Cryoablation kills tumor cells with low temperature environment, which changes the osmotic pressure and causes tissue ischemia. The critical temperature for cell death is -20°C. Various factors influenced tumor death rate, such as freezing duration, freeze-thaw cycle and local blood flow changes (16). Cryoablation technology uses an ablation probe to introduce liquid nitrogen or argon into the tumor tissue, forming an “ice ball” near the cryoprobe, with the center temperature reaching -170°C and about 0°C in the periphery of “isotherm”. The high-pressure gas induces a freezing cycle process, in which the temperature rapidly decreases and then increases in the tumor tissue according to the Joule-Thomson principle (17). Due to the rapidly decreased temperature inside and outside cells, ice crystal leads to cell dehydration in adjacent tissues, inducing cell permeability damage and cell membrane irreversible damage, and ultimately tumor necrosis (18). Low temperature also damages the intima of microveins and arterioles in the tumor tissues. The damaged endothelial cells contact platelets to form thrombus after ice crystal thawing, which will further develop ischemia and hypoxia in the tumor tissues (19, 20).

The advantages of cryoablation are summarized as the following aspects: ① Ice crystal can be imaged by computed tomography (CT) to achieve accurate treatment area and intuitive monitoring and evaluation of the therapeutic effects during cryoablation (16). ② Cryoablation effectively reduces the recurrence rate of tumor. In SOLSTICE study, 128 post-cryoablation patients were followed up at the 12th and 24th month respectively. The local recurrence-free survival (RFS) rate after cryoablation was 85.1% and 77.2% after initial treatment. The local RFS rate of the recurrent tumor after the secondary cryoablation was even increased to 91.1% and 84.4% (21). ③Cryoablation shows significant pain alleviation effects. Previous studies indicated that cryoablation decreased the analgesics dosage in the treatment of localized tumor foci (≤4cm), which was better than RFA (22). Matthew R et al. summarized the most severe cancer pain assessment of 69 patients with bone metastasis before and after cryoablation, which found that the average score within 24 hours prior to cryoablation was 7.1/10, with a range of 4/10 to 10/10. The mean scores of 1, 4, 8, and 24 weeks after treatment decreased to 5.1/10 (p<0.001), 4.0/10 (p <0.001), 3.6/10 (p <0.001), and 1.4/10 (p <0.001) respectively. Cryoablation treatment provides sustain relief for bone metastatic pain (23). In a palliative treatment of bronchial cancer, cryoablation also effectively relieved the severe pain and hemoptysis which caused by bronchus blockage, and restored the bronchial ventilation again (24). Cryoablation has been widely used in the treatment of patients with early hepatocellular carcinoma (HCC), non-small cell lung cancer (NSCLC), pleural lesions, palliative treatment of lung metastatic foci, which achieved good clinical outcome (25, 26). A phase I clinical study enrolled 160 patients with NSCLC after cryoablation therapy. The average local tumor progression-free survival (PFS) interval was extended to 69 ± 2 months (27). Another study of cryoablation in recurrent pleural malignancy also showed a 3-year recurrence-free rate of local disease as 73.7% (28).

RFA is a physical ablation that can be performed with percutaneous puncture, laparotomy or laparoscopy (29). The radiofrequency applicator is inserted into a target lesion with imaging guidance, such as ultrasound or computed tomography (CT). The current induces ions agitation at a high frequency to generate frictional heat (90-120 ℃), which causes coagulation necrosis in tumor tissues (30). Meanwhile, RFA prevents blood supply and tumor metastasis with the blood vessels disruption. A large proportion of liver tumors are suitable for RFA treatment rather than surgery resection, because of multiple intrahepatic lesions and other extrahepatic diseases (31). RFA is an optional technique for treating primary HCCs, as well as intrahepatic metastatic foci of colorectal or gastric cancer (32).

RFA exhibits significant advantages of minimal invasion, short recovery period in clinical practice. HCC patients after RFA treatment show low recurrence and mortality rate, which achieves similar curative effect as surgery to some extent (33–36). Previous studies also supported low recurrence rate and only mild adverse reactions, including fever, neutrophils, local pain and minor thermal damage in peripheral organs (37–39).

