EXPERIMENTAL AND CLINICAL TRANSPLANTATION

Hepatocellular carcinoma is the most common primary liver malignancy. Liver transplantation has been a successful therapy for selected patients with hepatocellular carcinoma. Since 1996, Milan criteria have been universally recognized as the guidelines for selecting patients with hepatocellular carcinoma for orthotopic liver transplantation. However, the simple use of tumor size and number has been insufficient to indicate the biologic features of hepatocellular carcinoma and to predict the risk of tumor recurrence. The Milan criteria are quite strict because their rules can cause patients to be excluded from wait lists who could in fact benefit from transplant. Therefore, many expanded criteria are now incorporating different biologic markers, such as alpha-fetoprotein. 18F-fluorodeoxyglucose positron emission tomography-computed tomography could be a helpful diagnostic tool to decide the most suitable therapy, particularly for patients with hepatocellular carcinoma beyond the Milan criteria. Patients initially beyond Milan criteria can be downstaged to reduce tumor size to fulfill Milan criteria. Locoregional therapies are used for downstaging, including transarterial chemoembolization, radiofrequency ablation, and percutaneous ethanol injection. Good responses to downstaging therapy and then waiting at least 3 months after locoregional therapy to reevaluate the decision of liver transplantation for patients with hepatocellular carcinoma beyond Milan criteria can result in better survival rates in these patients. Sorafenib and mammalian target of rapamycin inhibitors are promising agents for reducing tumor recurrence rate after liver transplantation. With cautious patient selection criteria and the use of locoregional therapy before liver transplantation, good results can be obtained for patients beyond Milan criteria who had no better chance other than liver transplant.

Key words : Downstaging, Expanded criteria, Locoregional therapy

Hepatocellular carcinoma (HCC) is the most common primary liver malignancy, the sixth most common cancer, and the second cause of cancer-related deaths all over the world.^1,2 Early results after liver transplantation for HCC had been dismal, with high recurrence rates and poor outcomes.³ After Milan criteria (single HCC ≤ 5 cm or up to 3 HCCs ≤ 3 cm, without vascular invasion) had demonstrated good results with liver transplantation (LT) in patients with HCC, these criteria were universally recognized as the guideline to list patients for orthotopic LT since 1996.⁴ The use of Milan criteria worldwide has resulted in better outcomes in LT for HCC. The overall predicted survival after LT for patients with HCC meeting Milan criteria is nearly 75% at 5 years. However, the limited number of donor livers usually prevents extension of the oncologic criteria beyond the Milan criteria for LT, since extending these criteria is usually associated with an increased risk of tumor recurrence and shortened survival.^1,5 On the other hand, the simple use of tumor size and number has been insufficient to indicate the biologic features of HCC and to predict the risk of tumor recurrence. In addition, Milan criteria for LT are thought to be too strict by many investigators, particularly during the last 2 decades, and they prevent many patients from receiving LT, a life-saving treatment.

Expanded criteria for liver transplantation for hepatocellular carcinoma
There are several expanded criteria for HCC beyond the Milan criteria. One of the most popular and best known is the University of California, San Francisco (UCSF) criteria.⁶ In addition, recently, an “up to 7 criteria” was proposed by Mazzaferro and associates in 2009.⁷

Marsh and colleagues revealed that vascular invasion, tumor size, lobar distribution, and lymph node status but not tumor number were independent predictors of tumor-free survival, and these factors affect the prognosis of LT recipients with HCC. However, lymph node metastasis and tumor vascular invasion are difficult to diagnose before surgery. Therefore, this modified tumor-node-metastasis staging system, known as the Pittsburg criteria, has been less used because of this limitation.⁸

Surgeons from the University of California proposed expanded criteria (which included solitary tumor ≤ 6.5 cm or ≤ 3 nodules with the largest lesion ≤ 4.5 cm and a total tumor diameter ≤ 8 cm) for listing patients. The original UCSF criteria were developed based on explant histopathologic analyses; subsequently, these criteria are validated with the use of pre-LT imaging. Five-year survival rates of patients within the Milan versus UCSF criteria were comparable (79% vs 64%).⁶

Herrero and associates revealed that the Navarro criteria (a single lesion ≤ 6 cm or 2-3 lesions ≤ 5 cm each) as expanded criteria had good results after LT. After LT, 5-year overall survival and recurrence-free survival rates were 79% and 70%.⁹

