Impact of successful local ablative bridging therapy prior to liver transplantation on long-term survival in patients with hepatocellular carcinoma in cirrhosis

Background It has been shown that local ablative procedures enable downsizing, reduce drop-out from the waiting list and improve prognosis after liver transplantation. It is still unclear whether a response to the local ablative therapy is due to a favorable tumor biology or if a real benefit in tumor stabilization exists, particularly in complete pathological response. Method Data of 163 HCC patients who underwent liver transplantation were extracted from our prospectively maintained registry. We analyzed the tumor load, pre-transplant α-fetoprotein levels, child stage aside the application and success of local ablative therapies as bridging procedures before transplantation. Results 87 patients received multiple and/or combined local therapies. In 20 cases, this resulted in a complete remission of the tumor as observed in the explant histology. The other 76 patients underwent no bridging procedure. The observed 5- and 10-year survival rates for patients with bridging were 67% and 47% and without bridging 56% and 46%, respectively. Tumor-related 10-year survival showed a statistically significant difference between both groups (81% versus 59%). In the multivariate analyses bridging, number of lesions and α-fetoprotein level showed an independent statistically significant influence on tumor-related survival in these patients. Conclusions Successful local ablative therapy before liver transplantation is an independent statistically significant factor in long-term tumor-related survival for patients with HCC in cirrhosis and reduces tumor recurrences.


Introduction
Transarterial chemoembolization (TACE), radio frequency ablation (RFA), radioembolization (RE), percutaneous alcohol injection (PEI), microwave ablation, irreversible electroporation and stereotactic body radiation therapy (SBRT) in combination with thyrosinkinase inhibitor are being employed for local ablative treatment prior to liver transplantation (LT) in selected patients [1][2][3][4][5][6][7]. TACE is the most frequently employed bridging procedure. The role of bridging therapy was first reported by Manjo et al. in a study of the Hospital Paul Brousse, Villejuif, France [8]. The procedures are often combined and/or repeated for increased efficiency [9]. It has been shown that local ablative procedures enable downsizing, reduce drop-out from the waiting list and improve prognosis after liver transplantation [5,[10][11][12][13][14]. A statistically significant improvement of survival has been documented in a multi-center study of the European Liver Transplant Registry (Pommergard 2018). It is still unclear whether a response to the local ablative therapy is due to a favorable tumor biology or if a real benefit in tumor stabilization exists, particularly in complete pathological response.
Here, we present the effect of local ablative bridging procedures on the 10-year recurrence rate and the tumor-related 10-year survival after liver transplantation for HCC in our patient population. To the best of our knowledge, this is the first long-term observation in this field.

Materials and methods
From our prospectively maintained tumor register, we extracted data of HCC patients who underwent liver transplantation between 1996 and 2017. Patients who died within 3 months after LT and patients transplanted for recurrence of HCC after partial hepatectomy were excluded. All patients were followed-up until death or until August, 1st 2018. 163 patients were included in the study.
In cases of sufficient liver function bridging procedures, such as liver resection, local ablative procedures (transarterial chemoembolization (TACE), radio frequency ablation (RFA), Yttrium 90 radio embolization (Y 90 RE), tomotherapy, in combination with systemic therapy with thyrosinkinase inhibitor were employed since 2004. All these interventions were continued for as long as residual tumor was identified and monitored radiologically in 90 days intervals. In cases of residual vital tumor, the procedures were repeated and combined. TACE was the most frequently applied local ablative therapy. In 87 patients (54%), TACE, RFA or Y 90 radioembolisation was performed, some of them repeatedly. 67, 13 and 7 cases, respectively, (77%, 15%, und 8%) were TACE, RFA or Y 90 radioembolisation. 79 patients did not receive any bridging therapy (transplantation before 2004, or functionally unsuitable for local ablative procedures).
133 patients received a deceased donor liver. 30 patients received a split from a living donor (all of them received segments SV-VIII). The waiting time prior to living donation was median 6 months (range 0-20 months) and, thus statistically significantly shorter than prior to deceased donor transplantation (median 9 months, range 0-46). mTOR-based long-term immunosuppression was administered since 2010 in patients who had no contraindications.
Follow-up consultations after LT for HCC are standardized including follow-up for tumor status. As long as laboratory tests including AFP were within normal ranges, a CT scan was performed every 3 months for the first 2 years and then annually. If tumor recurrence was confirmed, therapeutic options were discussed in the interdisciplinary hepatobiliary tumor board. We performed surgical resections with curative intent for intra-and extra-hepatic recurrence, applied local therapy (TACE, Y 90 RE, RFA) in non-resectable intra-hepatic recurrences and radiation for bone metastases. Whenever possible, a systemic therapy with thyrosinkinase inhibitors followed.
We analyzed the morphological data of the tumor load in pre-transplant contrast computed tomography (CT) or magnetic resonance imaging (MRI) scans, α-fetoprotein (AFP) (ng/ml) level, stage of underlying liver disease (Child stage) and use of loco-regional therapy.
All patients gave their consent for clinical registration. We have only used data from the clinical data registry. The study in humans has been carried out with approval of the local ethics committee (Nr. 4337-02/15), in accordance with national law and the Declaration of Helsinki of 1975 (in the current revised form).
The statistical univariate analysis was performed with SPSS Software version 19. Differences in the distribution of variables have been tested with Fisher's exact test or with Chi square test for statistically significant differences. Survival rates were calculated with the Kaplan-Meier procedure and significance testing was performed with the log-rank test. Starting point for survival was date of transplantation. End point for observed and tumor-related survival were death of any cause and death of hepatocellular carcinoma, respectively. Cox regression analysis was used for the multivariate analysis.

