Impact of an immediate short waiting period on ultrasound-based ablative margin assessment following radiofrequency ablation for hepatocellular carcinoma
Article information
Abstract
Purpose
This study aimed to evaluate whether an immediate short waiting period after radiofrequency ablation (RFA) can improve the accuracy of ultrasound (US)-based assessment of the ablation zone in patients with hepatocellular carcinoma (HCC).
Methods
A prospective cohort study was conducted involving 41 patients who underwent US-guided RFA for HCC. Tumor margin conspicuity, electrode tip visibility, and operator confidence in assessing the ablative margin were recorded immediately following electrode deactivation and at 1-minute intervals for 5 minutes. Post-ablation computed tomography was performed to confirm the sufficiency of the ablative margins. The Friedman test and post-hoc Conover analysis were used to assess changes over time.
Results
Over time, significant improvements were observed in tumor margin visibility, electrode tip visualization, and operator confidence in ablative margin assessment (all P<0.001). Repositioning and additional ablation were required in 29.3% (12/41) of patients, with all achieving sufficient ablative margins. Larger tumor size was associated with decreased operator confidence (P=0.008). No major complications occurred.
Conclusion
A short waiting period following RFA enhances the visibility of tumor margins and electrode tips on US, thereby increasing operator confidence in assessing ablative margin sufficiency. Implementing an immediate short waiting period may improve the accuracy of treatment.
Introduction
Radiofrequency ablation (RFA) can be performed successfully under ultrasound (US) guidance, especially with advanced techniques such as contrast-enhanced ultrasound (CEUS) or computed tomography/magnetic resonance (CT/MR) fusion imaging. These methods provide real-time visualization and facilitate the effective ablation of tumors that are otherwise poorly visualized on conventional US [1]. The importance of achieving an adequate ablation zone during RFA is well established, with multiple studies demonstrating that insufficient ablative margins are associated with increased rates of local tumor progression, ultimately impacting patient prognosis [2,3]. Conversely, an excessively large ablation zone can unnecessarily raise complication rates without offering additional benefit in reducing recurrence [4]. Therefore, precise assessment of the ablation zone immediately following the procedure is critical for achieving optimal patient outcomes.
Despite advances in US image quality and imaging technologies, including US and CT/MR fusion techniques, immediate post-ablation evaluation with US prior to concluding the procedure remains challenging for several reasons [5]. The presence of gas and charred tissue within the ablation zone can obscure sonographic images, complicating the assessment process [6-8]. In addition, confirming sufficient ablative margins in all directions and distinguishing viable tumor tissue from necrotic areas requires significant expertise and operator experience. Underestimating the ablation zone may result in unnecessary repeated procedures, as a sufficient ablative margin might be misjudged as insufficient. Conversely, overestimating the zone increases the risk of local tumor progression due to an inadequate margin [2,9].
Recent clinical experience with US-guided ablation indicates that assessment of the ablation zone becomes more reliable several minutes after ablation is completed, as gas bubbles dissipate and the contrast between viable tumor tissue and ablated parenchyma becomes more distinct [10]. This gradual improvement in visibility facilitates identification of both the tumor margin and the radiofrequency (RF) electrode tip. Given this observation, a short "waiting" period after ablation may be important for enabling a more accurate assessment of the ablation zone before finalizing the procedure and removing the electrode, ultimately supporting better local tumor control.
However, to the authors’ knowledge, this issue has not been systematically or scientifically evaluated. Therefore, this study aimed to determine whether a short waiting period after RFA enhances the accuracy of ultrasound-based assessment of the ablation zone in hepatocellular carcinoma (HCC) patients. Specifically, it investigated whether delayed evaluation improves visualization of tumor margins, the electrode tip, and the ablation zone, thereby increasing operator confidence and accuracy in confirming adequate ablation.
Materials and Methods
Compliance with Ethical Standards
The study was approved by the Samsung Medical Center institutional review board (IRB) (SMC 2023-04-026-001), and informed consent was obtained from all participants.