Another highly expanded staging system (no invasion of major vascular structure and no evidence of extrahepatic disease) independent of tumor size and number of tumor nodules was suggested by Haberal and associates.¹⁰ In this center-based system, the overall 5-year and 10-year survival rates of patients after LT for HCC were 50.3 % and 43.1% versus rates of 78.4% and 72.6% within Milan criteria.¹¹ As expected, outcomes were significantly better in the Milan criteria group. However, these rates were promising in patients beyond Milan criteria who otherwise had no better chance of survival other than LT.^10,11

Sugawara and associates suggested a selection rule (called the 5 to 5 rule) for potential candidates with HCC for living-donor LT. Criteria for the 5 to 5 rule included having 5 nodules with a maximum diameter of 5 cm.¹² On the basis of results from 78 patients who had living-donor LT, recurrence-free survival at 3 years for patients who met the 5 to 5 rule criteria was 94% versus 50% for patients outside of this rule.

The up-to-7 staging system (the sum of the tumor number and size of the largest tumor [in cm] to not greater than 7), proposed by Mazzaferro and associates, was obtained from analyses of 1556 patients undergoing deceased-donor or living-donor LT (121 patients) for HCC from 36 transplant centers.⁷ The patients without microvascular invasion in the up-to-7 criteria achieved a 5-year overall survival of 71.2%, which is comparable to Milan criteria. The difficulty of this system is the impossibility to verify microvascular invasion before LT.

Biomarkers used to predict hepatocellular carcinoma recurrence and survival after liver transplant
Large or multiple HCCs are not always associated with poor prognosis, as tumor size and number are not completely able to predict vascular invasion and biologic behavior of the tumor. Although some studies have added the histopathologic characteristics of the tumor when evaluating the risk of tumor recurrence, the histopathologic results are usually difficult to obtain before LT.^13,14 Therefore, tumor markers are used for pretransplantation decisions regarding tumor biology. Alpha-fetoprotein (AFP) is the most commonly used prognostic marker for invasion and treatment decisions for patients with HCC. Alpha-fetoprotein and des-γ-carboxy prothrombin (DCP) both have established correlations with posttreatment prognoses.^1,13,14

Toso and associates proposed that total tumor volume of ≤ 115 cm³ and preoperative AFP levels of ≤ 400 ng/mL could independently predict patient survival, as demonstrated in 6478 recipients from the Scientific Registry of Transplant Recipients in 2009.¹⁵ The group demonstrated acceptable and similar recurrence rates (4.5% vs 9.4%; P = .138) and posttransplantation survival rates (78.7% vs 74.6% at 4 years; P = .932) in 134 patients within Milan criteria versus 32 patients within the total tumor volume-AFP criteria.¹⁶ A pre-LT AFP level of > 1000 ng/mL has been demonstrated as a significant predictor of HCC recurrence after LT and associated with poorer prognosis.¹⁷

The Hangzhou system (absence of macrovascular invasion and total tumor diameter of ≤ 8 cm) is the most popular extended criteria for LT in patients with HCC that includes AFP levels. When the tumor is larger than 8 cm, tumor biopsy should be performed; biopsy results should show nonpoorly differentiated HCC and AFP level should be ≤ 400 ng/mL. In a study of 195 patients, those who met Hangzhou criteria had 5-year survival of 70.7%. In a study of 6012 patients from China with HCC, survival rates after LT in those beyond Milan but fulfilling Hangzhou criteria were 89.5% at 1 year, 70.8% at 3 years, and 62.4% at 5 years.¹⁸ Transplantable patients were expanded by 16.3% for UCSF, 19.6% for Navarro, and 51.5% for Hangzhou criteria.¹⁹

Another important biologic marker for HCC is DCP, also known as protein induced by vitamin K absence or antagonist II. Correlations between DCP levels and HCC microvascular invasion and metastasis have been reported in some studies.^20,21 Another study involving 653 patients from 49 centers revealed that recipients beyond Milan criteria but with serum AFP levels of ≤ 200 ng/mL and serum DCP levels of ≤ 100 mAU/mL had 5-year disease-free survival rate of 84.3%.²² A new scoring system (AFP of ≤ 250 ng/mL, DCP of ≤ 450 mAU/mL, and the Tokyo criteria) for HCC patients with living-donor LT was suggested. The patients who have all or 2 of the 3 factors of this scoring system had better 5-year disease-free survival rates than patients fulfilling only 1 or none of the 3 factors.²³

