieved with optimal RF alone or cryoablation alone (1.5 cm +/- 0.3 and 1.6 cm +/- 0.3, respectively; F = 95; P < .01). The coagulation diameter with simultaneous combination RF/cryoablation was related in parabolic fashion to argon pressure and current with a multivariate r(2) of 0.68. For in vivo liver, optimal combination RF/cryoablation achieved 3.3 cm +/- 0.2 of coagulation, which was significantly larger than that achieved with RF alone (1.1 cm +/- 0.1; P < .01) or cryoablation alone (1.1 cm +/- 0.1 and 1.3 cm +/- 0.1; F = 203; P < .01). The greatest contiguous coagulation was achieved with multiple-applicator arrays. For ex vivo liver, short-axis coagulation measured 5.3 cm +/- 0.1, 6.4 cm +/- 0.1, and 7.6 cm +/- 0.1 for two-, three-, and four-applicator arrays, respectively. For in vivo liver, two-, three-, and four-applicator arrays produced 5.1 cm +/- 0.2, 5.8 cm +/- 0.5, and 7.0 cm +/- 0.5 of confluent coagulation, respectively.

CONCLUSION

Simultaneous combination RF and cryoablation with use of a novel applicator design yielded significantly larger zones of coagulation than either modality alone. The large ablation diameters achieved warrant further investigation of the device.

Bipolar Radiofrequency Ablation Using Wet-Cooled Electrodes: An In Vitro Experimental Study in Bovine Liver

OBJECTIVE

Our aim was to evaluate the performance of hypertonic saline (HS)-enhanced bipolar radiofrequency ablation using wet-cooled electrodes versus monopolar radiofrequency ablation to create coagulation necrosis in explanted bovine liver.

CONCLUSION

HS-enhanced bipolar radiofrequency ablation using the wet-cooled electrodes shows better performance in creating coagulation necrosis than the monopolar mode.