Patient Enrollment
This prospective cohort study was designed to evaluate the assessment of tumor margins and ablation zones following RFA in patients with HCC. Screening for participant enrollment took place from August 2023 to October 2024, including patients clinically diagnosed with a single HCC according to the 2022 Korean Liver Cancer Association National Cancer Center (KLCA-NCC) Korea practice guidelines [11] and referred for US-guided RFA.
Inclusion criteria were as follows: (1) patients older than 20 years with risk factors for HCC, such as hepatitis B virus, hepatitis C virus, cirrhosis, or a history of HCC; and (2) patients with a histological or clinical diagnosis of HCC undergoing RFA. Exclusion criteria consisted of: (1) multiple tumors or tumors larger than 3 cm, (2) infiltrative-type HCC, (3) histology other than HCC (e.g., cholangiocarcinoma, mixed-type histology), (4) change of treatment to an alternative therapy, and (5) contraindications for RFA. Contraindications included coagulopathy (international normalized ratio ≥1.5 or platelet count <50,000/mm³), poor patient cooperation, portal vein tumor thrombosis, tumors abutting the portal vein or bile ducts larger than segmental branches, and unfeasibility for sedation.
Ablation Procedure and Evaluation
The ablation procedures were performed by two experienced radiologists with 20 and 5 years of experience in liver tumor ablation, respectively. During the RFA planning session, patients were screened for eligibility. If eligible, informed consent was obtained on the day of the procedure. RFA was performed according to conventional protocols, with the operator selecting either the no-touch or tumor puncture technique. The ablation zone was determined by the outer margin of the peripheral echogenic rim on US, and ablation sufficiency was assessed either directly, based on visualized tumor margins, or indirectly using surrogate indicators, such as the position of surrounding vessels when tumor margins were not visible.
Pre-procedure evaluations included assessment of tumor echogenicity and tumor margin conspicuity before ablation. Once the operator judged that sufficient ablation had been achieved, the following variables were recorded immediately after electrode deactivation: tumor margin conspicuity, electrode tip visualization, operator confidence in ablative margin assessment, ablative margin sufficiency, shortest margin length (mm), and need for electrode repositioning. The shortest margin length (mm) was defined as the minimum distance between the tumor surface and the ablative margin. These evaluations were performed every minute for up to 5 minutes after electrode deactivation. A five-minute waiting period was incorporated to allow for the washout of gas bubbles while minimizing any significant increase in overall procedure time. Additional electrode repositioning was performed if the anticipated ablation margin was less than 5 mm, except when the margin abutted large vessels (defined as vessels >3 mm in diameter [12,13]) or the liver surface. If repositioning was required, it was documented, but no further evaluations were performed thereafter.
Immediate post-ablation CT with dynamic contrast enhancement was performed to assess technical success, detect early complications, and compare findings with US images. An identical three-phase dynamic liver CT protocol—comprising late arterial, portal venous, and delayed phases—was used for immediate post-ablation evaluation. Complications and ablation sufficiency were assessed by the operator using multiplanar reconstruction images in axial, coronal, and sagittal planes. The recorded variables included technical success and ablative margin sufficiency based on post-ablation CT findings. Technical success was defined as complete coverage of the tumor by the ablation zone. Ablative margin sufficiency was defined as a margin of ≥5 mm in all directions, except in cases involving the liver surface or where the margin abutted large vessels.
The assessment scale for tumor margin conspicuity was as follows: (1) the echogenicity of the index tumor was distinctly different from the surrounding liver, with more than 90% of the tumor having a well-defined margin; (2) the echogenicity was slightly different, with more than 50% of the tumor having a well-defined margin; (3) the tumor was nearly isoechoic to the surrounding liver, with less than 50% having a well-defined margin; and (4) the tumor was completely unidentifiable [14]. Electrode tip visualization was categorized as: (1) well visualized, (2) poorly visualized, or (3) unidentifiable. Operator confidence in ablative margin assessment was recorded using a 3-point scale: 2 for confident, 1 for equivocal, and 0 for not assessable. The ablative margin itself was classified as: (1) sufficient, (2) equivocal, or (3) insufficient.
