Long-term efficacy and safety of microwave ablation for hepatocellular carcinoma adjacent to the gallbladder with a diameter ≤ 5 cm: a multicenter, propensity score matching study

Abstract Objective To compare the long-term efficacy and safety of microwave ablation (MWA) as first-line therapy for hepatocellular carcinoma (HCC) adjacent versus nonadjacent to the gallbladder. Materials and Methods From 2006 to 2018, 657 patients with ≤5 cm HCC who underwent percutaneous ultrasound-guided MWA as first-line therapy from 5 hospitals were enrolled in this retrospective study. Patients were grouped into the adjacent group (n = 49) and the nonadjacent group (n = 608) according to whether the tumor was adjacent to the gallbladder. Propensity score matching (PSM) was used to balance baseline variables between the two groups. Results Forty-eight patient pairs were matched after PSM. For the PSM cohort, during a median follow-up time of 60 months, there were no differences in PFS (hazard ratio [HR], 1.011; 95% confidence interval [CI], 0.647-1.578; p = 0.963) or OS (HR 0.925; 95% CI 0.522-1.639; p = 0.789) between the adjacent and nonadjacent groups. Univariate and multivariate analyses revealed that the tumor adjacent to the gallbladder was not an independent risk factor for PFS or OS (all p > 0.05). Subgroup analysis showed comparable PFS and OS between the two groups in the <3 cm subgroup and the 3-5 cm subgroups (all p > 0.05). In addition to more use of assistive technology (p < 0.05), the adjacent group shared comparable local tumor progression, complications, technical success rate, and hospital stay (all p > 0.05) to the nonadjacent group. Conclusion There were comparable long-term efficacy and complications between patients with HCC adjacent and nonadjacent to the gallbladder treated with MWA. HIGHLIGHTS The application of MWA to HCC adjacent and nonadjacent to the gallbladder resulted in comparable PFS and OS and complications. For both cohorts, MWA shared comparable complications (immediate and delayed), LTP, hospitalization, and operative time. MWA might be a first-line alternative for ≤5 cm HCC adjacent to the gallbladder with the use of assistive technologies and advances in technology.


Introduction
According to the latest Global Cancer Statistics 2020, primary liver cancer, a common malignant tumor of the digestive tract, has the sixth highest incidence of malignancies and the third highest fatality rate in the world and is a serious threat to human life and health [1].As the most common subtype of liver cancer (85% $ 90%), hepatocellular carcinoma (HCC) is highly aggressive and heterogeneous [2].Thus, one of the key issues in HCC management is effective therapy for tumors at an early stage, which is crucial for improving the patient's prognosis [3].As an alternative therapy, liver transplantation has been a phased success in the field of liver cancer treatment.However, most patients are unable to undergo liver transplants due to a shortage of donor organs, strong posttransplant rejection, severe complications, and elevated treatment costs [4].In addition, patients with advanced HCC tend to have a poor general condition and impaired hepatic function due to cirrhosis or portal thrombosis, which makes curative resection impossible [5].Since 80% of HCC patients are not candidates for liver resection and transplantation, a variety of nonsurgical locoregional interventional therapies have been developed over the past 20 years [6].Among those, image-guided percutaneous ablation, such as radiofrequency ablation (RFA) and microwave ablation (MWA), has been considered a potentially curative treatment for patients with early-stage HCC because of its advantages of preserving liver function and achieving the effect of similar surgical resection [7,8].The Barcelona Clinic Liver Cancer (BCLC) 2022 UPDATE suggests ablation as the first treatment approach for HCC patients with very earlystage BCLC 0 [9].As a new direction in thermal ablation technology, MWA has shown numerous advantages over other technologies, including higher intratumoral temperatures, as well as reduced operation time and electrical conductivity dependence [10][11][12].
However, thermal ablation remains a controversial treatment option for HCC in high-risk locations due to the risks associated with direct puncture, such as hemorrhage, metastasis, pulmonary gas interference, and unsatisfactory localization.For instance, the use of thermal ablation for HCC adjacent to the gallbladder may result in reversible thermal damage to the gallbladder, perforation, and acute cholecystitis due to heat conduction [13].Previous studies have demonstrated the efficacy of percutaneous thermal ablation for the treatment of HCC adjacent to the gastrointestinal tract [14], large blood vessels [15], bile ducts [16], gallbladder [17], diaphragm [18], etc.Recent studies have shown that MWA does not differ significantly in terms of technical efficacy, local tumor progression (LTP), or complication rates between high-risk and safe locations [19][20][21].However, these studies included several different high-risk sites (gastrointestinal tract, bile duct, gallbladder, diaphragm, etc.).Therefore, confounding factors may have influenced the findings.In addition, only a few single-arm studies have shown that MWA is safe and effective in treating HCC adjacent to the gallbladder [13,22].However, these studies were single-center, with short follow-up periods and unbalanced essential clinical characteristics of patients in the adjacent group and the nonadjacent group.
Herein, we conducted a multicenter, propensity score matching (PSM) study to compare the long-term efficacy and safety of MWA as first-line therapy for HCC adjacent versus nonadjacent to the gallbladder.