MWA is another novel tumor ablation technology which developed from biologic thermal effects, which is mainly applied in the treatment of solid tumors in liver (40), lung (41), kidney (42), adrenal gland (43), spleen (44), thyroid (45) and breast (46). The therapeutic effects of MWA depends on the dipolar molecules (primarily water) to align and realign according to the variable electromagnetic field. The frictional heating induces tissue degeneration attributed to highly interdependent antenna–tissue interactions, which is different from RFA. So MWA is suitable for increased maximal local ablation deposition in clinical practice (47).

MWA treatment is safe and effective in local lesion treatment, which showed well tolerance for the patients. The incidence of serious complications after MWA is only 3.65%, mild complications as 8.03%, and the mortality rate is 0% according previous study (48). Both MWA and RFA are widely used in the treatment of liver tumors, while MWA is more appropriate for larger diameter liver malignancy than RFA. RFA treatment causes local tissue dehydration and carbonization to prevent heat conduction to the adjacent tumor tissues, while WMA oscillates water molecules to cause tissue heating, which expands the coagulation zone in tumor tissues (49). Medhat et al. evaluated 26 HCC patients with large lesions (5-7cm in diameter) with MWA treatment and found that 19/26 patients (73.1%) achieved complete ablation, while no ablation-related major complications or death (50).

The tumor-specific immune response caused by cryoablation is commonly known as “cryoimmune response” (52). Released tumor cell contents during ablation treatment stimulate autoimmune response, and eliminate immunosuppression effect to kill tumors. Meanwhile, the antigenicity of tumor tissue necrosis activity also stimulates immune response in vivo to cure distant tumor foci, which commonly known as “distal immune effect”. Previous studies have shown that tumor-specific immune responses contribute to spontaneous regression of distant metastases in patients received cryoablation treatment (53). Zhu et al. reported that cryoablation treatment causes DAMPs release in the center of tumor lesion by ultra-low temperature to induce tissue destruction (54), including DNA, heat shock protein (HSP), tumor antigens, cytokines and inflammatory factors (55). DAMPs phagocytosis activates NF-κB pathway to induce anti-tumor immune response of immature dendritic cells (DCs). Mature DCs translocate to lymph nodes with the help of MHC class I molecules, activating CD8⁺ T cells and costimulatory factor CD80/86 expression by presenting antigen. The immune system finally eliminates residual tumor lesions and distant metastases (56). Kato et al. analyzed the renal tumor samples after cryoablation treatment and found a significant increase in the proportion of T cell β receptor (TCRB) clones (defined as clones with a frequency ≥ 1% of total TCRB reads). The TCRB clones were from 27.3% ± 27.4% to 46.4% ± 31.0% after ablation (P = 0.024), the transcription levels of granase A and CD11c in CD4⁺ T cells were also significantly increased in tumor tissues (57). Other study also supported increased levels of interleukin-1 (IL-1), IL-6, NF-κB and tumor necrosis factor α (TNF-α) after cryoablation (30). Of note, IL-6 shows the most significant increase after cryoablation than RFA and MWA (58). The main reasons are summarized as follows: ① Thermal ablation causes protein denaturation in tumor tissues, and reduces the tumor antigen amounts. Meanwhile, tissue coagulation by thermal ablation reduces the release of intracellular content into circulatory system. ② Cryoablation treatment increases cell membrane permeability while maintains the intact cell structure. ③ Cryoablation increases the release of inflammatory intracellular fragments to activate immune cells, Systemic inflammatory response syndrome (SIRS) maybe observed in some serious cases, whereas rarely observed in thermal ablation technologies (30). Therefore, cryoablation showed a stronger immune response than the thermal ablation.