Statistical Analysis
For statistical analysis, the Friedman test was used to assess changes over time for variables with a non-normal distribution, and the Wilcoxon signed-rank test with Holm correction was applied for post-hoc analysis. Univariable and multivariable regression analyses were conducted to determine the factors influencing confidence in ablative margin assessment and the final ablative margin. Variables for multivariable analysis were selected based on clinical relevance, even if they did not achieve statistical significance in univariable analysis. All statistical analyses were performed using Python version 3.13.1 (PSF, Beaverton, OR, USA), with P<0.05 considered statistically significant.
Results
Study Population
Of the 50 participants initially enrolled, one was excluded due to a change in clinical diagnosis (probable eosinophilic abscess), and eight were excluded after switching to alternative treatments (cryoablation or transarterial chemoembolization). The remaining 41 participants proceeded with the procedure and were included in the final analysis (Fig. 1). Demographic and clinical characteristics, including liver disease etiology, presence of cirrhosis or ascites, Model for End-Stage Liver Disease scores, tumor size, location, and tumor markers, are summarized in Table 1.
Flow diagram of patient enrollment.
HCC, hepatocellular carcinoma; RFA, radiofrequency ablation.
Basic demographic features of the study population (n=41)
Ablation Results
Technical success was achieved in all participants (41/41, 100%). Ablative margin sufficiency (≥5 mm) on immediate post-ablation CT was observed in 90.2% (37/41) of cases. Repositioning with additional ablation after a 5-minute observation period was performed in 29.3% (12/41) of patients, all of whom subsequently achieved a sufficient ablative margin. In eight patients, the initial assessment of the ablative margin was equivocal or insufficient. Of these, seven underwent additional ablation with electrode repositioning, while one patient with poor liver function did not receive further ablation due to a conservative management approach.
Five patients with an initially sufficient ablative margin after the 5-minute observation period also underwent further ablation. In two of these cases, reassessment after a prolonged observation period (>5 minutes) indicated insufficient margins. In the other three cases, additional ablation was performed to achieve a wider ablative margin (approximately 1 cm), based on imaging features suggestive of aggressive tumor biology—specifically, arterial phase rim enhancement, non-smooth tumor margins, arterial phase peritumoral enhancement, and peritumoral hypointensity on hepatobiliary phase magnetic resonance imaging [15,16]. A wider ablation zone was pursued in these patients because this approach has been shown to significantly reduce the risk of local tumor progression, particularly in tumors with aggressive imaging characteristics [2,17]. No major procedure-related complications were observed following ablation. Subsegmental portal branch occlusion was detected in two patients; however, these events were clinically insignificant.
Tumor Margin Visibility, Electrode Tip Visibility, and Ablative Margin Confidence
Friedman analysis revealed significant differences in tumor margin visibility (χ2=181.98, P<0.001), electrode tip visibility (χ2=185.30, P<0.001), and confidence in ablative margin assessment (χ2=160.01, P<0.001) over time following electrode deactivation. As illustrated in Fig. 2, both tumor margin and electrode tip visibility increased progressively over time, accompanied by a corresponding increase in operator confidence when assessing ablative margin sufficiency (Figs. 2, 3). Post-hoc analysis demonstrated significant differences in tumor margin visibility, electrode tip visibility, and confidence in ablative margin assessment across consecutive time points (all P<0.05) (Supplementary Tables 1-3).
Temporal changes in tumor margin visibility, electrode tip visibility, and operator confidence following ablation.
Box plots and a trend line illustrate the changes in tumor margin visibility (A), electrode tip visibility (B), and confidence in assessing ablative margin sufficiency (C) over time after electrode deactivation. The trend line (mean value with standard deviation [SD]) (D) demonstrates a progressive improvement in both tumor margin and electrode tip visibility, accompanied by an increase in operator confidence in evaluating ablative margin sufficiency over time.
