Repeat ablation and single-session ablation in patients with multiple colorectal cancer liver metastases after chemotherapy failure.

INTRODUCTION
Colorectal cancer ranks as the third most common malignancy worldwide, with nearly 1.9 million new cases diagnosed each year[1,2]. 20%-35% of patients present with colorectal liver metastases (CRLM) at the time of diagnosis, 50%-70% develop metastases during the course of their disease. The median survival of untreated patients with CRLM is only 6-12 months[3-5]. Surgical resection remains the standard of care for CRLM[4,6]. However, only 20%-30% of patients with liver-limited disease are eligible for surgery[7]. The majority are excluded due to comorbidities, unfavorable tumor distribution, extrahepatic spread, or excessive tumor burden[8,9]. Despite the integration of systemic chemotherapy and targeted therapy into treatment, the primary goal of systemic therapy for patients with unresectable CRLM remains palliative care. Therefore, there is a clinical pursuit for a more aggressive local treatment approach[4,10,11]. Radiofrequency ablation (RFA) destroys tumor tissue by inducing coagulative necrosis through high-frequency alternating current and has become widely applied in oncology[12]. In 2009, the American Society of Clinical Oncology acknowledged that thermal ablation for CRLM is safe and effective in providing local tumor control[13]. Since then, multiple studies have demonstrated that RFA can be a valuable local treatment option, particularly in patients with oligometastatic CRLM[14-17]. Of particular note in a series of studies is a European intergroup randomized phase II study which demonstrated that in patients with unresectable CRLM, combining systemic therapy with local ablation significantly prolongs both progression-free survival (PFS) and overall survival (OS)[4,18]. Based on current expert consensus, RFA is most effective when applied to patients with three or fewer tumors, each measuring ≤ 3 cm[19]. Although the feasibility of RFA for oligometastatic CRLM have been well established, previous studies tend to mix patients with varying tumor burdens, and frequently fail to address patients who experience systemic chemotherapy failure. To address this gap, our study is the first study to exclusively focus on patients with ≥ 2 CRLM after chemotherapy failure. This research aims to assessing long-term survival and factors affecting outcomes of single-session ablation and repeat ablation in patients with multiple CRLM after systemic chemotherapy failure.

MATERIALS AND METHODS

Study population
We retrospectively reviewed all patients who underwent RFA for CRLM at our institution between January 2015 and June 2024. Inclusion criteria: (1) Patients diagnosed with limited multiple CRLM (generally 2-5 lesions); (2) Patients who underwent first-line and at least one second-line systemic therapy regimen (e.g., FOLFOX, FOLFIRI, + bevacizumab or cetuximab), but demonstrated either progressive disease or no objective response (stable disease or partial response not achieved) according to Response Evaluation Criteria in Solid Tumours version 1.1 criteria, indicating inadequate response or resistance to the therapeutic strategy employed; (3) Imaging assessments performed prior to the procedure confirm that the lesions’ characteristics (quantity, distribution, and size) allowed for a 5-10 mm ablation margin; (4) Patients deemed ineligible for surgery after multidisciplinary team or who declined surgery; and (5) Availability of relevant imaging or follow-up data. Exclusion criteria: (1) Patients assessed by surgeons as unsuitable for ablation (due to unfavorable location, quantity or ultrasound invisibility); (2) Initial treatment with other thermal ablation techniques; (3) Cardiopulmonary function, coagulation issues, or other comorbidities; and (4) Patients lost to follow-up.
A total of 80 patients met the criteria, of whom 42 underwent single-session ablation and 38 underwent repeat ablation. Single-session ablation referred to treatment of all target lesions in a single procedure. Repeat ablation was performed for patients with residual tumors after the first session or with recurrent liver metastases during follow-up. Clinical and imaging data were collected for all patients. Extrahepatic metastases were categorized as regional (e.g., peritoneal lymph nodes, peritoneum) or distant (e.g., lung, bone). The flowchart of patients enrolled for multiple CRLM ablation is depicted in Figure 1.