RFA technology provides a high temperature in local tumor tissue lesions for therapeutic effects. The release of large amounts of DAMPs from necrotic tissue further affect tumor microenvironment, including RNA, DNA, HSP-70, HSP-90, High Mobility Group Protein B1 (HMGB1), C-reactive protein and uric acid, which is similar to cryoablation. Zerbini et al. found that cell fragments released from tumor tissues after RFA contributed to the maturation of antigen-presenting cells (APCs) and DCs in vivo, which further promoted the activation of the immune system (59). Increased plasma levels of IL-1, IL-6, IL-8 and TNF-α were also reported after RFA treatment (60). Dramatic morphological changes were observed in tumor tissues after RFA, which were characterized as four areas: application, central, transition, and reference tissue areas (61). High temperature from RFA induces cell necrosis and increased HSP-70 expression in the transition zone (61, 62). Elevated expression levels of HSP-70 and HSP-90 are also observed in animal tumor models after RFA treatment (63). HSP-70 plays a key role in the activation of innate and adaptive immune cells (64). Ryan Slovak et al. indicated that innate immune response in the early stage after RFA contributed a majority part in RFA related immune response, which was associated with recurrence rate (65). Increased HSP-70 expression in tumor peripheral tissues after RFA attracts NK cells infiltration and activation, constituting the innate immunity response induced by RFA (66).

The immunosuppressive microenvironment within the tumor tissue contributes to the poor prognosis of malignancy. Among them, regulatory T cells (Tregs) induce immunosuppressive effects and mitigate the function of cytotoxic T cells (CTLs) (67). RFA treatment improves the tumor immune microenvironment by increasing the infiltration of CD4⁺ and CD8⁺T cells and reducing Tregs proportion (68, 69). The proportions of Tregs in the tumor microenvironment and blood show significantly decrease in pancreatic ductal adenocarcinoma after RFA (70). RFA treatment attenuates the immunosuppressive effects in tumor microenvironment.

Unlike cryoablation and RFA, no significant difference in HSP-70 expression after MWA treatment (51). However, the phenotype and number of T cells exhibit dramatic changes. Increased IL-12, IL-18 activates Inducible T Cell Co-stimulator (ICOS) pathway after MWA, which synergistically promotes Helper T cells (Th) differentiation into Th1 and increases IL-2 secretion, thus inducing the expansion of cytotoxic T cells (CTLs) related immune effect (71). Previous study showed increased percentage of T cells in peripheral blood at 1 week after MWA compared with surgery group (3.37 ± 30.31% vs. 19.42 ± 31.82%, p=0.033). All the studies support that WMA enhances the innate immune response (71).

The study of Zhang et al. observed decreased IL-4 and IL-10 levels after MWA, with IL-4 decreased from 0.13 ± 0.02 to 0.07 ± 0.01 pg/mL (p <0.05), and IL-10 decreased from 9.17 ± 2.97 to 3.93 ± 1.13 pg/mL (P <0.05) (72). So MWA inhibits Th2 cytokines secretion and reduces immunosuppressive factors in tumor microenvironment. Furthermore, ICOS expression on cell membrane after T cell activation also promotes antigen presentation of CD4 + T cells. Activated ICOS⁺ CD4⁺ T cells produce higher levels of interferon-γ (IFN-γ) (73). IFN-γ and other inflammatory factors contributes to the formation of Th1 type environment around tumor tissue, leading to immune response to eliminate tumor cells (74).