Patient selection
All patients signed an informed consent form before the initiation of treatment.Owing to patient confidentiality being maintained, the Human Subjects Committee approved this study and waived the requirement for additional informed consent for this retrospective study (identifier number: S2020-464-01), which was in line with the 1975 Helsinki Declaration.We used data from a multicenter database of 2,395 HCC patients who received MWA from January 2006 to December 2018 at five hospitals across the country, from which 657 patients were screened and included in this study.The diagnosis of HCC was based on puncture biopsy (n ¼ 465) or typical imaging findings (n ¼ 192).The inclusion criteria were as follows: (a) number of nodules 3 and diameter 5 cm; (b) Child-Pugh class A or B; (c) BCLC stage 0 to B; (d) initial onset of HCC; and (e) perioperative laboratory results meeting the operation criteria.The exclusion criteria were as follows: (a) vascular and/or bile duct invasion on imaging, (b) extrahepatic metastasis (EM), (c) ascites, (d) history of malignant tumor of other organs, (e) history of liver transplantation, (f) preoperative chemotherapy and/or transcatheter arterial chemoembolization (TACE), (g) tumor near the gastrointestinal tract and/or diaphragm muscle and/or the liver capsule and/or pericardium, (h) tumors near large blood vessels and/or the hepatic hilum and/or primary to secondary branching bile ducts and (i) presence of both adjacent and nonadjacent gallbladder tumors.The location adjacent to the gallbladder was defined as the position where the tumor was located within 5 mm of the gallbladder.Patients were grouped into the adjacent group (n ¼ 49) and the nonadjacent group (n ¼ 608) according to whether the tumor was adjacent to the gallbladder (Figure 1).

Ablation equipment and procedures
MWA uses a commercial microwave ablation system (KY2000, Nanjing Kangyou Medical, China) under the guidance of realtime ultrasound, which consists of a 2.4-GHz microwave generator, a flexible coaxial cable, and a cold axis antenna with a diameter of 15 mm to establish an ablation treatment cold axis and generate sufficient ablation areas to surround the target lesion and an ablation edge of at least 5 mm.Before treatment, a detailed plan was developed for each patient, including antenna placement, power output settings, ablation time duration, and ablation zone prediction.When necessary, auxiliary ablation techniques were used, including ethanol injection [23], artificial pleural effusion [24], artificial ascites [14], temperature monitoring [25], and contrast-enhanced ultrasound (CEUS) guidance [26], to improve the visibility of the tumor and avoid damage to essential structures such as the intestinal tract, diaphragm muscle, blood vessels, bile duct, and gallbladder.Hydrodissection, also known as artificial pleural effusion, artificial ascites, and biliary water injection, was also utilized.Under the guidance of ultrasound, the microwave antenna was implanted and confirmed to reach the deep edge of the tumor after local anesthesia with 1% lidocaine.Depending on the thermal field effect of the antenna, the antenna tip was at least 3 mm away from the gallbladder (vertical approach, the antenna was aimed at the gallbladder), and the antenna body was at least 5 mm away (parallel approach, the antenna was placed parallel to the gallbladder).During the procedure, multiple overlapping ablations were performed on the tumor, and the high-echo region was monitored in real time by conventional gray ultrasound.The ablation was terminated when the hyperechoic region created by the overlapping ablation covered the entire tumor and the surrounding hepatic parenchyma along the antenna axis.While the antenna was being removed, the needle channel was routinely ablated to prevent the spread and bleeding of tumor cells.Conscious sedation was achieved with intravenous administration of midazolam (0.06 mg/kg), with continuous vital sign monitoring being performed.After ablation, flumazenil (0.5 mg/kg) was injected intravenously to aid in postoperative recovery from sedation.Antibiotic treatment was given for 2 days to prevent infection after the operation.For those patients with fever, their treatment was continued with extended antibiotic therapy.All treatments were performed by two experienced interventional ultrasound doctors according to the preoperative protocol.