A representative case illustrating changes in tumor margin visibility over time following electrode deactivation.
A 65-year-old man presents with clinically confirmed hepatocellular carcinoma in liver segment 7. A. Fusion imaging of real-time ultrasound (US) and pre-acquired magnetic resonance imaging reveals a 1.9 cm hypoechoic nodule in segment 7 (arrow) on the US image (left), with the corresponding magnetic resonance image displayed on the right. B. Gas bubble formation within the ablation zone initially obscures assessment of ablative margin sufficiency. C. Three minutes after electrode deactivation, the tumor margin (arrowheads) becomes discernible. D. At 5 minutes post-deactivation, the ablative margin (arrowheads) is clearly visible and determined to be sufficient (>5 mm), allowing conclusion of the procedure. E. An immediate post-ablation image (right) confirms successful ablation with adequate margins (arrowheads) covering the target tumor (arrow) noted in the pre-procedural imaging (left).
Factors Affecting the Confidence for Assessing Ablative Margin
Table 2 presents the variables influencing operator confidence in assessing the ablative margin after a 5-minute observation period. In the univariate analysis, none of the included variables demonstrated a significant effect (P>0.05). However, in the multivariate regression analysis including all variables, larger tumor size was found to have a significant negative impact on operator confidence (β=-0.08; 95% confidence interval, -0.14 to -0.02; P=0.008).
Factors affecting the confidence for assessing ablative margin
Discussion
This study systematically evaluated the effect of a short waiting period on the accuracy of ablative margin assessment and operator confidence. The results demonstrated that waiting for 5 minutes before assessing the ablation zone significantly improved the visibility of tumor margins and the RF electrode tip, ultimately increasing the operator’s confidence in ensuring adequate ablation. Notably, operator confidence in ablative margin assessment increased progressively over time following electrode deactivation, and larger tumor size was identified as a significant negative factor impacting this confidence.
According to the findings, implementing a short waiting period before electrode removal can be a practical approach to improving ablative margin assessment. Previous studies have highlighted the importance of post-ablation margin monitoring to reduce tumor recurrence, but few have systematically examined the methodology or the impact of timing on this process [6,7,18]. Traditionally, immediate post-ablation evaluation has relied heavily on operator experience, with additional ablation often performed empirically when the margin is unclear. Although immediate CT imaging is typically performed after the procedure, detecting an insufficient ablative margin at this stage necessitates repeated ablation, increasing procedural time, resource utilization, and complication risk due to repeated punctures [1,19]. Establishing a structured waiting protocol can make margin assessment more objective and reliable. Incorporating such a protocol into clinical practice may minimize the need for empirical additional ablations and confirmatory imaging, thereby streamlining workflow and potentially reducing complication risks and resource consumption.
Regarding factors influencing operator confidence in assessing the ablative margin, variables including tumor size were included in the multivariable model despite not reaching statistical significance in univariable analysis. This decision was based on their clinical relevance and potential role as confounders. In observational studies, true associations may be obscured in univariable analysis due to confounding effects or small sample size, leading to possible false-negative results. Therefore, including clinically important variables in multivariable models, even when not statistically significant in univariable testing, is a widely accepted methodological approach [20]. Practically, it is intuitive that operators may feel less confident when ablating larger tumors, as achieving an adequate margin is generally more challenging in such cases.
Some studies have proposed the use of CEUS approximately 10 minutes after RFA to assess the ablation zone [21-23]. CEUS improves visualization of the ablation zone by differentiating viable tumor tissue from necrotic areas [24]. However, this approach requires additional contrast agents, increases procedure time, and may not be readily available in all clinical settings. By contrast, the findings of the present study suggest that simply waiting for a short period, without the use of CEUS, can improve the accuracy of ablative margin assessment. This provides a more cost-effective and widely applicable alternative for real-time intra-procedural decision-making, particularly where CEUS is unavailable or impractical.
