Clinical features of extrahepatic recurrence after curative hepatectomy for hepatocellular carcinoma: simple parameters predicting extrahepatic recurrence

Background Extrahepatic recurrence (EHR) after curative hepatectomy for hepatocellular carcinoma (HCC) is associated with a poor prognosis. We investigated the features of EHR and identied its predictive factors. Methods This retrospective study included 398 treatment-naive patients who underwent curative hepatectomy for HCC at two tertiary hospitals. Multivariate analysis via Cox-regression was performed to identify the variables associated with EHR. Results EHR was diagnosed in 94 patients (23.6%) over a median follow-up period of 5.92 years, most commonly in the lungs (42.6%). The 5-/10-year cumulative rates of HCC recurrence and EHR were 63.0%/75.6% and 18.1%/35.0%, respectively.. The median time to EHR was 2.06 years. Intrahepatic HCC recurrence was not observed in 38.3% of patients on EHR diagnosis. On multivariate analysis, bile duct invasion, tumor necrosis, sum of tumor size > 7 cm, macrovascular invasion, rst recurrence free survival < 1 year, and serum alpha fetoprotein > 400 IU/mL during recurrence were predictive of EHR. Four risk levels and their respective EHR were dened: very low risk, 2-/5-year, 0.7%/14.2%; low risk, 2-/5-year, 6.4%/31.0%; intermediate risk, 2-/5-year, 21.9%/73.1%; and high risk, 2-/3-year, 70.8%/100.0%. Our predictive model claries the clinical course of EHR and could improve the follow-up strategy to improve outcomes.


Introduction
Despite recent advances in the diagnosis and treatment of hepatocellular carcinoma (HCC), HCC continues to be associated with poor prognosis, ranking third among cancer-related mortality worldwide [1,2]. Curative hepatectomy remains the treatment of choice for HCC, especially in settings where liver transplantation is not feasible [3]. However, the long-term prognosis after curative hepatectomy remains unsatisfactory, with the 5-year rate of HCC recurrence ranging between 60% and 70% [4,5]. Therefore, identifying risk factors of HCC recurrence and standardizing the perioperative management protocol could be important to improve long-term prognosis after curative hepatectomy for HCC.
According to current practice, curative hepatectomy is indicated over other local therapies, such as radiofrequency ablation (RFA), for patients with advanced HCC, having larger size tumors and/or presence of microvascular tumor invasion. The more advanced disease status of patients who undergo curative hepatectomy could explain the comparatively higher risk of HCC recurrence after curative hepatectomy than RFA. Current treatment guidelines recommend surveillance after treatment, with either curative hepatectomy or RFA, including abdominal computed tomography (CT) and measurement of serum alpha fetoprotein (AFP) levels [6,7]. This recommendation, however, does not consider differences in the risk of recurrence between patients treated using curative hepatectomy and those treated with RFA [8]. Moreover, although intrahepatic recurrence (IHR) is the most common type of recurrence, extra-hepatic recurrence (EHR) is possible, with the most common sites of EHR being the lungs, lymph nodes, and bones, which are di cult to evaluate using conventional abdominal CT imaging [9][10][11]. Considering the aggressive nature of metastatic hepatic tumors and the limited treatment options for recurrent HCC, the prognosis for patients with EHR is generally worse than that for those with IHR. Despite the dismal prognosis of EHR, few studies showed improved outcomes with mestastasectomy in selected patients [12,13]. Thus, early identi cation would be important to improve oncological outcomes and survival.
However, at present, there is insu cient data on the clinical course and pathological progression after curative hepatectomy for HCC to identify predictive factors of EHR. Accordingly, our aim in this study was to determine the risk factors of EHR among patients who had undergone curative hepatectomy as the initial treatment for HCC and to use these risk factors to construct a simple parametric model to predict EHR.

Baseline Characteristics of Enrolled Patients
Among 398 patients, EHR was identi ed in 94 (23.6%). The 10-year cumulative rate of HCC recurrence was 75.6%, with a rate of 35.0% for EHR (Fig. 1). Compared to those without EHR, those with EHR were younger and had a higher serum alkaline phosphatase level, a lower serum albumin level, absence of fatty change in the liver, and a more advanced HCC stage (Table 1). Serum AFP level at the time of rst recurrence of HCC after curative hepatectomy was higher and the time interval to the rst recurrence was also signi cantly shorter in the EHR group.

Comparison of Surgical Findings Between Patients With and Without EHR
Microvascular and serosal invasion were more prevalent among patients with EHR than in those without (16.9 versus 29.8% and 2.1 versus 6.5%, respectively) ( Table 2). Moreover, the presence of satellite nodules and tumor necrosis in resected specimens was more prominent in patients with EHR than in those without (13.0 versus 31.2% and 46.9 versus 68.8%, respectively). Comparison of Characteristics Between Patients with Early and Non-early EHR Both the radiologic and pathologic mUICC stages were more advanced in the early EHR than in the nonearly EHR group (Supplementary Table S2). The early EHR group also had a markedly shorter RFS and shorter time interval to EHR development (Fig. 2). Moreover, the proportion of tumors with an mUICC stage ≥ III at the time of rst recurrence was larger in the early EHR than in the non-early EHR group. Of note, in 54.8% of patients in the early EHR group, EHR was the rst presenting recurrence after curative hepatectomy.