Follow-up care
CEUS or contrast-enhanced magnetic resonance imaging (MRI) was performed within 3 days after the operation to evaluate the integrity and immediate complications of ablation early.If the target tumor was completely necrotic with a sufficient ablative margin, the treatment was regarded as a technical success (TS).If a residual tumor was found, additional therapy was given to achieve complete necrosis of the target tumor.The follow-up period for all patients was calculated from the first MWA treatment.One month after the first MWA treatment, chest X-ray enhanced CT or enhanced MRI and laboratory examination were performed every 3 months for the first 2 years and every 4-6 months thereafter.If the patient was not suitable for undergoing enhanced CT or MRI, CEUS was performed.We assessed postoperative complications based on clinical symptoms, imaging tests, and laboratory records.All postoperative complications were recorded, whereas the occurrence of tumor metastasis was recorded as a delayed event.If a recurrence was detected during follow-up, the best second-line treatment, such as ablation, surgery, TACE, radiotherapy, chemotherapy, or targeted therapy, was used, depending on the characteristics of the recurrence, the patient's liver function, and the general condition of the patient.

Definitions of ablation terminology and index of therapeutic efficacy
Definitions are based on the standardization by the International Working Group on Image-Guided Tumor Ablation [27].LTP referred to the occurrence of new tumor lesions in or adjacent to the ablation zone after at least one contrast medium follow-up study has proven complete ablation, intrahepatic metastasis (IDM) was defined as the appearance of new tumors in the liver parenchyma far from the ablation area, and extrahepatic metastasis (EM) was defined as the emergence of new tumors in other organs.One of the primary outcomes was progression-free survival (PFS), which was defined as the interval from the first MWA to the occurrence of any event, such as LTP, IDM, EM, or death.Another primary endpoint of this study was overall survival (OS), which was calculated as the interval from the initial MWA treatment to the death of the patient or the last follow-up before 5 September 2022.The secondary endpoints included TS rate, LTP, complication rates, hospitalization, and operative time.Complications referred to clinical events that resulted in additional treatment interventions, prolonged hospital stays, or unexpected increases in the level of care.We carried out an analysis of immediate and delayed complications using the standard classification for complications published by the Society of Interventional Radiology (SIR) [28].Patients who were lost to follow-up were censored at the last contact.