Neutrophil-to-lymphocyte ratio and serumC-reactive protein are systemic inflammation markers; both have been reported to be associated with the prognosis of HCC.¹ A study of 130 patients with HCC showed that elevated preoperative serum gamma-glutamyltransferase (γ-GGT) was significantly associated with high AFP, large tumor size, and macro- and microvascular invasion. In that study, Fu and colleagues found the optimal cut-off value of γ-GGT to be 128 U/L. The 1-, 3-, and 5-year disease-free survival rates and overall survival rates of HCC patients in the γ-GGT > 128 U/L group were poorer than rates of patients in the γ-GGT ≤ 128 U/L group.²⁴

Some studies have demonstrated that 18F-fluorodeoxyglucose positron emission tomography-computed tomography (18F-FDG-PET/CT) findings are powerful prognostic markers in patients with HCC after LT. Results from this test have shown good correlation with pathologic tumor characteristics, such as microvascular invasion and differentiation. Positron emission tomography imaging could be a good preoperative tool for estimating the post-LT risk of tumor recurrence because histologic grade and vascular invasion cannot be determined preoperatively.^25,26

Lee and associates demonstrated the clinical impact of 18F-FDG PET/CT in 280 patients who underwent living-donor LT at the National Cancer Center. Of 280 patients, 147 (52.5%) had HCC staged beyond the Milan criteria on the basis of pathologic reports. Patients with HCC beyond the Milan criteria with a PET/CT-negative status and total tumor size < 10 cm showed similar overall and disease-free survival compared with those with HCC within the Milan criteria.²⁷ In a recent study, the combination of AFP and 18F-FDG PET/CT showed a better ability to predict prognosis than Milan criteria in recipients of living-donor LT.²⁸ Low sensitivity in detecting HCC and high cost may be major obstacles to using PET/CT routinely in the patient selection process for LT. The expanded staging systems beyond Milan criteria are summarized in Table 1.

Downstaging of hepatocellular carcinoma
Patients with HCC beyond Milan criteria can be downstaged by locoregional therapy (LRT). Loco-regional therapy serves 2 objectives for HCC candidates of LT. First, it can serve as a bridge to transplant to prevent dropout from a wait list due to tumor progression in patients within Milan criteria. Second, in patients initially beyond Milan criteria, it can serve to reduce tumor size to downstage patients and fulfill Milan criteria. Neoadjuvant therapies such as transarterial chemoembolization, radiofrequency ablation, percutaneous ethanol injection, and transarterial radioembolization are used for these purposes.^1,13,19 The treatment response has been evaluated radiologically by the modified Response Evaluation Criteria in Solid Tumors.²⁹ Tumor response to LRT, observation period after LRT before LT, and HCC biomarkers have been described for selecting the most favorable tumor biology in patients with HCC beyond Milan criteria.^29-31

Yao and associates investigated 118 patients with HCC who were undergoing downstaging to within Milan criteria and United Network for Organ Sharing T2 criteria (1 lesion at 2-5 cm or 2-3 lesions at ≤ 3 cm) before LT since 2002. Results were compared with 488 patients with HCC on LT wait lists meeting T2 criteria at listing during the same period. In the downstaging group, 64 patients (54.2%) had received LT, with 5 (7.5%) developing HCC recurrence. The 5-year survival rate in the intent-to-treat patients in the downstaging group was comparable (56.1%) versus the patient group (63.3%) meeting T2 criteria without downstaging (P = .29). In that study, a low rate of HCC recurrence and excellent posttransplant survival were achieved after successful downstaging of HCC to within T2 criteria, similar to that shown in patients meeting T2 criteria without downstaging.³² Assessment of the response to LRT is useful for selection of patients with favorable tumor characteristics (well differentiated and without vascular invasion). A minimal observation period from successful downstaging to LT is usually required. This observation period is 3 months in that protocol, but the optimal wait time for tumor biology is unknown (“ablate and wait” strategy).³⁰ A baseline AFP level of ≥ 1000 ng/mL was identified to be a predictor of dropout in multivariate competing risk analyses in a downstaging group and control group with T2 HCC.³² A US national conference proposed adding AFP > 1000 ng/mL as an exclusion criterion for LT unless the AFP level decreases to < 500 ng/mL after downstaging treatment.³³

A systematic review of 13 studies (which included 950 patients) revealed that the success rate of downstaging therapy was between 11% and 77%. Therapy with transarterial chemoembolization or transarterial radioembolization had similar results for success of downstaging therapy. The review reported that post-LT recurrence rates could be up to 16%.³⁴

