**1. Introduction**

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third most common cause of cancer-related mortality [1]. The incidence of HCC has progressively increased during recent decades due to the increased number of patients with liver cirrhosis caused by chronic hepatitis C (HCV), hepatitis B (HBV), and alcohol abuse, as well as patients with metabolic syndrome-induced liver disease [2]. Fewer than half of HCC cases are diagnosed at an early stage, which allows them to receive curative treatments such as surgical resection, locoregional ablative therapy, and liver transplantation (LT) [3]. LT is considered the best treatment option for HCC because removal of the native liver simultaneously eliminates the tumor and the underlying liver disease [4].

In the second half of the 1960s, LT programs were developed in humans with the aim of offering radical treatment of unresectable liver tumors [5]. However, the initial enthusiasm for these experiences quickly dwindled, as it appeared evident that post-transplant survival was unsatisfactory due to the unsustainable tumor recurrence rates in the transplanted liver, which predicted poor survival [6]. These unsatisfactory results stemmed from a lack of precise selection criteria for patients to undergo transplantation, which were based on the tumor type and on the intrahepatic burden of the neoplastic disease [7]. Thus, the dramatic change that allowed improvement of the post-transplant survival in patients with HCC was the introduction of more accurate selection criteria. In 1996, Mazzaferro et al. published for the first time the Milan criteria (MC), which are still largely used as

**Citation:** Toniutto, P.; Fumolo, E.; Fornasiere, E.; Bitetto, D. Liver Transplantation in Patients with Hepatocellular Carcinoma beyond the Milan Criteria: A Comprehensive Review. *J. Clin. Med.* **2021**, *10*, 3932. https://doi.org/10.3390/jcm10173932

Academic Editor: Tatsuo Kanda

Received: 24 July 2021 Accepted: 29 August 2021 Published: 31 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the reference benchmark to select patients with HCC for LT in many countries. The MC were based on HCC morphological characteristics evaluable before LT (up to three HCC nodules the largest < 3 cm in diameter or a single HCC nodule up to 5 cm in diameter), without macrovascular invasion or extrahepatic spread of the tumor [8]. Patients fulfilling MC experienced a 4-year survival rate of 75%. Several studies confirmed these results and demonstrated that the overall post-transplant survival of HCC patients transplanted within MC was not unlike that of patients transplanted for decompensated liver cirrhosis without HCC [9]. However, even in patients transplanted within MC, HCC recurrence was described in 10–16% of cases and was the main cause of death [10,11]. These data showed that despite the careful selection of HCC patients for transplantation, based on tumor morphological criteria, the risk of HCC recurrence remained consistent. This may be explained by the dissemination of circulating cancer cells and micrometastases before or during transplant operation [12]. Thus, the key issue in the success of LT for HCC is to select candidates that present as the least likely to experience tumor recurrence after transplantation and who maintain a comparable post-LT survival expectancy to that of non-HCC recipients [13].

Although the MC are still largely applied to candidate LT patients with HCC, a growing number of studies have shown that acceptable post-LT may be obtained in patients exceeding the MC at baseline [14–17]. This evidence is of paramount importance, suggesting that the strict application of the MC in all LT candidates could take away the possibility of transplantation in some patients who instead would have an important clinical benefit of LT [18]. The reasons why the MC did not allow accurate prediction of the outcome after LT in all patients stem from the fact that they were based exclusively on tumor morphological characteristics, such as the size and number of nodules. To overcome this limitation, several selection criteria that sought to expand the MC were proposed [19]. Some of these criteria were constructed using morphological and biological variables of the tumor obtainable pretransplant (Tables 1 and 2), others on variables obtainable only after transplantation (for example, the full histology of the tumor), and others by combining variables obtainable in both the pre- and post-transplantation periods. More recently, it has been proposed that the response of HCC to locoregional treatment (LRT) before transplantation may represent a surrogate marker of biological aggressiveness of the tumor to be used to improve patient selection for transplantation and overcome the limitations of criteria based exclusively on tumor morphology. This concept shifts the current paradigm to select for LT-only patients fulfilling the MC at baseline and offers the possibility of considering all patients with an HCC outside the MC at baseline that may be downstaged with locoregional treatments to within the MC as potentially suitable for LT (Figure 1).


**Table 1.** The preoperative selection criteria for liver transplantation (LT) in patients with hepatocellular carcinoma (HCC) based on morphological characteristics of the tumor. Only the externally validated selection criteria are reported in the table.

RFS: recurrence-free survival; MVI: macrovascular invasion; EHS: extrahepatic spread; UCSF: University of California, San Francisco.


**Table 2.** The preoperative selection criteria for liver transplantation (LT) in patients with hepatocellular carcinoma (HCC) based on the addition of the biological serum markers and/or histological differentiation grade to the morphological characteristics of the tumor. Only the externally validated selection criteria are reported in the table.

RFS: recurrence-free survival; AFP: alpha-fetoprotein; MVI: macrovascular invasion; DCP: des-gamma-carboxyprothrombin. \* The criteria were evaluated in living donor liver transplantation.

**Figure 1.** Evolving concepts in the selection of patients with HCC for liver transplantation. After the discovery of the Milan criteria in 1996, the subsequently expanded criteria for liver transplantation in patients with HCC were mainly based on the morphological characteristics of the tumor. Starting in 2008, with the Hangzhou criteria, the addition of biological markers to tumor morphologic criteria allowed expansion of the original Milan criteria, maintaining a good clinical outcome of liver transplanted patients with HCC. More recently, the new way to select patients to be transplanted for HCC who are beyond the Milan criteria at presentation has been to qualify those who can be traced back to the Milan criteria after successful downstaging after locoregional and/or systemic treatments. UCSF: University California, San Francisco; TTV: total tumor volume; AFP: alpha-fetoprotein; DS: downstaging; LRT: locoregional treatment.

> It should be considered that the expansion of the transplant criteria for HCC will progressively increase the number of potential candidates on the waiting list. This imposes three main questions when contemplating expansion of the MC for LT in patients with HCC: (a) What upper limit of the tumor burden beyond the MC could be accepted for LT? (b) What could be the minimal acceptable overall survival after LT? (c) How could markers

of tumor biology, in addition to the tumor burden, be incorporated to better select patients for LT?

This essential review aims to present the current data on the pretransplant selection criteria for LT in patients with HCC exceeding the MC at presentation based on morphological and histological characteristics of the tumor and to critically discuss those that have been validated in clinical practice. Moreover, the role of HCC biological markers and the tumor response to downstaging procedures as new tools for selecting patients for LT with a baseline tumor burden outside of the MC will be evaluated.