Results
Patient age at transplantation was median 61 years (range 38-72 years). Morphological tumor load was "inside Milan" in 70 (43%) patients and "outside Milan" in 93 (57%) patients. Further data on patients and tumor load are being shown in Table 1. Child-and UICC stage were different in patients with and without bridging. Bridging resulted in complete remission (no evidence of vital tumor tissue in the explant specimen) in 20 cases. This was most frequently observed in single lesions (13/42 = 31% vs. 7/45 = 16%) and after RFA (5/10 vs. 15/77 = 20%), respectively.
Median follow-up time after LT was 55 months (range 8-264). By now, 71 patients died, 34 of them of other causes and 37 (22%) of them from HCC recurrence. 5 patients died from malignant second tumor (lung cancer in 3, pancreatic cancer in one and urinary bladder cancer in one), and 29 died from other causes.
Of the 92 living patients, 10 developed a second carcinoma after LT (3 each in the ENT area or on the skin, one each B-cell lymphoma, stomach GIST, small cell renal carcinoma or prostate cancer).
Observed 5-and 10-year survival rates were 62% and 47%, respectively. Observed 5-year survival rate was higher with bridging than without (67% vs. 56%). The difference did not reach statistical significance, though. The respective 10-year survival rates were 47% and 46%, respectively. For tumor-related survival, there was a statistically significant difference in long-term survival (Fig. 1). In addition to bridging, tumor-related survival was statistically significantly related by the level of pre-operative α-fetoprotein levels, Milan and UICC classification (Table 2, Fig. 2).
Sex, patient age, type of bridging procedure, existing portal vein thrombosis and child stage had no statistically significant influence on tumor-related 10-year survival in the univariate analysis. The statistically significant influence of bridging was maintained with stratification according to Milan classification (Fig. 2).
Of the 20 patients with complete pathological response after bridging, none has died from tumor recurrence so far. 12/20 had been histologically confirmed pre-LT, the others were classified as HCC from CT/MRT scans and AFP.
Bridging therapy, level of pre-operative α-fetoprotein and number of lesions showed an independent statistically significant impact on tumor-free survival in the multivariate analysis (Table 3).
Following LT with prior bridging, one patient developed an intra-hepatic recurrence, 16 patients (18%) developed extra-hepatic recurrence. Without prior bridging, intrahepatic recurrence was observed in 12 patients (16%), extra-hepatic recurrence in 17 patients (22%). The rates of Duration of recurrence diagnosis from LT was median 12 months (range 2-62). Where possible, recurrences were treated with curative intent. In cases of intra-hepatic recurrence (IHR), patients survived for median 2 months and in cases of extra-hepatic recurrence (EHR) 18 months. Patients treated with curative intent survived the recurrence for median 38 months, patients treated with palliative intent 6 months.