Immune checkpoint inhibitors (ICIs) achieves indelible mark in the field of tumor therapy, which exhibits remarkable safety and effects in clinical practice (75). The combination of ICIs and ablation aims to increase and prolong the antitumor immune effects. Present clinical studies usually selected the combination of anti-PD-1/PD-L1 or CTLA-4 mAb with ablation, which showed feasible safety and efficacy (76). McArthur et al. evaluated the safety of combinate treatment of cryoablation and Ipilimumab (CTLA-4 inhibitor) in 19 breast cancer patients. Only 1 patient suffered grade III toxicity symptoms (maculopapular rash unrelated to Ipilimumab treatment) (77). The combined treatment of cryoablation and Ipilimumab induced higher proportion of Ki67⁺ T cells and ICOS^hi T cells in peripheral blood than those only with cryoablation (p=0.05; p=0.005). The results suggest that combination therapy may bring on auto immune reaction and synergistic antitumor effect. A cervical cancer patient received Pembrolizumab after cryoablation, which showed a significant reduction of metastatic lesions (from 7.2 cm×6.8cm to 2.9 cm×1.5 cm) after two months treatment. Then the lesion was undetectable 3 months later and maintained complete response (CR) for 7 months (78). A retrospective study of cryotherapy combined with Pembrolizumab for liver metastatic melanoma, achieves superior efficacy to cryotherapy or immunotherapy alone. Cryotherapy improves the efficacy of anti-PD-1/PD-L1 mab in the 15 enrolled metastatic melanoma patients, with 1 patient (6.7%) achieved a complete response and 3 patients (20.0%) achieved a partial response. The objective response rate (ORR) of the combination regimen was 26.7% (95%CI 4.3-49.0%) and median PFS time was 4.0 months (95%CI 2.5-5.5) (79), which were significantly higher than only Pembrolizumab treatment (2.7 months) as Tumeh et al. reported (80). Moreover, grade 3-4 adverse events (AEs) were not observed in the combine treatment (79), while grade 3-5 AEs attributed to only Pembrolizumab treatment every two or three weeks occurred in 13.3%, 10.1% of advanced melanoma patients according to Caroline Robert et al. ‘s report (81). Cryoablation combined with Pembrolizumab is safe and effective than immunotherapy only.

DCs connects innate and acquired immune responses in tumor therapy. Toll-like receptor agonists activate and maturate DCs to uptake and present specific tumor antigens in microenvironment, which enabling acquired immune response (82). So combinate Toll-like receptor agonists with ablation technology means a promising strategy for enhancing anti-tumor immune response. Gaitanis et al. studied the combination treatment of cryoablation with Imiquimod (Toll-like receptor 7 agonist) for basal cell carcinoma, and only 1 patient (n = 21) suffered relapse after at least 18 months of follow-up with a cumulative efficacy of 95% (83).

Adoptive cell immunotherapy developed rapidly these years. The combination of tumor ablation and immune cell therapy is worth expecting. The metastatic pancreatic cancer patients treated with the combination of cryoablation and DCs immunotherapy showed significantly higher median survival time than those with cryoablation alone (13 vs. 7 months, p<0.001) ( 84). Lin et al. evaluated the clinical efficacy of cryoablation combined with allogeneic NK cell immunotherapy in 60 renal cell carcinoma patients. The combination therapy group showed significantly lower CT values of tumor lesion and higher response rate than cryoablation alone group at 3 months post-treatment (85).

Preclinical studies indicated that RFA combined with ICIs significantly reduced the tumor burthen and prolong survival time in mice models (86). Further investigation also explored their clinical application. Duffy et al. evaluated Tremelimumab (CTLA-4 inhibitor) in combination with RFA in the patients with advanced hepatocellular carcinoma (87). Definite partial remission was achieved in 5 of 19 patients (26.3%; 95%CI:9.1-51.2%), and favorable PFS and OS estimation. The median viral load was decreased from 1275 x10³ IU/ml to 351 x10³ IU/ml in 14 patients with quantifiable data. Besides, a metastatic squamous cell lung cancer patient received Atezolizumab (PD-L1 inhibitor) treatment following RFA in left lower lung lesions. The left lung lesion showed a definite therapeutic response, while the right upper lung lesions without RFA treatment showed poor response to Atezolizumab (88). Thus, combined RFA therapy with ICIs maybe an optional regimen when the patient showed poor response to immunotherapy.

