Volumetric assessment of the periablational safety margin can be used as an intraprocedural tool to evaluate local treatment success in patients with colorectal liver metastases referred to stereotactic radiofrequency ablation (SRFA), a recent study in European Radiology concludes. This could replace the visual, side-by-side evaluation of periablational safety margins, the investigators suggest, which is “challenging, even for experienced radiologists”. Authored by Gregor Laimer, Reto Bale (both Medical University Innsbruck, Innsbruck, Austria) and colleagues from Dresden (Germany), Milan (Italy), Jerusalem (Israel), and Boston (USA), the report offers an objective tool that enables the immediate, real-time determination of the extent and percentage of the safety margin requiring ablation, enabling the reliable prediction of procedural outcome success.
“Ablations with [a] 100% three-dimensional (3D) periablational safety margin of 3mm and ablations with at least [a] 90% 3D safety margin of 6mm can be considered indications of treatment success,” the study authors report. They further conclude that their results indicate image fusion of pre- and post-interventional computed tomography (CT) scans with the Ablation-fit software is feasible, and “could represent a useful tool in daily clinical practice”.
SRFA takes a multiple-needle approach, utilising 3D treatment planning, stereotactic needle placement, and image fusion for the intraoperative assessment of the periablational safety margin. According to Laimer, Bale et al, in some centres, this technique can overcome the limitations associated with standard radiofrequency ablation (RFA).
They write: “Several studies have reported that the periablational safety margin, defined as the shortest distance between tumour border and margin of the necrosis zone, independently predicts the local tumour progression in colorectal liver metastases, whereby ablations with safety margins >5–10mm exhibit lower local tumour progression rates [a better outcome]. In conventional CT or ultrasound-guided RFA, the creation of large necrosis zones extending 5–10 mm beyond tumour borders is often hampered by technical limitations, especially for larger tumors (i.e. >3cm), which may result in local tumour progression ranging from 4–70%. In contrast, these limitations have been overcome in some centres with stereotactic thermal ablation.”
In total, 45 patients with 76 colorectal liver metastases between them were treated with SRFA and analysed in this study. During image fusion of pre- and post-interventional contrast-enhanced CT scans, Laimer, Bale et al used a software with non-rigid registration (called Ablation-fit) to assess the percentage of predetermined periablational 3D safety margin and colorectal liver metastases successfully ablated (i.e. with no evidence of residual tumour tissue).
Periablational safety zones (1–10mm) and percentage of the periablational zone ablated were calculated, analysed, and compared with subsequent tumour growth to determine an optimal safety margin predictive of local treatment success.
Presenting their results, Laimer, Bale, and colleagues relayed that the mean overall follow-up was 36.1±18.5 months. Nine of 76 colorectal liver metastases (11.8%) developed local tumour progression; the mean time to local tumour progression was 18.3±11.9 months. Overall one-, two-, and three-year cumulative local tumour progression-free survival rates were 98.7%, 90.6%, and 88.6%, respectively. The periablational safety margin assessment proved to be the only independent predictor (p<0.001) of local tumour progression for all calculated safety margins. The smallest safety margin that was 100% ablated and that displayed no local tumour progression was 3mm, and at least 90% of a 6mm, circumscribed 3D safety margin was required to achieve complete ablation.
“With this work, we confirm prior reports demonstrating the importance of achieving a periablational margin for ablation, as we too established the periablational safety margin as the only significant predictor of local tumour progression in patients with colorectal liver metastases treated with SRFA,” the study investigators write in their discussion. “Apart from the periablational safety margin, no other conventional risk factor such as age, gender, tumour size, tumour location, or previous therapies significantly influenced local tumour progression. Importantly, this observation held true for all calculated 3D safety margins, namely, 1–10mm. Yet, in our study, the smallest safety margin displaying no local tumour progression for ablations with 100% circumscribed 3D safety margin was 3mm (0/55 [0%] target tumours).
“This indicates that an ablation with a 100% circumscribed 3D safety margin of 3mm can be considered successful at the time of the intervention. Given that larger volumes of ablation are more difficult to achieve, this may represent an improvement when considering that previous studies recommend a periablational safety margin of at least 5mm, with better results only seen with a 10mm periablational safety margin for colorectal liver metastases. One possible explanation for this discrepancy might be the exact image fusion and volumetric safety margin assessment achieved with the non-rigid registration software used for this study. Regardless, the small but precise margins in our study may actually represent larger volumes given known tissue shrinkage during ablation.”
Speaking to this newspaper, Bale adds: “Another explanation for this discrepancy may be related to the fact that deformation of the datasets for image fusion during stereotactic thermal ablation (intraprocedural arterial and portal venous phase planning and control CTs) is mimimised by respiratory triggering. This is achieved by temporary disconnection of the endotracheal tube and full muscle relaxation.”
Noting that this study is retrospective in design and is reliant on the experience of a single centre, and also that there is “scarce use of stereotactic approaches in the ablation of liver malignancies” outside of the Medical University in Innsbruck, Laimer, Bale, et al hope that their work aids in generating further interest in this technique.