RFA
All patients gave written informed consent prior to RFA. The procedures were conducted under general anesthesia, with evaluations of patient conditions and positioning performed by interventional and anesthesia physicians. Real-time ultrasound-guided RFA was performed using a 3.5 MHz probe (MyLab Twice; Esaote, Italy) and The Cool-tip RFA system (Covidien, Mansfield, MA, United States). The system consisted of a radiofrequency generator with a maximum power of 200 W and a 17 g internal cool straight electrode. For tumors < 3 cm, a single electrode was used. For larger lesions or tumors in challenging locations, multiple electrodes or overlapping ablations were performed. In all cases, a safety margin of 5-10 mm beyond the tumor boundary was targeted (vary depending on lesion location). Contrast-enhanced computed tomography (CT) was performed within 24 hours of the procedure. Intraoperative or immediately postoperative contrast-enhanced ultrasound (CEUS) was conducted to evaluate tumor ablation extent and activity of tumor ablation. All patients were hospitalized for 1-2 days for observation. Representative ablation cases are shown in Figures 2 and 3.

Assessment of treatment efficacy
Technical success was defined as the absence of residual contrast enhancement within the ablation zone and the planned 5-10 mm margin on CEUS and on the confirmatory CT scan performed within 24 hours. In cases of discrepancy, CT findings were considered definitive. Patients subsequently underwent contrast-enhanced CT every 3-4 months, with magnetic resonance imaging added when needed. Based on intrahepatic imaging findings, treatment outcomes were categorized as follows: Local control: No residual enhancement and no new intrahepatic lesions; local tumor progression (LTP): New or recurrent enhancement at or adjacent to the ablation zone; new metastases (NM): New intrahepatic lesions detected on follow-up imaging or confirmed by biopsy. In the repeat-ablation group, treatment efficacy was evaluated from the date of the last ablation. No evidence of disease (NED) refers to the absence of detected tumor lesions within liver during follow-up.

Statistical analysis
The primary study endpoints were OS, defined as the time from RFA to death from any cause or last follow-up, and PFS, defined as the time from RFA to LTP or NM. For patients undergoing repeat ablation, survival time was calculated from the date of their first RFA. Categorical data are expressed as n (%). Continuous variables were presented as median or mean ± SD. Group comparisons were made using Student’s t-test or the Mann-Whitney U test for continuous variables and χ2 or Fisher’s exact test for categorical variables. Survival probabilities were estimated with Kaplan-Meier curves. Univariate Cox regression was applied to assess the association of clinical and tumor characteristics with OS/PFS and cut-off value. Multivariate analysis was included for variables with a P value < 0.1 in the Cox univariate analysis. Variables with P < 0.1 in univariate analysis were included in multivariate Cox regression models. For categorical outcomes with limited sample sizes or unstable hazard ratio (HR) estimates, Firth regression was applied. R software (version 4.0.2) was utilized for statistical analysis.

RESULTS

Basic information
We performed 133 RFA procedures on 233 CRLM, with 230 cases confirmed as technical success. In the single-session ablation cohort, 100 lesions were treated (2-5 per procedure), whereas the repeat-ablation cohort included 133 lesions treated across up to five separate sessions. Lesion size ranged from 0.8 to 6.3 cm, with a median maximum diameter of 1.85 cm. Basic information is presented in Table 1. All patients received first-line systemic chemotherapy at least after primary colorectal lesion resection, with 11 individuals having undergone neoadjuvant chemotherapy. Common first-line systemic chemotherapy included mFOLFOX6, FOLFIRI. Additionally, 8 patients received additional combined immunotherapy regimens after ablation. No peri-procedural mortality was observed. 12 complications occurred across 133 procedures, including 2 cases of small intraperitoneal hematoma with no sign of active bleeding, 3 cases of pleural effusion less than 3 cm did not require drainage, and 7 cases of fever relieved after drugs.