Analysis of Factors Associated with EHR
In the multivariate analysis, the following factors were retained as independent predictors of EHR: bile duct invasion identi ed on pathological assessment of resected specimens [odds ratio (  Table 3). Regarding factors associated with early EHR, serum alkaline phosphatase (ALP) > 120 U/L, a pathological mUICC stage III or IVa, surgical margin involvement, absence of fatty change in the liver, and macrovascular invasion were found to be closely associated on multivariate analysis (Table 4).  Recent studies reported improved prognosis for recurrent HCC based on the pattern of IHR [5,18].
However, these studies did not clarify the clinical features and pathological course of EHR, with the absence of models to predict EHR after curative treatment for HCC, limiting the early detection of EHR. In our study, we identi ed the risk factors of EHR after curative hepatectomy for HCC and used these factors to stratify the risk for EHR into four levels. Our ndings highlight the necessity for a predictive score based on risk strati cation to inform optimal surveillance for prompt detection of EHR to improve patient outcomes.
Current results of HCC recurrence and EHR development rates suggest some distinction from our previous study of HCC patients who underwent RFA [19]. In that study, the median time to rst HCC recurrence and EHR after RFA was 1.75 years and 2.68 years, respectively, with 1-, 3-, 5-, 8-, and 10-year rates of EHR development of 1.0%, 2.9%, 8.1%, 15.7%, and 33.7%, respectively. These rates were comparably lower than those we now report after curative hepatectomy.
With regard to the pattern of HCC recurrence, the most common initial site of recurrence was within the intra-hepatic area, which was consistent with a previous report [20]. Another study identi ed IHR as being the most common initial site of recurrence, with EHR developing after several treatments for IHR [21]. Uchino et al. reported that 82.2% of HCC patients with EHR presented with IHR, a nding comparable to those of our previous study [19]. Therefore, multiple IHRs can be an indication of EHR risk in patients with HCC [22].
In our study cohort, the lungs were the most common site of EHR (42.6%). Thoracic metastases, which included the lungs and the vertebrae of the thoracic spine, were thus, relatively common as previously reported [23]. Thoracic metastases re ect a systemic involvement with poor prognosis as they are largely not curable. Of further concern is the fact that thoracic metastases may not be detected using conventional surveillance methods that rely on abdominal imaging. Therefore, the use of chest CT images should be included in the surveillance strategy for patients at risk for EHR after curative hepatectomy for HCC for the early detection of thoracic metastases. In addition, the rate of EHR at the time of the initial recurrence of HCC after hepatectomy was 34%, with 38.3% of these patients having no sign of IHR at the time of EHR diagnosis. Therefore, even if intra-hepatic HCC lesions are stable, close surveillance for possible development of EHR may be necessary.  [27]. Moreover, Shao et al. reported that HCC patients with bile duct invasion developed early recurrence (< 1 year) after resection more frequently (recurrence rate, 70.3%) than those without bile duct invasion [28]. Our ndings were consistent with those of these latter studies, with bile duct invasion being signi cantly associated with EHR after curative hepatectomy.
A recent study revealed that the presence of tumor necrosis is associated with an advanced tumor stage, HCC recurrence, and patient survival after curative hepatectomy for HCC [29]. In agreement with these ndings, we also identi ed necrotic HCC to be associated with a high rate of EHR. HCC tumors > 6 cm in size were also predictive of EHR after curative resection for HCC [30], with the predictive cut-off in our study being close at > 7 cm. Similarly, our nding of a signi cant association between rst recurrence free survival of < 12 months and EHR was consistent with the report by Kim et al. that reported EHR to develop more frequently among patients with early HCC recurrence [31]. They suggested that aggressive tumor pathology was, therefore, a risk factor of early HCC recurrence.
Recent studies have shown high AFP level to be an independent risk factor of HCC invasiveness [32][33][34][35][36]. However, only one previous study has reported an association between AFP level and EHR after curative HCC treatment [37]. In relation to the same, our previous study on RFA in EHR for HCC demonstrated an association between AFP levels and HCC recurrence when the AFP level was > 400 IU/mL. Similarly, this study also identi ed a serum AFP level > 400 IU/mL at the time of rst recurrence to be predictive of high risk (OR: 2.409, 95% CI: 1.421-4.084). We strati ed the risk for EHR into four levels based on the number of predictive factors present for EHR. We demonstrate that the cumulative rates of EHR and the median time to EHR correlated with these four risk levels. Therefore, our novel parametric model, albeit simple, could assist clinicians in identifying patients at high risk for EHR before surgery.
We identi ed involvement of the margin of resection as a signi cant risk factor of early EHR (OR: 4.035, 95% CI: 1.28-12.725), which is consistent with a previous study [38]. The frequency of diffuse steatosis in early HCCs peaks at a diameter of about 1.5 cm, decreasing as a function of increasing tumor size and grade [39]. Thus, diffuse fatty change is uncommon in HCCs > 3 cm and with progressing HCC disease status, and is also not usually observed in patients with poorly differentiated HCC [39,40]. Therefore, the absence of fatty change in the liver with HCC is associated with the tumor aggressiveness. In our study, the proportion of patients with the absence of fatty change in the liver was higher for the early EHR than for the non-early EHR group.
We nally note that 54.8% in the early EHR group exhibited EHR at the time of rst recurrence. This underlines the importance of identifying patients who are at high risk for early EHR, which may inform on the best strategy for surveillance after surgery for the rapid identi cation of EHR development.
The limitations of our study should be noted in the interpretation of results. First, the diagnosis of EHR was mostly based on medical imaging and, thus, the possibility of other primary cancers from an origin other than the liver could not be completely ruled out. However, in circumstances when the origin of the tumor was not certain, pathologic con rmation was performed at the discretion of the treating physician.
Secondly, this was a retrospective study based on medical records from patients enrolled from two tertiary hospitals. Therefore, the effect of bias on results cannot be denied. Third, there was no uniform post-treatment or surveillance schedule, and the surveillance modality used for each patient was at the discretion of the treating physician. Moreover, patients underwent different treatment modalities for local HCC recurrence depending on the tumor and patient status. There may be diverse conditions in terms of tumor stage, liver reserve function, and patients' physical performance status. However, we tried to overcome these limitations by using a considerable number of patients with a long-term follow-up duration in multiple tertiary centers. There is a need to conduct a well-organized prospective study to validate more effective methods of EHR prediction and management.
In conclusion, we present a simple parametric model to predict EHR after curative hepatectomy for HCC.
This tailored approach may be useful for the early detection of EHR and permit a more re ned estimation of risk on an individual basis.