Statistical analysis
All statistical analyses were carried out using SPSS software (version 26; IBM Corp., Armonk, NY).The missing values were imputed using various methods depending on the type of data.Logistic regression was used to estimate the propensity score.Known or suspected confounding factors (sex, ageadjusted Charlson codisease index (aCCI), Child-Pugh class, etiology, liver cirrhosis, tumor number, tumor size, aspartate aminotransferase (AST), alanine aminotransferase (ALT), serum albumin (ALB), serum alpha-fetoprotein (AFP), total bilirubin (TBIL), blood glucose (GLU), hemoglobin (HB), platelet count (PLT) and prothrombin time (PT)) were used as independent variables for the logistic regression model fitting, and then the prediction probability was calculated as the propensity score.The grouping variable (if HCC was adjacent to the gallbladder) was used as a dependent variable to fit the model.To reduce the selection bias and confounding factors between the two groups, 1:1 nearest-neighbor matching with an optimal caliper of 0.05 without replacement was used to generate 48 pairs of cases (Figure 1).
For the baseline comparison, continuous variables with normal distribution were statistically described as the mean-± standard deviation and compared by Student's t test.Nonnormal data were described by median and quartile spacing, and compared with the Mann-Whitney U test.Categorical variables were represented by numbers and/or percentages and analyzed using Pearson's chi-square test or Fisher's exact test.The survival curves were plotted using the Kaplan-Meier method and compared using the log-rank test.To assess the independent risk factors for PFS and OS, hazard ratios (HRs) with 95% CIs were estimated using the Cox proportional hazards model.Statistical significance was defined as p < 0.05 (2-sided).

Baseline features
Before PSM, 657 patients were included, of whom 528 were males and 129 were females, and the patients were divided into two groups: adjacent (n ¼ 49) and nonadjacent (n ¼ 608) to the gallbladder.The average age was 57.25 ± 11.33 (range 15-91) years.The median follow-up time for all patients was 61.4 months (range: 0.6-184.5 months).As shown in Table 1, significant differences were observed in four factors (all p < 0.05), including AFP (p ¼ 0.031), HB (p ¼ 0.006), PLT (p ¼ 0.025), and red blood cell (RBC) count (p ¼ 0.011), between the two groups.
After PSM, we included 96 patients (76 males and 20 females) who were divided into the adjacent group (n ¼ 48) and nonadjacent group (n ¼ 48), with an average age of 60.43 ± 12.26 years (range 34-91 years).Additionally, the median follow-up time for all patients was 60 (range 6-171) months.After performing PSM, the baseline characteristics were similar between the two groups (Table 1), including age, sex, etiology, number and size of tumors, liver function, and additional laboratory test results (all p > 0.05).

Analysis of risk factors affecting treatment outcome after PSM
In both univariate and multivariate analyses, whether the tumor was adjacent to the gallbladder was not found to be an independent risk factor for PFS (p ¼ 0.963) or OS (p ¼ 0.789).Specifically, in the univariate analysis, the results indicated that both PLT and AST were significant risk factors for PFS (both p < 0.05) and that age, aCCI, HBV, AST, GGT, DBIL, ALB, GLU, RBC, and PLT were significant risk factors for OS (all p < 0.05).Additionally, the multivariate analysis identified AST as a significant risk factor for DFS (p < 0.05), as well as GGT and ALB for OS (both p < 0.05) (Tables 2 and 3).