The combination of RFA and adoptive cellular immunotherapy also achieved remarkable clinical efficacy. RFA in combination with cell immunotherapy decreased the recurrence risk and disease progression estimation, as compared with RFA alone in HCC patients (recurrence rate: 72% vs. 33%). Notably, 2-year OS rates in the combination treatment and RFA alone group were 100% and 76.6%, respectively (p<0.05) (89). Another clinical study also found that a combination of RFA and RetroNectin activated killer (RAK) cells effectively reduced recurrent the risk of HCC, and no patients suffered recurrence at 7 months follow-up in the 7 HCC patients. The proportion of CD3⁺/CD8⁺ cells showed a continuous increase after combinate RFA and RAK treatment (36.08 ± 9.44% to 48.75 ± 9.44%, p=0.03), while the ratio of CD4⁺/CD8⁺ decreased (1.14 ± 0.42% to 0.65 ± 0.26%, p=0.02) ( 90). Increased IFN-γ concentration in peripheral blood was observed in 5 patients (4.73 ± 2.50 pg/ml~7.64 ± 1.10 pg/mL, p=0.02). Zhao et al. followed-up HCC patients who received CIK combined with RFA after transcatheter arterial chemoembolization (TACE). The combination treatment group showed lower recurrence rate than control group (12% vs. 25.8%). Meanwhile, most patients showed decreased HBV DNA content after combined treatment, whereas only one patient decreased from 1.1×10⁵ to less than 10³ in the control group (91). Thus, RFA combined with CIK therapy or TACE provides an option to reduce recurrence rate of HCC and anti-HBV effect.

ICIs are still the most common option for combination treatment with MWA. A report evaluated MWA combined with Tremelimumab (CTLA-4 inhibitor) treatment in 16 refractory biliary tract cancer patients. Two patients achieved partial remission (12.5%), 5 stable disease (31.3%), and a maximum duration of 6.2 months (92). Notably, the median PFS was 3.4 months in this study, which was higher than second-line chemotherapy as previous report (2.8 months) (93). Another study of Camrelizumab (PD-1 inhibitor) with MWA for non-small cell lung cancer also achieved an ORR of 33.3% and a DCR of 61.9% (94).

The combination of adoptive cell therapy with MWA also demonstrates efficacy and safety in tumor treatment. A phase I clinical study evaluated the safety of percutaneous MWA combined with adoptive cell immunotherapy (DCs and CTLs) in HCC patients. CIK injection was performed at 5 days after MWA, DCs and CTL at 11 days after MWA. No class III/IV serious AEs was observed during the study. Decreased percentage of CD4⁺CD25^high Treg cells was observed after 1 month of combination therapy (2.40 ± 0.61 to 1.67 ± 0.48, p<0.05) and CD8⁺CD28^- effector cells increased significantly (11.41 ± 6.63 to 15.13 ± 7.16, p<0.05) ( 95).

Staphylococcal enterotoxin C (SEC) exhibits superantigen activity to stimulate the proliferation of peripheral blood monocytes (PBMCs) and release of cytokines, such as IL-2, IFN-γ and TNF-α in a dose-dependent manner (96). Previous study indicated that SEC1 promoted the differentiation of CD4⁺ and CD8⁺ T cells to inhibit tumor growth in mice models (97). So combinate SEC with ablation is speculated to enhance antitumor effects. A phase II clinical trial evaluated MWA combined SEC intratumoral injection treatment in HCC. Totally 2000 U SEC was administrated in the edge of the tumor area after different days after MWA (24 days, 28 days, 2 months, 5 months and 7 months, respectively). Compared with the MWA treatment alone group, combined treatment group showed prolonged OS estimation (p=0.032), while no serious AEs associated with SEC injection (98).

No unified evaluation system was established to access for the safety and efficacy of different combination strategies, in consideration of different combinations of ablation and immunotherapies. For example, RFA is considered as an effective regimen for HCC lesion less than 3cm in diameter (99). Then various combinations of RFA with anti-CTLA-4, anti-PD-1 mAb agents and DC-cytokine-induced killer (CIK) further improved a variety of survival estimation related parameters. However, we cannot provide evaluation criteria to measure the efficacy of different combination, which limits further exploration of their clinical application value.