OS
During follow-up, 42 patients (52.5%) achieved tumor-free survival in liver, 28 patients (35%) had tumor-survival at the endpoint, and 10 patients (12.5%) died. Of the deaths, eight were related to metastatic progression (one patient had intrahepatic NED but succumbed to uncontrolled pulmonary disease), and two were due to pulmonary infection. Within 24 hours of ablation, 57 patients (71.2%) achieved local control, while 23 patients had residual intrahepatic disease. During follow-up, 12 patients (15%) experienced LTP and 25 patients (31.3%) developed NM. The survival distribution at each phase is shown in Figure 4. Among these patients, 3 with recurrent lesions following ablation were transitioned to an alternative combined immunotherapy, 1 underwent laparoscopic resection of a solitary lesion, and all achieved a tumor-free status at the endpoint. Additionally, 5 other patients received combined immunotherapy regimens post-ablation; of these, 4 maintained tumor-survival at the endpoint, while 1 patient died.
The median follow-up for OS was 29.5 months (range, 8-105). The 1-year and 3-year OS rates were 97.5% and 86.6%, respectively. The distribution of OS is presented in Table 2 and Figure 5A. No difference in OS was observed between the single- and repeat-ablation cohorts (P = 0.69; Figure 5B). Cox regression analysis indicated that lesion size [HR = 3.0, 95% confidence interval (CI): 1.7-5.4; P = 0.00], patient age (HR = 1.1, 95%CI: 1.0-1.2; P = 0.02), extrahepatic distant metastases (HR = 7.86, 95%CI: 2.06-29.9; P = 0.007) and recurrence (HR = 11, 95%CI: 1.5-91; P = 0.02) were significantly associated with OS. Corresponding Kaplan-Meier curves for OS stratified by lesion size, age and recurrence are shown in Figure 5C-E. It indicated that a notable survival difference for lesions larger than 2.7 cm and for patients older than 64 years. OS was not correlated with sex, tumor origin, tumor differentiation, and prior liver resection. In multivariate analysis, tumor size (HR = 3.09, 95%CI: 1.6-6.0; P = 0.003) and age (HR = 1.1, 95%CI: 1.0-1.27; P = 0.01) were independent predictors of shorter OS.
In subgroup analysis, only extrahepatic distant metastases (HR = 5.83, 95%CI: 1.09-31.1; P = 0.048) displayed significant differences in univariate analysis of OS for single-session ablation. In repeat ablation cohort, differences were observed in lesion size (HR = 6.4, 95%CI: 1.4-29; P = 0.00), types of colorectal origin (HR = 0.1, 95%CI: 0-2.7; P = 0.04), extrahepatic distant metastases (HR = 20.7, 95%CI: 2.0-215; P = 0.018) and recurrence (HR = 12.6, 95%CI: 0.48-332; P = 0.02). Only lesion size (HR = 2.4, 95%CI: 1.1-9.5; P = 0.02) emerged as an independent risk factor, with a cutoff value of 2.7 cm. The cumulative incidence curve is illustrated in Figure 5F.

