Colorectal Liver Metastases Microwave Ablation: 1-Year CIEMAR Results

A large European registry published on September 6, 2026 in CVIR, the journal of the Cardiovascular and Interventional Radiological Society of Europe (CIRSE), shows how microwave ablation for colorectal liver metastases performs in everyday practice. Across 30 hospitals in 11 countries, 1,083 liver metastases up to 3 cm were treated, and one year after the last ablation 88.9% of evaluable lesions showed no regrowth at the treated site (local tumour control).

Results were best when the tumour was ablated with a margin of more than 10 mm and when the margin was checked during the procedure with dedicated confirmation software. For patients and families asking whether a needle-based treatment can replace or complement liver surgery, the study points to the right questions to ask.

What is new: a 30-hospital European registry

The liver is one of the most common sites to which colorectal cancer spreads. When liver metastases are small and few, surgical removal has long been the standard, but thermal ablation, in which a thin needle (antenna) is placed into the tumour through the skin and the tumour is destroyed with heat, has become a main option alongside surgery. Thermal ablation includes radiofrequency ablation (RFA) and microwave ablation (MWA); microwaves heat a larger zone in a shorter time and are widely used for liver metastases. See our RFA guide and IVR guide for how these work.

CIEMAR (CIRSE Emprint Microwave Ablation Registry) enrolled patients from September 2019 to January 2023 and analysed follow-up data up to December 2025. It was led by Professor Martijn Meijerink's team at Amsterdam UMC and Professor Philippe Pereira of SLK-Kliniken Heilbronn, with hospitals in Italy, Austria, Germany, Norway, Turkey and other countries. Its value is that the results come from routine practice at each hospital, not from the tightly controlled setting of a trial.

Who was included

Patients qualified if microwave ablation was part of a curative-intent plan for up to nine untreated liver metastases of 3 cm or less, with no disease outside the liver except up to five small lung nodules.

• Patients: 491 (median age 66, 64.6% men); 91% had ECOG performance status 0–1
• Lesions: 1,083; 47.3% of patients had one lesion treated and 29.1% had three or more
• Size: 67.4% under 2 cm, 32.3% 2–3 cm
• Approach: percutaneous 85.8%, during open surgery 9.8%, laparoscopic 4.4%
• Prior systemic therapy: 51.7% after diagnosis of liver metastases

The registry therefore covered a broad range of patients for whom ablation was planned with or instead of surgery, not only those unable to have an operation. 19.2% of lesions lay close to critical structures such as vessels, bile ducts or bowel.

Results: one-year local control and safety

Single-arm results without a comparison group. Figures are taken directly from Meijerink M, Schulz H, et al., Cardiovasc Intervent Radiol 2026;49(10):2004–2017.

The 88.9% figure counts only lesions with one-year follow-up. When the authors included lesions lost to follow-up under different assumptions, local control was 90.3% in the best case and 77.7% in the most conservative case. Many lesions that regrew could be treated again the same way, so 90.9% were controlled at one year when re-ablation is included. Being repeatable is one of the advantages of ablation over surgery.

What made the difference: a 10 mm margin and confirmation software

Successful ablation means destroying not only the tumour but also a safety margin of normal liver around it. This study put numbers on how much that margin matters.

The rest: incomplete ablation 1.5%, missing data 6.2%. Margins were reported by each centre using its own method.

Confirmation software, which overlays pre- and post-ablation CT to check whether the ablation zone fully covers the tumour, was used for 34.1% of lesions. With software, a margin over 10 mm was achieved in 75.1% of lesions versus 29.6% without it. Twelve-month local control including re-ablation was 99.2% with software and 86.3% without, and every centre using software reached a one-year local control rate of at least 90%. The authors note that this part is exploratory and needs confirmation, but their conclusion was clear: routine verification of adequate margins during the procedure.

Lesion size and image guidance also mattered: lesions larger than 2 cm and procedures guided by percutaneous ultrasound alone, without CT or MRI, had higher odds of local failure. These are two points worth asking about when choosing a hospital abroad.

Compared with surgery: the COLLISION phase 3 trial

Whether ablation is as good as surgery was addressed by COLLISION, an international phase 3 randomised trial led by the same Dutch group. At 14 hospitals in the Netherlands, Belgium and Italy, 300 patients with resectable liver metastases of 3 cm or less (up to ten) were randomised to ablation or surgery. The results were published in The Lancet Oncology in January 2025.

van der Lei S, et al., Lancet Oncol 2025;26(2):187–199. Median follow-up at interim analysis 28.9 months.

The investigators concluded that the idea of reserving ablation for unresectable metastases needs re-evaluation. CIEMAR shows that high local control can be reproduced in everyday practice across many hospitals outside a trial, and that an adequate margin is the key.

Where it is available and what guidelines say

Coverage and costs vary by country, hospital and insurance. International patients are often not covered by the host country's public insurance, so check in advance.

When looking at hospitals abroad, ask whether ablation decisions are made by a multidisciplinary team of surgeons, oncologists and radiologists, whether CT guidance and confirmation software are used, and whether microwave or radiofrequency ablation is their main technique.

Points to check and questions for your doctor

• Type of study: CIEMAR is an observational study without a control group; the comparison with surgery comes from COLLISION.
• Who it applies to: up to nine metastases of 3 cm or less. Larger or more numerous tumours call for discussion of surgery and systemic therapy as well.
• Margins were site-reported: methods differed between hospitals, so ask how your margin will be checked.
• Funding: the registry was funded by a grant from the device maker (Medtronic), and author interests are disclosed in the paper.

Questions to ask your care team:

• Given the size, number and location of my metastases, is ablation, surgery or a combination right for me?
• What margin do you aim for, and how is it checked during the procedure? Do you use confirmation software?
• Is guidance by CT or ultrasound? Percutaneous or laparoscopic?
• How will ablation be timed around my chemotherapy?
• When are follow-up CT or MRI scans done, and can the area be re-treated if it regrows?

For consultation tips see Five questions before an RFA consultation, for the procedure day and recovery see Liver cancer RFA procedure and recovery, and for reading study figures see How to check the study phase of overseas papers.

Frequently asked questions

Can colorectal liver metastases be treated with radiofrequency or microwave ablation?

Yes. Small (usually 3 cm or less) and limited liver metastases can be treated with thermal ablation. In the European CIEMAR study, one-year local control after microwave ablation of metastases up to 3 cm was 88.9% per lesion, and the COLLISION phase 3 trial found no difference in overall survival compared with surgery.

Is ablation or surgery better?

In COLLISION, overall survival was similar for metastases up to 3 cm, and adverse events were fewer with ablation (19%) than with surgery (46%). Depending on location, size and number, surgery may still be the better choice, so the decision is best made with a multidisciplinary team.

Why does the ablation margin matter?

In CIEMAR, lesions ablated with a margin over 10 mm had about one-tenth the odds of local failure (OR 0.1) of lesions with a margin under 5 mm. Lesions whose margin was checked with confirmation software had 99.2% local control at 12 months including re-ablation.

How common are side effects?

In CIEMAR, procedure-related adverse events occurred in 16.3% and serious (grade 3–4) events in 5.3%, with no procedure-related deaths. In COLLISION, serious adverse events were 7% with ablation versus 20% with surgery. Ask before the procedure how the hospital manages complications such as bleeding or infection.

This article summarises published papers and public information for general understanding and is not medical advice. Please discuss treatment choices with doctors who have examined the patient.