National Health and Family Planning Commission Expert Group on tumor ablation technology management of China established an evaluation criterion in 2017, “National restricted medical technology–Expert interpretation: Standardized management and clinical application’s quality control index of tumor ablation technology” ( 100), which was updated in 2021 and 2022. The evaluation criteria included a variety of operational accuracy and security index and related calculation formula, such as local lesion control after tumor ablation, serious complication rate within 30 days after tumor ablation. However, no evaluation item was provided to calculate tumor immunotherapy-related indicators, even which kinds of immune responses should be included in the efficacy evaluation system. Moreover, a reasonable evaluation system for immunotherapy efficacy should include reasonable evaluation time-point, and the correlation between immune response and clinical efficacy. Ablation-based combine immunotherapy is deficient in evaluation at present, because of a late start, few clinical studies and a lack of corresponding evaluation parameters. It is suggested to add corresponding immunotherapy evaluation parameters on the basis of the evaluation criteria in tumor ablation technology quality control. Perhaps including anti-tumor immune response indicators as other combination therapies into the efficacy evaluation system. In addition, it should be reminded that the safety assessment of ablation combination therapy is also necessary for clinical application, in view of the novel therapeutic effects and adverse reactions in clinical trials. Moreover, standardized technical and management training for the medical operators will be of great value to the combination therapy evaluation.

Personalized treatment plan for cancer patient should not be ignored in the research of ablation combinate therapy. The ablation efficiency is affected by tumor size, location, ablation equipment, ablation duration and frequency in clinical practice, while immunotherapy efficiency is also affected by various factors, such as different forms and dosages in administration. Therefore, the influencing factors for ablation combine therapy are more complex, including the optimal timing of immunotherapy, the choice of adjuvant immunotherapy, and application order of ablation and immunotherapy (101). Previous study indicated that ICIs treatment after MWA, PBMCs showed higher T-cell activity than that of pre-MWA treatment (102). Besides, Imiquimod treatment after cryoablation resulted in better tumor clearance (10/10 vs 3/7 tumors; p=0.0147) and overall treatment efficacy (9/10 vs 2/7 relapse-free tumors; p=0.0345) than immunotherapy before cryoablation in basal cell carcinoma according to Georgios Gaitanis et al. ( 103). The advanced HCC patients received intravenous NK cell therapy after cryoablation also achieved therapeutic effects (104). These results support immunotherapy after cryoablation exhibits superiority in eliminating tumors and reducing recurrence. However, more clinical trials are needed to explore the specific time and regimen of immunotherapy administration. It is necessary to continuously optimize and explore combined treatment strategies and develop accurate treatment plans.

Tumor tissue destruction caused by cryoablation depends on longer freezing time and slow thawing (105), with a maximum of 15 minutes for freezing and five minutes for thawing each cycle (106). The triple freeze-thaw protocol shows remarkable advantage in time consuming and tumor ablation zones than double freeze-thaw protocol in pulmonary ablation (107). In a study of cryoablation combinate with DC-CIK, the patients with recurrent or advanced HCC received repeated cryoablation showed longer median OS than those received single treatment (106). Treatment frequency is suggested to be an important factor for combination therapy efficiency.

Researchers also attempted to add chemotherapy and targeted therapy to the ablation combine therapy for further efficacy. Aquaporin (AQP) mediates adaptation effects associated with freezing injury, which was supported by increased AQP3 expression in tumor cells at 2 hours after freezing (108). AQP3 participates in the plasma membrane permeability and intracellular water retention, which increases the critical freezing temperature to maintain cell survival advantage with cryoablation treatment. AQP inhibitors (mercuric chloride) combinate with cryoablation significantly reverses resistance and improve treatment effects (109, 110), which supported AQP inhibitors in combination with cryoablation-immunotherapy in clinical practice. Combined cryoablation with Toripalimab (anti-PD-1) and Lenvatinib (anti-angiogenic agent) in a metastatic HCC patient also achieved CR at 7 months after treatment, and PFS as 24 months (111). Compared with cryoablation-chemotherapy and cryoablation-DC-CIK cell therapy, combinate treatment of cryoablation-immunotherapy-chemotherapy showed the longest median OS estimation in metastatic NSCLC (27 months, 95%CI, 26.6-37.0, P <0.001) (112). The combination of cryoablation, NK cell therapy and Herceptin significantly prolonged the PFS of patients with recurrent HER-2 positive breast cancer (113). Moreover, most reports of ablation combine with immunotherapy focus on cryoablation, while few studies choose MWA and RFA. Further clinical trials of MWA and RFA combination therapy are still needed to develop more accurate and efficient treatment plan.