PFS
The median PFS for the entire cohort was 15 months (range, 1-47), with 1- and 2-year rates of 65.7% and 52.3%. The median PFS was 24.5 months in the single-session cohort and 12 months in the repeat-ablation cohort, with no significant difference between cohort, as shown in Figure 6A and B. Disease progression occurred most frequently during the first year, when 45% of patients experienced LTP or NM. Lesion size (HR = 2.2, 95%CI: 1.5-3.1; P = 0.00) and extrahepatic distant metastases (HR = 4.39, 95%CI: 1.73-11.15; P = 0.007) were associated with shorter PFS. Multivariate analysis identified lesion size (HR = 2.01, 95%CI: 1.35-2.98; P = 0.002) as an independent risk factor affecting PFS. Patients with lesions < 1.4 cm had significantly longer PFS. Corresponding Kaplan-Meier curves for PFS are depicted in Figure 6C-E.
As shown in Table 3, only extrahepatic distant metastases (HR = 4.29, 95%CI: 1.4-12.74; P = 0.019) showed significant differences in univariate analysis of PFS for single-session ablation. In repeat ablation cohort, lesion size (HR = 3.8, 95%CI: 1.9-8; P = 0.00), quantity of lesions (HR = 1.2, 95%CI: 1.0-1.5; P = 0.041) and times of ablations (HR = 1.8, 95%CI: 0.98-0.31; P = 0.04) emerged as significant risk factors. The cutoff value for lesion size was 2.6 cm. The relevant Kaplan-Meier curves are presented in Figure 6F. Having more than 4 lesions or undergoing 2 procedures was associated with poorer PFS in the repeat ablation cohort. In multivariate analysis, tumor size was the only independent predictor of shorter PFS in repeat ablation cohort.