Immediately after ablation, gas bubbles within the treated area can obscure the US image, making it difficult for the operator to identify tumor margins and the electrode tip. As these bubbles dissipate over several minutes, the contrast between ablated and viable tissue increases, improving visibility and allowing for more intuitive assessment of ablative margin sufficiency [10]. Beyond margin assessment, enhanced tumor visibility facilitates more precise electrode repositioning, thereby improving the accuracy of supplemental ablation [25]. Notably, improved margin visibility during the waiting period led to additional ablation in 29.3% of patients in this study to achieve sufficient margins. Additionally, the washout of gas bubbles reduces post-acoustic shadowing, enabling better visualization of surrounding vital structures and potentially minimizing excessive ablation and related complications [26]. In this study, no major complications were observed, apart from minor portal branch occlusions, which were clinically insignificant. However, limited sonographic access to deeper or poorly visualized margins may still result in underestimation of the actual ablation margin in certain cases, as was observed in approximately 10% of patients in this study. This finding highlights the inherent limitations of ultrasound-based assessment, particularly in anatomically challenging regions.
Despite these promising findings, this study has several limitations. First, it was a single-center study with a relatively small sample size, limiting the generalizability of the results. Due to the small sample size, further subgroup analyses based on tumor location and echogenicity were not extensively performed. Multicenter studies with larger sample sizes are warranted to validate these findings. Second, assessment of ablative margin confidence was subjective, potentially introducing observer bias. Nevertheless, there is a lack of a validated reference standard for determining the true ablation margin on ultrasound, which is represented as an echogenic rim. Future studies might consider incorporating objective tools or metrics, such as quantitative US texture analysis or artificial intelligence-assisted margin detection, to improve the reproducibility and standardization of ablative margin assessment. Finally, long-term oncologic outcomes, such as local tumor progression and recurrence rates, were not included in this analysis. Future studies should focus on multicenter validation of these findings, as well as long-term follow-up to determine whether improved ablative margin assessment translates into better clinical outcomes. Additionally, a direct comparison between CEUS and waiting time-based evaluation should be conducted to determine the most effective strategy for intra-procedural margin assessment.
In conclusion, this study demonstrates that incorporating a 5-minute waiting period following RFA significantly improves the accuracy of ablative margin assessment, enhances operator confidence, and offers a cost-effective alternative to CEUS-based evaluation. Given the clinical significance of accurate ablative margin assessment in reducing local tumor recurrence, adopting a structured waiting protocol may optimize RFA outcomes and improve decision-making in real-time clinical practice. Further research is needed to determine the optimal waiting period and to evaluate its long-term impact on oncologic outcomes.
Notes
Author Contributions
Conceptualization: Han S, Lee MW, Rhim H. Data acquisition: Han S, Lee MW, Gu K. Data analysis or interpretation: Han S, Lee MW. Drafting of the manuscript: Han S, Lee MW. Critical revision of the manuscript: Han S, Lee MW, Gu K, Rhim H. Approval of the final version of the manuscript: all authors.
Conflict of Interest
Min Woo Lee reports personal fees from STARmed outside the submitted work. The remaining authors declare no conflicts of interest.
Acknowledgments
This study was supported by Research Fund of the Korean Society of Ultrasound in Medicine for 2023.
Supplementary Material
P-values for the differences in tumor margin visibility over time after electrode deactivation (https://doi.org/10.14366/usg.25055).
P-values for the differences in confidence of electrode tip visibility over time after electrode deactivation (https://doi.org/10.14366/usg.25055).
P-values for the differences in the confidence of ablative margin assessment over time after electrode deactivation (https://doi.org/10.14366/usg.25055).
References
Article information Continued
Notes
Key point
Immediate waiting after ultrasound-guided radiofrequency ablation improves tumor margin and electrode tip visibility. Confidence in assessing ablative margin sufficiency increases over time during the waiting period. Larger tumor size negatively impacts the operator’s confidence in margin evaluation.