Improvement of the primary efficacy of microwave ablation of malignant liver tumors by using a robotic navigation system

The indications for percutaneous tumor ablation were established by a multi-disciplinary tumor board. The following exclusion criteria were applied: coagulation disorders not amendable to substitution; portal vein, hepatic vein or inferior vena cava invasion; extrahepatic metastases; and multifocal hepatic disease not amenable to complete ablation.

A total of 192 patients underwent either freehand or robotic guided microwave ablation from 08/2011 to 11/2018, inclusive. All procedures were performed by the same three experienced interventional radiologists (blinded).

This single-center retrospective observational study was approved by the local ethics committee. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments and the guidelines for Good Clinical Practice from the International Conference on Harmonization. Informed consent was obtained from all individual participants included in the study.

All microwave ablations were performed under general anesthesia. During CT scans and antenna positioning, control of the respiratory movement was performed by temporary tube disconnection. Arterial and portal venous helical CT scans (Somatom 16 or Definition Egde, Siemens Healthcare, Forchheim, Germany) with a slice thickness of 1 mm were acquired.

CT fluoroscopy was used for ablations without navigation support, an acquisition mode that allows continuous image update using in-room table control. After the initial 2-phase planning CT, the antenna was placed during repeated temporary breath holds. To verify the correct antenna placement, one unenhanced CT was obtained before starting the ablation. If necessary, the antenna was repositioned until the whole tumor volume was covered.

When using robot navigation, the initial CT data was sent to the navigation system (Maxio, Perfint Healthcare, Chennai, India).^18,19 The desired ablation area and the antenna entry point were defined using the planning software, and the trajectory was visualized. If necessary, multiple antenna positions were planned with overlapping ablation zones. After approval of the plan, the robotic arm was automatically positioned over the patient and the antenna was positioned using the targeting device during breath hold. The probes were pushed forward manually along the preplanned path while held by the robotic needle holder. Before ablation, a CT scan was performed and the antenna position was verified by overlaying it with the planned trajectory. Consistent docking and absolute registration of the robotic device was performed using a base plate fixed on the ground. The navigation system is connected to the local PACS as a DICOM node. The images are automatically pushed to the navigation system by an auto transfer task after successful reconstruction of the 1mm images in the CT scanner.

For ablation, either the Acculis Microwave Tissue Ablation (MTA) System (AngioDynamics, Latham, NY, USA; Accu2i pMTA Applicator 1.8 mm diameter in 14 or 19 cm length) or the Emprint Ablation System (Medtronic, Minneapolis, USA; Emprint™ Percutaneous Antennas 1.8 mm diameter in 15 or 20 cm length) was used, depending on tumor configuration and relationship to the surrounding tissue. By comparison of the expected ablation zone in the unenhanced scan (typically hypodense) to the initial tumor in the planning scan. If there was suspicion of insufficient ablation margin repositioning was performed.

After ablation and track ablation, all patients underwent a noncontrast multislice CT scan of the liver to detect complications.

All patients underwent our standard follow-up scheme after 6 weeks including CT, MRI with hepatospecific contrast agent and ultrasound. Further follow-up investigations were only carried out using MRI and ultrasound for radiation protection. The radiographic adjudication/visual assessment of the complete success of the ablation was retrospectively determined in consensus by two experienced radiologists (blinded). The primary technique efficacy was defined as the percentage of the target tumors that were successfully eradicated following the initial procedure as evidenced in the 6-week follow up according to the standardization of terminology by Ahmed et al. ²⁰

R 3.51 was used to perform all statistical calculations. A p-value of p ≤ 0.05 was considered the cutoff point of statistical significance. For multivariate analysis of primary efficacy using nested data (multiple ablations per patient in some cases), we applied generalized estimation equations (GEEs).

A total of 192 patients (156 male) were included in the study (Table 1). The median age was 64 years (range: 57–72). In total, 264 ablation sessions were performed with a median number of treatment sessions per patient of 1 (range: 1–4). 137 patients required one session, 41 patients required two sessions and the remaining patients required three or more sessions. The median number of tumors treated per patient was 1 (range: 1–9).

A total of 368 tumors spread across all liver segments were treated using MWA and either robotic-assistance or CT fluoroscopy (Table 2). The two most frequent tumor entities were hepatocellular carcinoma (n = 271) and liver metastasis of colorectal carcinoma (n = 54), followed by cholangiocellular carcinoma (n = 18). The median tumor size was 16 mm, with 59 tumors larger than 30 mm.

The primary efficacy rate using robotic guidance was 88%, i.e., 219 of the 249 tumors were covered completely by the ablation volume. Needle repositioning was necessary in 92 of 249 ablations (37%). In contrast, the primary efficacy rate for freehand ablation was 76% (91 of 119 tumors). Logistic regression was performed to investigate whether tumor characteristics (size, entity and location) and the type of guidance (robotic or freehand) can impact primary technique efficacy (Table 3).

Compared with tumor size ≤ 3 cm, tumor size > 3 cm was a significantly unfavorable prognosticator of primary technique efficacy (odds ratio 0.42; p = 0.02). Compared with freehand antenna placement, robotic guidance was a significant favorable prognostic factor (odds ratio 2.24; p = 0.01). Table 4 shows estimations of the primary technique efficacy for robotic and freehand guidance.

141 (80.11%) of the robotic-guided and 62 (70.45%) of the CT-fluoroscopy-guided procedures were performed without any adverse events. Grade I (mild), II (moderate) and III (severe) adverse events occurred in 9 (5.11%), 6 (3.41%) and 1 (0.57%) of the robotic-guided procedures, respectively, and 3 (3.41%), 3 (3.41%) and 1 (1.14%) of the freehand-guided procedures, respectively.

Grade IV (life-threatening) adverse events occurred in 1 (0.57%) of the robotic-guided procedures and 2 (2.27%) of the freehand-guided procedures. The patient in the robotic-guided group suffered an injury to the 10th intercostal artery during ablation, which led to persistent bleeding and had to be treated with embolization. One of the patients in the freehand group, who had previously undergone partial liver resection and a consecutive Chilaiditi situation, had a perforation of a prolapsed intestinal loop that had to be surgically overstitched. The other patient in the freehand group suffered from bleeding from the 7th and 8th intercostal artery after ablation, which had to be closed by embolization.

Treatment-related patient death (Grade V) occurred in 1 (0.57%) of the robotic-guided procedures and 0 (0.00%) of the freehand guided procedures. A patient that had a previous liver and kidney transplant developed severe cholangitis two days after ablation and subsequent liver and kidney failure with lactate acidosis, which could not be controlled despite ultima ratio crush hepatectomy.

There was no significant difference in the frequency of adverse events (p = 0.07) between the two groups.