DISCUSSION
To the best of our knowledge, this is the first study to investigate long-term survival outcomes and prognostic factors of ablation therapy in patients with multiple CRLM after systemic chemotherapy failure. Our findings reveal that RFA increased the proportion of patients achieving tumor-free status to 52.5%, with 1-year and 3-year OS rates of 97.5% and 86.6%, respectively. The data support the potential for RFA not only to enhance survival outcomes but also to fulfill an essential role in palliative care, aiming to improve patients’ quality of life even in advanced disease stages. Thus, this study presents a novel therapeutic approach for refractory CRLM. An early study employed CT-guided RFA to treat recurrent CRLM after hepatic resection, proposing that RFA may serve as a salvage therapeutic option for recurrent CRLM following liver resection[20,21]. Our findings extend these observations by evaluating a broader population with multiple chemotherapy-refractory lesions, thereby offering more comprehensive insights into the clinical role of RFA in CRLM.
The potential advantages of RFA include its applicability to a broader patient population. For resectable CRLM, surgery remains the preferred treatment with reported 5-year survival rates ranging from 40% to 60%[4,6]. However, only a limited proportion of patients with CRLM are candidates for surgical treatment. For those with unresectable or marginally resectable CRLM, various treatment strategies have been developed. Systemic chemotherapy and interventional embolization are established options to convert patients to a resectable state[21,22]. According to a study in 1384 patients, synchronous ablation combined liver resection may expand the eligible patients with surgical treatment[23]. For patients still ineligible for surgical resection, palliative systemic chemotherapy represents the most frequently utilized therapeutic strategy. A multinational phase II study showed a median PFS of 18.6 months (bevacizumab-mFOLFOXIRI) vs 11.5 months (bevacizumab-mFOLFOX-6)[24]. Subsequent phase III research reported median OS of 29.8 months (FOLFOXIRI + bevacizumab) vs 25.8 months (FOLFIRI + bevacizumab)[25]. Furthermore, when systemic therapy is combined with aggressive local treatment such as RFA, survival benefits may be further enhanced. A phase II randomized controlled trial involving 119 patients reported a 3-year OS rate of 56.8% for RFA combined with chemotherapy, with prolonged OS compared to systemic therapy alone (45.6 months vs 40.5 months)[4]. Our results are consistent with these findings, demonstrating that RFA can help to achieve survival outcomes comparable to those of combination chemotherapy regimens, but also added advantage of rapid tumor response and a favorable safety profile with markedly lower systemic treatment-related toxicity[26]. However, it should be emphasized that RFA remains a palliative strategy for unresectable or refractory CRLM, with the primary goal of achieving local control and quality-of-life improvement, rather than replacing curative resection.
Previous studies have investigated that the number of metastatic tumors (particularly more than three) is an independent predictor of tumor recurrence and poor long-term prognosis[27,28]. For patients with unresectable CRLM enrolled in this study, a certain tumor burden (≥ 2) and a likelihood of recurrence are present. The tumors subjected to ablation fall into three categories: Those not responding to chemotherapy, those recurring after chemotherapy, and those recurring after ablation. Given that these patients had already undergone at least one laparotomy and prolonged systemic therapy, RFA was considered a suitable option due to its lower invasiveness and shorter treatment duration[26]. Our study demonstrated that > 4 metastases were significantly associated with worse PFS in the repeat ablation cohort, but not with OS. However, differing opinions exist. A phase II clinical trial involving 56 patients receiving combined ablation therapy demonstrated no significant heterogeneity in outcomes between patients with ≤ 4 liver metastases and those with > 4 metastases[4]. This discrepancy may be attributable to differences in ablation techniques, patient selection, and tumor characteristics. Notably, the suitability of ablation is influenced not only by the quantity of lesions but also by their spatial distribution, as variations in lesion distribution may compromise the adequacy of ablation margins. In particular, the presence of multiple lesions within the same liver segment may reflect a more infiltrative or aggressive tumor phenotype, which could affect local control effectiveness. Therefore, we suggest that accurate assessment of both lesion count and spatial distribution is essential for determining ablation eligibility and optimizing treatment planning. Furthermore, according to prior expert consensus, interventional physicians typically select a judicious number of tumors for ablation during preoperative assessment, rather than proceeding indiscriminately. In our cohort, the median number of ablations was similar between single-session and repeat ablation (2 vs 3), which may help explain the lack of significant difference in OS between the two groups. For patients with a higher tumor burden or diffuse lesion distribution, multidisciplinary teams generally prioritize novel combination chemotherapy as the preferred therapeutic approach.
Another significant advantage of ablation is its repeatability. Recurrence remains a major focus in management of refractory CRLM, and repeated ablation provides a feasible strategy for disease control. An early study demonstrated that RFA reduced the proportion of repeat liver resections from 100% to 39%[20]. In the study conducted by Solbiati et al[17], patients who underwent repeat ablation due to NM or LTP experienced longer OS compared to those did not. Shady et al[29] also found that patients without liver recurrence and those who received further ablation due to LTP and/or NM had significantly longer OS compared to those for who was no longer suitable for ablation. Consistent with these findings, our study identified recurrence as a major risk factor, with patients who achieved and maintained NED showing superior OS compared to those with recurrence. Although the likelihood of achieving intrahepatic NED after a single ablation session (73.8%) was slightly higher than after repeat ablation (68.4%), OS and PFS did not differ significantly between these groups. This suggests that survival benefits can be realized even in the absence of complete tumor eradication. This perspective aligns with findings from previous studies[17,30]. A study involving 123 patients with a maximum tumor diameter of ≤ 5 cm and ≤ 5 metastatic lesions without extrahepatic disease reported a median survival of 36 months, with corresponding 3-year and 5-year OS rates of 49% and 24%, respectively[30]. Similarly, a 2016 study involving 126 patients indicated a median OS of 36 months, with 1-year, 3-year, and 5-year OS rates of 90%, 48%, and 31%, respectively[29]. In our study, the median OS had not been reached at endpoint, but the survival rate seems to be improved. This may be attributed, in part, to the inclusion of cases that underwent repeat ablation, providing a period of relative NED survival for many patients. Additionally, advancements in chemotherapy and immunotherapy and related interventional procedures in recent years likely contributed to this improvement.
Previous studies have indicated that tumor size exceeding than 3 cm is associated with reduced ablation efficacy and higher risk of LTP[18,31]. Our results support these findings, demonstrating that both OS and PFS correlate with tumor size, with optimal cut-off values at 2.7 cm and 1.4 cm, respectively. This correlation may be attributed to several factors: Firstly, the heat dissipation effect during thermal ablation can diminish effectiveness, especially for tumors located near larger blood vessels are subject to heat loss from the circulating blood. Additionally, the need to avoid bile ducts and major vessels during ablation, combined with the spatial constraints of ablating multiple tumors, may result in a situation where the safety margin fails to reach the recommended ≥ 5 mm safety margin. Margins smaller than 5 mm are associated with poorer OS and PFS[29,32,33]. Therefore, particular consideration is warranted for ablating lesions larger than 2.7 cm. Moreover, achieving a lower risk of recurrence necessitates better control of tumor size, as smaller tumors are generally associated with improved PFS. Our study found that larger tumors were found to be correlated with a higher incidence of local recurrence. In this study, two cases with tumors exceeding 3.0 cm experienced LTP within three months post initial ablation. These patients underwent hepatic arterial infusion chemotherapy prior to the second ablation and achieved complete ablation based on CEUS and enhanced CT. However, LTP and recurrence in other liver segments were still observed during follow-up, potentially related to a larger tumor burden and increased tumor aggressiveness. The potential tumor cells in blood vessels adjacent to liver metastases have always been a difficulty in clinical treatment. The largest tumor in this study was 6.3 cm, and complete ablation was not achieved after the first procedure. Despite undergoing transarterial chemoembolization before the second ablation and utilizing microwave ablation in the third procedure, a satisfactory ablation margin could not be obtained, and LTP occurred within a month.
Although extrahepatic disease is less frequently analyzed in RFA studies, our data suggest that its presence negatively impacts both OS and PFS. Despite the limited sample size, we observed worse outcomes in patients with distant metastases. Prior reports support this trend, indicating that extrahepatic disease significantly reduces OS (25 months vs 44 months), and patients with lung-only extrahepatic disease had a longer median OS than those with multi-site metastases[29]. These findings highlight the importance of carefully selecting patients for ablation, with consideration not only of intrahepatic disease burden but also of extrahepatic involvement.
Surprisingly, previous liver resection did not influence OS or PFS, despite the common belief that resection affects liver reserve function. RFA effectively destroys CRLM locally while preserving surrounding healthy liver tissue, offering local control for unresectable disease and serving as an alternative treatment for lesions in patients with insufficient liver reserve post-resection or those with comorbidities[34]. Although the number of patients with prior hepatectomy was imbalanced between cohorts, multivariable analysis confirmed that hepatectomy was not an independent prognostic factor (P > 0.05). Similarly, distant extrahepatic metastases were associated with worse OS and PFS in univariate analysis, but their effect was not retained in multivariable models, likely due to confounding from other clinical variables. These discrepancies may reflect differences in patient selection, disease characteristics, and treatment strategies, which are inherent to retrospective analyses. This study identified only a few independent risk factors, from another perspective, further suggesting that ablation is suitable for a broader a wider population regardless of gender, pathological differentiation, or liver resection.
This study has notable limitations. Firstly, the small sample size and limited follow-up duration restricted the accurate calculation of the 5-year survival rate. In addition, ablation therapy for patients with multiple metastatic lesions was initiated only after significant technological advancements and the broadening of treatment guidelines. Given the exploratory nature of the study, the findings could be interpreted as preliminary observations that provide valuable insights and research leads. Secondly, several confounding treatment factors were noted, including extrahepatic metastases, which was difficult to ascertain their occurrence and timing. Additionally, systemic therapy regimens were frequently modified, and some patients received hepatic arterial infusion chemotherapy/transarterial chemoembolization or novel immuno-chemotherapy combinations. These factors limit the generalizability of our findings. Future randomized controlled prospective studies may be necessary to better manage these variables.

CONCLUSION
For patients with multiple refractory CRLM, both single-session ablation and repeat ablation can enhance OS and PFS while increase the proportion of patients achieving tumor-free status. However, lesions larger than 2.7 cm require careful selection and technical planning.

ACKNOWLEDGEMENTS
The authors thank all members from the Department of Ultrasound Interventional.