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The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.Open Access *Correspondence: lamp197311@126.com; xiaoloong.liu@gmail.com †Zhixiong Cai, Xiaoping Su, Liman Qiu and Zhenli Li contributed equally to this work as the joint first authors. 1 The United Innovation of Mengchao Hepatobiliary Technology Key Laboratory of Fujian Province, Mengchao Hepatobiliary Hospital of Fujian Medical University, Xihong Road 312, Fuzhou 350025, Fujian Province, People’s Republic of China Full list of author information is available at the end of the article Page 2 of 13 Cai et al. Molecular Cancer (2021) 20:164 Introduction Hepatocellular carcinoma (HCC) is an increasingly seri - ous global pandemic. Especially in China, where new HCC cases account for 55% of the world’s total cases, approximately 422,100 people die from HCC progression per year [1]. Presently, a large number of HCC patients were diagnosed with different degrees of vascular inva - sion, resulting in low surgical resection rate and poor prognosis. However, previous clinical studies have shown that some HCC patients with vascular invasion in por - tal venous branch can still benefit from surgery treat - ment [2, 3], and such patients could be recommended for surgery treatment following the guidelines for Diag - nosis and Treatment of Primary Liver Cancer in China (2019 Edition) and EASL Clinical Practice Guidelines [4, 5]. Unfortunately, these patients still suffer a high risk of recurrence and metastasis after surgery. At present, pro - phylactic anti-recurrence treatments after surgery are very limited clinically. Transcatheter Arterial Chemoem - bolization (TACE) is the main prophylactic strategy in China, but still lack sufficient supportive evidence while there are no recommended treatments in EASL and NCCN guidelines. Therefore, it is very urgent to develop new and effective strategy for preventing postoperative recurrence. Neoantigens, epitope peptides solely derived from nonsynonymous mutations of malignant tumor cells, could be present to the cell surface with the major his - tocompatibility complex (MHC) on tumor cells and thus could be specific recognized by T cells to elicit strong antitumor immune responses. Comparing with tumor- associated antigen based vaccine, neoantigen based can - cer therapeutic vaccine has been proved as a promising anti-tumor immunotherapy strategy with maximized therapeutic efficacy and minimized risk of autoimmunity, since such neoantigens are only found in tumor tissues [6]. Neoantigens usually originate from somatic DNA point mutations, RNA editing events, insertion-deletion mutations, gene fusions and so on [7]. Recently, numbers of neoantigen vaccine based clinical trials showed that neoantigens derived from somatic point mutations could induce strong antitumor immune responses in patients with melanoma, small cell lung cancer, as well as glio - mas [8–10]. However, whether neoantigen vaccine could induce anti-tumor immune responses in HCC, which tumor mutation burden only ranked in the intermedi - ate position among different types of cancers, with ~ 2.0 mutations/megabase [11, 12], is still in doubt. Previous reports have pointed out that neoantigen burden in HCC patients could reflect tumor evolution during HCC pro - gression and associated with patient’s prognosis, suggest - ing neoantigens might serve as ideal immunotherapeutic targets for HCC immunotherapy [13, 14]. In addition, RNA editing events have been reported with capability to produce neoantigens [15], providing complementary targets for patients with low mutation burden. However, whether it could be used as potential therapeutic targets to induce robust anti-tumor immune responses in HCC patients also needs to be explored. Furthermore, real-time and accurate evaluation of treatment efficacy and prognosis is still a great challenge in HCC surveillance. Due to the limited sensitivity and specificity, imaging (CT/MRI) and serum protein bio - markers fail to acutely provide real-time evaluation of tumor burden, especially in monitoring minimal resid - ual disease (MRD) after radical operation. Our previ - ous study has indicated that circulating tumor DNA (ctDNA), a short DNA fragment derived from tumor cell in plasma, could function as noninvasive and sensi - tive biomarker for monitoring real-time HCC burden by tracking personalized tumor mutations [16]. Jia et al. also indicated that by tracking personalized neoantigen sites in ctDNA could accurately reflect the clinical response during checkpoint blockade immunotherapy in non- small cell lung cancer [17]. Thus, ctDNA might provide a potential strategy to sensitively and accurately assess clinical response during neoantigen based immunother - apy and should be thoroughly investigated. In this prospective clinical study, we enrolled 10 patients with resectable HCC and vascular invasion in portal venous branch, which indicated high risk of recur - rence after radical operation. Following personalized neoantigen identification for each patient, we compre - hensively evaluated whether personalized neoantigen long peptide vaccine could serve as a safe and effective strategy for preventing postoperative recurrence. Fur - thermore, we designed personalized somatic mutation panel, including neoantigen mutation sites, for ctDNA sequencing to monitor the clinical response during neo - antigen based immunotherapy. Methods Clinical trial design and treatment This study was investigator initiated, single-arm, open- label clinical trial at Mengchao Hepatobiliary Hospital of Fujian Medical University in China and was registered at Chinese Clinical Trial Registry (http:// www. chictr.