IRE has emerged as a promising non-thermal tissue ablation technique in the last few decades. Due to the advantages of achieving tumor ablation without heat sink and ECM destruction, IRE is drawing more and more attention for its potential in the treatment of tumors of hollow viscera or close to major anatomical structures [23–26]. In the present study, the effect of IRE on normal rabbit common bile duct was evaluated directly. In vitro cell experiments were conducted to determine the appropriate electric field parameter for simulation and in vivo bile duct IRE ablation. We found that bile duct IRE ablation was feasible without dramatic temperature increase using an appropriate parameter, however the risk of biliary dilatation and stricture may reduce the biliary patency in the long-term run.
Results from in vitro experiments of this study confirmed the sensitivity difference of different cell lines to the effect of IRE. This observation was consistent with previous studies. Gianulis et al. found that the difference in LD50 spanned as much as nearly 80-fold among different cell types. Besides, they also found that the different sensitivity showed no correlation with cell or nuclear size, cell morphology, and metabolism level . Kodama et al. observed that cancer cell lines seemed more sensitive to cell death from IRE compared to normal human bronchial epithelial cell line . Our results showed that the normal cell line HIBEpiC seemed more resistant to IRE treatment than cancer cell line HCCC-9810 and QBC-939. However, HUCC-T1 cancer cell were the least sensitive to IRE. Thus, the different sensitivity may not be determined simply by benign or malignant characteristics. The mechanism of IRE susceptibility needs to be further investigated to provide evidence for optimal parameter selection.
Previous studies have evaluated the effects of IRE on vital structures adjacent to the target ablation area [29–31]. Kevin et al. found that IRE was safe and effective for liver ablation in the liver hilum without the presence of heat sink, and that major bile ducts and portal veins were more resistant to IRE ablation . Furthermore, long-term increase in serum bilirubin or transaminases levels was not identified, which was similar to our findings. Choi et al. evaluated the chronic effects of IRE on bile ducts and found that 6/7 animals in the periductal (electrode-to-duct distance ≤ 2 mm) electrode placement group developed bile duct narrowing, but no bile duct narrowing was observed when the electrode was placed more than 2 mm from the bile duct . However, the parameters used in these studies were based on the clinical experience, and the electric field distribution and temperature changes during IRE procedure were not evaluated. Our study demonstrated that the electric field strength should be above 1500 V/cm to achieve cholangiocarcinoma cell ablation. The simulation models showed the uneven distributions of electric field and temperature in different bile duct layer because of the difference in electric and thermal properties , this may illustrate the resistance discrepancy of different tissues. In addition, computer simulation revealed that the temperature was not dramatic increased during the ablation process, which may not cause thermal injury.
As for catheter-directed biliary ablation, Ueshima et al. performed normal porcine common bile duct ablation with IRE with endoscopy. Their results illustrated the feasibility and early safety within one week after catheter-directed IRE . However, the chronic effects were not evaluated in their study. Srimathveeravalli et al. assessed the effects of IRE on normal porcine ureters through catheter-mounted electrode. They found that the intraluminal catheter-directed IRE ablation was safe and effective in normal ureter ablation over a relatively short follow-up period. However, the ureteral strictures were observed on day 7 post-IRE and did not resolve by day 28 [33, 34]. Even though the effects of IRE may vary due to the variations in the geometry of the electrode and the parameters of IRE, the presence of biliary stricture and dilatation in the present study and in that by Choi et al. suggests that bile duct injury always manifested as a delayed effect and the long-term effects of catheter-directed IRE on the bile ducts need further investigations.
There are several limitations to this study. The parameters of the in vitro experiments only changed the voltage, the pulse duration and number should also be assessed to obtain optimal treatment parameter in future studies. Besides, the computer simulation models can partly reveal the temperature distribution, however the change of tissue temperature was not measured in real time. The thermal injury may also exist because of the complex environment of in vivo study as reported in previous studies . Furthermore, we performed our study on normal rabbit common bile duct rather than a tumor model, electrical properties and microenvironments are different between the tumor and normal tissue; tumor ablation with IRE may require different parameters and have different effects. Therefore, further studies are needed to optimize the treatment parameters tumor ablation.
In conclusion, the normal common bile duct retains the lumen wall integrity and function following IRE with immediate periductal placement of the electrode. However, the parameters of cholangiocarcinoma IRE ablation need to be determined more precisely to ensure the treatment efficacy and reduce the risk of collateral damage.