Discussion
Ultrasound-guided percutaneous thermal ablation is an increasingly popular and effective minimally invasive treatment for liver tumors.Among the available thermal ablation technologies, MWA is particularly popular due to the 'heat sink' effect being less of an issue and a more uniform ablation zone in both shape and size [30,31].However, the guidelines do not explicitly recommend MWA for the treatment of HCCs in high-risk sites due to a lack of sufficient evidence [9,32,33].Previous studies have shown that tumors located at high-risk sites were independent risk factors for LTP and were associated with the prognosis of HCC patients [34,35].Moreover, smooth puncture and thermal ablation of tumors adjacent to critical structures are challenging procedures, as they may result in incomplete necrosis or additional damage to the surrounding organs [36].Therefore, it is crucial to explore the efficacy and safety of MWA for treating  HCC in high-risk locations, such as those adjacent to the gallbladder.Percutaneous ablation for HCCs that are adjacent to the gallbladder is a controversial procedure for the following reasons: (1) there are risks of gallbladder perforation and acute cholecystitis accompanying thermal ablation of tumors adjacent to the gallbladder [37], and (2) to avoid thermal damage to the gallbladder, complete ablation of the tumor and the surrounding 5-10 mm of the hepatic parenchyma are difficult to achieve, which may increase the incidence of LTP [38][39][40].Although a gallbladder injury is rarely life-threatening, it can be somewhat painful for the patient.With the rapid development of ablation techniques, guidance methods, and auxiliary technologies, recent studies have shown that MWA is a safe and effective treatment for HCCs near the gallbladder [13,22].However, these studies were single-center with short followup periods and did not balance the basic clinical characteristics of patients in the adjacent and nonadjacent groups, which made it difficult to draw definitive conclusions.It is vital to further evaluate the efficacy and safety of MWA for the treatment of HCCs adjacent to the gallbladder, particularly in multicenter studies with longer follow-up periods and better-balanced patient selection criteria.
Our study used multicenter data with longer follow-up periods and used PSM balance to reduce selection bias and confounding factors between the two study groups.To the best of our knowledge, our study represents the first multicenter PSM analysis to compare the therapeutic outcomes of MWA as a first-line therapy in patients with small HCCs located adjacent to the gallbladder and those with nonadjacent HCCs.Our research showed that the position of the tumor adjacent to the gallbladder led to the application of adjuvant technology more frequently (p < 0.05), but the position near the gallbladder had no significant effect on the long-term efficacy of MWA in the treatment of HCCs (PFS or OS), the incidence of adverse events, the efficiency of the ablation technique, or postoperative hospital stay (all p > 0.05).Previous studies have documented that the size and location of HCC critically affect the possibility and difficulty of radical cure [7].Therefore, after PSM, a subgroup analysis was carried out according to tumor diameters <3 cm and 3-5 cm.The results showed that there was no significant difference in PFS or OS between the adjacent group and the nonadjacent group (all p > 0.05).
Tumors adjacent to the gallbladder were not found to be an independent risk factor for PFS (p ¼ 0.963) or OS (p ¼ 0.789).Furthermore, there was no significant difference in the TS rate (p ¼ 0.204) between the adjacent group and the nonadjacent group, which may be related to the use of more auxiliary techniques in the adjacent group.The combination of multiple adjuvant techniques, including ethanol injection, hydrodissection, temperature monitoring, and CEUS, offers greater possibilities for the radical treatment of HCCs adjacent to the gallbladder.Ethanol injection was used to chemically ablate tumor edge cells to achieve complete necrosis of the tumor nodule [23].Hydrodissection and realtime temperature monitoring technology can avoid thermal damage to adjacent vital organs during ablation [25,41].
CEUS can show lesions near the gallbladder more clearly than conventional ultrasound.In addition to using assistive technology, it may also be related to the use of straight needle electrodes to improve visibility and control of the tip [42].The two groups had a comparable complication rate, including immediate and delayed complications (p > 0.05).Therefore, our results indicated that MWA could be a firstline alternative for 5 cm HCC adjacent to the gallbladder with the use of advanced assistive technologies.
Despite the major advantages of our study, including the multicenter research design and the use of PSM to balance baseline characteristics, there are several limitations.First, potential selection and indication bias are inevitable due to the retrospective nature of the study design.Moreover, the different guiding methods used, including CT and US from multiple medical centers, may have contributed to the artificial discrepancies in ablation techniques.Finally, pharmacological therapies such as angiotensin receptor blockers may affect patient outcomes after ablative treatment for HCC [43].Therefore, a large prospective multicenter cohort study is needed to further validate these conclusions.

Conclusion
In summary, this 12-year multicenter study provided strong evidence for the efficacy of percutaneous MWA in treating HCCs adjacent to the gallbladder, with the longest follow-up period.With appropriate patient selection, preoperative planning, and assistive technology, there was no significant difference in the long-term effects (PFS and OS) and the incidence of complications of percutaneous MWA as first-line treatment of HCCs adjacent and nonadjacent to the gallbladder.

Table 1 .
Baseline characteristics of patients before and after PSM.

Table 2 .
Univariate and multivariate analyses of PFS after PSM.

Table 3 .
Univariate and multivariate analyses of OS after PSM.

Table 4 .
Perioperative treatment status of patients in the adjacent and nonadjacent groups after PSM.