Molecular signatures for hepatocellular carcinoma beyond Milan Criteria
A higher rate of tumor suppressor gene mutation correlated with worse post-LT outcomes. This was associated with tumor vascular invasion or tumor volume.³⁵ MicroRNA effects detected in serum exosomes could be potential biomarkers for HCC,³⁶ with 2 microRNA markers (miR-214 and miR-3187) shown to be important markers for tumor recurrence in 40 patients after LT. Risks of tumor recurrence and death in low- and high-risk groups were detected more accurately when the expression levels of miR-214 and miR-3187 and the Milan criteria were combined.³⁷

Living-donor liver transplant beyond the Milan Criteria
For patients with HCC exceeding the Milan criteria, whether recurrence and survival rates in liver recipients of living donors are different from those of deceased donors remains controversial. Living-donor LT patients usually do not have long wait times before LT, as they are usually transplanted immediately even if the tumor is beyond Milan criteria. Because tumor burden and tumor biology are not taken into account, results could be dismal. Soejima and associates reported that tumor diameter > 5 cm but not the number of tumors was associated with worse prognosis. In a cohort of 60 patients who underwent living-donor LT for HCC, 67% were beyond Milan criteria, with these patients showing 3-year post-LT survival rate of 68.6%.³⁸ Günay and associates reviewed the data for 109 patients with HCC who underwent living-donor LT, and they demonstrated that survival rates were not significantly different between patients with HCC who met and were beyond the Milan or UCSF criteria. In that study, only poorly differentiated tumors were associated with lower survival rate and within Milan or UCSF criteria not found to be a risk factor for HCC recurrence or patient survival.³⁹ Lin and associates reported that living-donor LT for HCC had good results with cautious selection and stepwise approaches. For good results and for a better judgment of transplantation, they found that HCC must be staged with USCF criteria, with high AFP levels as an indication for downstaging therapy and high uptake of FDG PET/CT necessitating liver biopsy.⁴⁰

Posttransplant adjuvant therapy for patients with HCC beyond Milan Criteria
Sorafenib is an oral multikinase inhibitor and has been verified to reduce the risk of recurrence as treatment for recurrent HCC after LT. A study by Huang and associates demonstrated that sorafenib, compared with capecitabine, reduced or delayed tumor recurrence after LT for patients with HCC beyond Milan criteria.⁴¹ In a pilot study from Taiwan, in 17 patients with HCC beyond Milan criteria after orthotopic LT, 5 patients treated with adjuvant sorafenib had better disease-free and overall survival rates than the palliative sorafenib group (6 patients) and control group (6 patients).⁴² Calcineurin inhibitors have also been widely used in LT, but their adverse effects, such as renal dysfunction and dose-dependent posttransplant risk of HCC recurrence, have been important issues for selection of their use.^43,44 The mammalian target of rapamycin inhibitors (mTORi), such as everolimus or sirolimus, are alternative immunosuppressive agents; these agents have antineoplastic effects.^45,46 In a systematic review that evaluated the recurrence of HCC in 3666 LT recipients from 42 studies, patients who received calcineurin inhibitors developed HCC recurrence significantly more frequently than patients who received mTORi agents (13.8% vs 8%; P < .001), although patients treated with calcineurin inhibitors had a better tumor profile than mTORi-treated patients before transplantation.⁴⁷ Recently, it was revealed that sirolimus treatment in LT recipients with HCC did not result in improved long-term recurrence-free survival beyond 5 years; however, yearly analyses had shown improvements in recurrence-free survival and overall survival in patients with sirolimus treatment up to 5 years, especially in low-risk patients.⁴⁸

Milan criteria have been universally recognized as the guideline to list patients for orthotopic LT since 1996. However, the simple use of tumor size and number was insufficient to indicate HCC biologic features and to predict the risk of tumor recurrence. Therefore, many expanded criteria have now incorporated different biologic markers, such as AFP. In addition, 18F-FDG-PET/CT could be a helpful diagnostic tool, particularly for patients with HCC beyond Milan criteria. Patients initially beyond Milan criteria are downstaged to reduce tumor size to fulfill Milan criteria. Responses to downstaging therapy and waiting at least 3 months after LRT to reevaluate the decision of LT for patients with HCC beyond Milan criteria can result in better survival rates. Sorafenib and mTORi have shown some good results in terms of recurrence of tumor after LT. If we can select patients cautiously and use LRT before LT, we can get promising results in such patients beyond Milan criteria who otherwise have no better chance other than LT.

Volume: 15 Issue: 2 March 2017 - Supplement - 2

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