Results from the Pan-European CIEMAR study: Local Tumour Control After Microwave Ablation of Colorectal Liver Metastases and Exploratory Associations with Ablation Margins and Confirmation Software

PURPOSE: The CIRSE Emprint Microwave Ablation (MWA) Registry (CIEMAR) was designed to evaluate real-world effectiveness of MWA for colorectal liver metastases (CRLM). This analysis reports on local tumour control and includes post-hoc analyses of ablation margin size and confirmation software use. MATERIALS AND METHODS: Patients were eligible if MWA was indicated as part of a curative-intent strategy for ≤9 local treatment-naïve CRLM measuring ≤3 cm, with no extrahepatic disease except ≤5 lung nodules. Baseline, treatment and follow-up data from September 2019 to December 2025 were analysed. Ablation margin categories were reported locally according to centre-specific practice. The primary endpoint was the 1-year local tumour control rate after the last MWA. RESULTS: In total, 491 patients with 1083 CRLM from 30 sites in 11 countries were included. Reported margins were <5mm (12.3%), 5-10mm (35.5%) or >10mm (44.5%); for 1.5% and 6.2% of lesions, ablation was incomplete and data were missing, respectively. The 1-year local tumour control rate was 88.9% (95% CI 83.2-94.6) on a per-lesion basis and 81.2% (95% CI 77.2-84.6) on a per-patient basis. Larger margins were independently associated with lower odds of local failure (5-10 mm: OR 0.30; >10 mm: OR 0.10; both p<0.01 vs <5 mm). Use of confirmation software was associated with wider reported margins (p<0.001). CONCLUSION: MWA for CRLM achieved high 12 month local tumour control. These findings support routine verification that adequate ablation margins have been achieved, while exploratory association between confirmation software use and wider margins warrants further confirmation. TRIALS REGISTRATION: NCT03775980.