Discussion
To our best knowledge, this is the first monocentric series that shows a statistically independent influence of bridging therapy on tumor-related 10-year survival after liver transplantation in patients with HCC in cirrhosis. In patients with bridging therapy, the rate of intra-hepatic recurrence decreased from 16 to 1%, but the rate of extra-hepatic recurrence was unchanged. 20 patients had a complete pathological response. None of them died of recurrence as yet. Only transplanted patients were enrolled in the study. The majority of patients which were removed from the transplant waiting list had distant metastases or intra-hepatic tumor progress. Rarely, patients were removed from the list for other reasons, like aggravation of comorbidities, noncompliance or newly diagnosed second primary tumors. If liver function was sufficient, loco-regional ablative therapy was carried out in these patients. Unfortunately, these data are missing due the retrospective data analysis.
EASL/EORTC guidelines and the international consensus conference for liver transplantation for hepatocellular carcinoma recommend bridging therapy if the waiting time exceeds 6 months [15,16]. The American guidelines from the AASLD states bridging therapy as the method to prevent tumor progression and drop out from the waiting list [17].
Studies on bridging therapies are extremely heterogenous. There are no recommendations concerning indications for the different procedures, choice of procedure in remaining vital tumor tissue as well as documentation and quality assurance of local ablative therapies. The choice of procedures often depends on local availability. Thus, comparability and reporting the results of these procedures are extremely difficult. In the literature as well as in our study, TACE was the most frequently employed bridging procedure. Currently, there are only small case series on 90 Y RE as successful bridging therapy prior to transplantation [6,[18][19][20][21]. Prospective randomized studies are missing, thus an evaluation of the 90 Y RE is not possible to date. In selected patients with a maximum of three tumor lesions, each with a maximum diameter of 3 cm, RFA can be performed safely and effectively. Also, the large proportion of complete histological tumor absence in 7/13 patients treated with RFA in our study results from the limited tumor burden.
Primary tumor size and number of lesions vary (Yao und Fidelman 2016;[22]). In our series, transplantation was performed in 93/163 patients with HCC "outside Milan". The efficiency of bridging therapy in large tumors for meeting the Milan criteria was reported [22,23]. In our series, too, the effect of bridging in small and large tumor burden was observed.

Short-term effect of bridging
Numerous studies on bridging prior to LT evaluate the success from imaging studies, employing different standards for remission. Successful radiological criteria of downstaging are tumor shrinkage by 30 or 50% [5,24], meeting the Milan criteria [6,25,26], meeting the Milan criteria with definition of AFP target levels [22,27], meeting the UCSF criteria [23] as well as complete or partial response according to the mRECIST criteria [28]. The pathological report of the explanted liver for evaluation of response behavior  In the literature, information on complete pathological response after bridging therapy ranges from 10 to 54% in different procedures. In our patients, we saw complete pathological response after bridging in 20 of 87 patients (23%) ( Table 4).
The efficiency of local ablative therapy has been documented by various radiological studies [30,31]. Lei et al. reported superior overall survival and tumor-free survival in the group of responders (patients with response (mRECIST) after local ablative therapy with TACE). In particular, the long-term results of patients with HCC outside Milan were after TACE with response better (p < 0.05). In the univariate analysis as well as in the multivariate analysis, complete and partial response after TACE was the best predictor for survival and tumor-free survival [32]. Discrepancies of 25% between radiological and pathological response have been reported [33]. Thus, there is an uncertainty to date.
Other factors, such as waiting time for the donor organ or immunosuppression may influence the recurrence rate after liver transplantation [34][35][36].

Long-term effect of bridging
Few studies address the impact of bridging therapy on longterm survival. Here, too, different target criteria are being used (overall survival, disease specific survival, recurrence rate). The observed 5-year survival in patients with bridging therapy prior to transplantation for HCC in cirrhosis ranges from 55 to 94% in the literature [25,37] Only Lee et al. reported 10-year survival of 42% with bridging therapy [38].
The significant impact of tumor number on long-term survival in our series has also been documented by Pawlik et al. and Llovet et al. [39,40]. Na et al. and Jang et al. reported in agreement with our series a statistically significant impact   (Table 5).

Summary
To reduce prognostically relevant recurrence after LT different options are being investigated. In our series, response behavior to bridging therapy prior to transplantation for HCC in the explanted liver had a positive impact on longterm survival, cumulative tumor recurrence rate and location of tumor recurrence after LT.

Lay summary
We report on a single-center analysis of all local ablative bridging procedures (TACE, RFA, RE, PEI) prior to liver transplantation with 10-year long-term survival.
Acknowledgements Open Access funding provided by Projekt DEAL.

Compliance with ethical standards
Conflict of interest The authors declare that they have no competing interests.

Ethical approval
The study in humans has been carried out with approval of the local ethics committee (Nr. 4337-02/15), in accordance with national law and the Declaration of Helsinki of 1975 (in the current revised form). Informed consent: All patients give their consent for clinical registration. We have only used data from the clinical data registry.
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