Ablation related complications and side effects still bring troubles to clinical application despite of its relatively mild and low incidence rate. Among them, cryoshock (114), immunosuppression and thrombocytopenia (115) are observed in cryoablation. Cryoshock is a cytokine release syndrome with a mortality rate of 40% (114), which was corelated with dysfunction of suppressor T cells (116). Cryoshock usually occurs in the patients received cryoablation with a large tumor volume or simultaneous treatment in multiple areas (117). However, the mechanism of cryoablation induced immunosuppressive response is still unclear. Some researchers suggest its relation with cytokines release of TNF-α, IL-1 and IL-6, as well as regulatory T cell proliferation (118). Thrombocytopenia is associated with double cryo-cycle therapy (119). Otto Kollmar et al. demonstrated that aprotinin prevented platelet capture induced by cryoablation, prolonged platelet survival time (3.3 ± 0.4 vs 2.4 ± 0.2 days), and significantly reduced platelet aggregation (local platelet activity: 14.0 ± 1.7% vs 1.9 ± 1.9%; P < 0.001) (120). Moreover, cryoablation may cause skin necrosis, fracture and other complications in the treatment of bone tumors (121). Clark Chen et al. irrigated the adjacent tissues of the lesion with warm saline and filled the defect after resection with bone graft substitute, which could reduce the complication rate from 25% to 2.34% (122). Therefore, improving the accuracy of cryoablation, reducing the range of cryoablation, selecting appropriate ablation points, adjusting the parameter design of freeze-thaw cycle and combining other adjuvant therapies may be effective strategies to reduce adverse reactions of ablation and improve the safety of combined immunotherapy.

Excessive RFA and ICIs are associated with impaired liver function and immune-related adverse events (IrAEs) in liver, which will cause a variety of adverse reactions, such as fever and fatigue (123, 124). High dose corticosteroids are used to alleviate the adverse reactions in clinical practice (125), which will also decrease immune response to influence the curative effect. It is suggested that short-term corticosteroid therapy may reduce the associated adverse effects during ablation combined with immunotherapy. Inadequate RFA (iRFA) is an inevitable defect in RFA treatment alone group, which will facilize tumor proliferation, migration, invasion, epithelial-mesenchymal transformation and angiogenesis by affecting the transcriptional and epigenetic regulation of residual tumor cells, ultimately leading to tumor recurrence and metastasis (126, 127). It is speculated that iRFA may be an important factor in affecting the efficacy of RFA combined immunotherapy. Previous studies have shown that XL888 (HSP90 inhibitor) combined with RFA significantly reduces STAT3 expression and phosphorylation in tumor cells, promotes tumor cell apoptosis, and ameliorates adverse effects of iRFA effectively (128). In addition, RFA may induce arterioportal fistula in the treatment of HCC cases, which limited the therapeutic outcome (129). Xu et al. embolized the fistula between the right hepatic artery and the right portal vein with transcatheter arterial embolization (TAE) in HCC patients. AFP (alpha fetoprotein) remained at normal levels within 3 months and intrahepatic bleeding area was reduced (130), which supported that TAE can be used as a complementary therapy in ablation combine strategies.

Previous studies have shown that thermal ablation induced DAMPs release activates tumor proliferation and development related signaling pathways in the periphery of ablation site (131, 132), which will increase the recurrence risk. Erik Velez et al. selected IL-6, vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF) to assess the recurrence risk after ablation. VEGF levels were significantly reduced in 20-W MWA (5-W MWA: 5952 ± 1068 pg/mL, 20-W MWA: 3915 ± 881 pg/mL, p<0.05) ( 133). Therefore, high power and short term MWA treatment will contribute to a reduced recurrence risk. Optimizing the parameters of MWA will be a new way to reduce the occurrence of adverse reactions in ablation combination therapy.