Patients
Between January 2004 and December 2013, 493 consecutive patients who underwent surgical resection for HCC at two tertiary hospitals were evaluated for study enrolment. The inclusion criteria were age ≥ 18 years and a diagnosis of HCC. Patients with hepatic tumors other than HCC were excluded. After screening, 398 patients were enrolled (Supplementary Fig. S1). Baseline clinical and tumor characteristics, resection method, pathological ndings, status of recurrence, and recurrence free survival (RFS) were assessed retrospectively.

Statement of Ethics
The design of our cohort study was approved by the Institutional Review Board of Chonnam National University Hospital (IRB No. CNUH-2019-203). Owing to the retrospective design of our study and the use of de-identi ed data, the requirement for informed consent was waived. The study was performed in compliance with the Helsinki Declaration of 1975.

Data Collection
The following information was extracted from patients' medical records for analysis: age, sex, underlying diseases (including hepatitis infection status and alcohol use), blood chemistry [including the Child-Pugh score and model for end stage liver disease (MELD) score], serum AFP level, pathological ndings (tumor size, histological grade, micro-vascular invasion, and presence of satellite lesions), and abdominal imaging for tumor staging at the time of HCC diagnosis. Data on blood chemistry, serum tumor markers, and abdominal imaging obtained at each follow-up visit were also obtained for analysis. Measured data at the time of rst recurrence were also evaluated.
Baseline HCC Staging and Follow-up HCC was diagnosed according to the guidelines of the Korean Liver Cancer Study Group and the National Cancer Center [14]. HCC staging at the time of diagnosis was determined using the modi ed Union for International Cancer Control (mUICC) staging system [15] and the Barcelona Clinic Liver Cancer (BCLC) classi cation system [16]. Abdominal CT or magnetic resonance (MR) imaging (MRI) and assessment of serological tumor markers were routinely performed at 1 month after surgical resection and at each 3-6month follow-up visit.
Tumor size was measured by radiologic modalities, with CT or MRI. The histological differentiation of HCC was graded according to the criteria of Edmondson and Steiner [17]. The presence of macrovascular invasion was detected on CT imaging or MRI, while microvascular invasion was de ned as invasion of vascular structures on microscopic analysis of resected tumor specimens. Bile duct invasion and tumor necrosis were con rmed from pathological ndings of resected tumor specimens.

Diagnosis of EHR
Most cases of EHR were diagnosed during routine follow-up studies, with few cases diagnosed during evaluation of new onset symptoms or based on signi cant elevation in AFP or serial AFP without de nite intra-hepatic lesions. The diagnosis of EHR was con rmed by contrast-enhanced CT imaging or MRI, as well as by pathological examination in some patients. In addition, bone scintigraphy, positron emission tomography (PET)-CT, and brain MR or CT imaging were also performed at the discretion of the treating physician. Chest radiographs were obtained routinely at the time of admission, as well as when pulmonary symptoms were present. Early EHR was de ned as EHR developing within the rst year after initial curative hepatectomy.

Statistical Analysis
The data were expressed as mean ± standard deviation or median and range, as appropriate for the data type and distribution. Univariate analyses were performed using chi-squared test or student's t-test, as appropriate. Variables with a p-value ≤ 0.05 on univariate analyses were included in a multivariate logistic regression analysis to identify factors predictive of EHR. The multivariable Cox regression model  Serosal invasion, n (%) 6 (2.1) 6 (6.5) 0.034