At ECIO 2012, David J Breen, Department of Radiology, University of Southampton School of Medicine, UK, shared with delegates 10 top tips for optimising outcomes in microwave ablation of the liver. The talk was part of a satellite symposium supported by Microsulis Medical.
Microwave ablation brings fast, effective, treatment volumes that are significantly less influenced by tissue perfusion. Breen said, “It is vital that interventional radiologists understand treatment dosimetry. Microwave enables more robust, iterative and modelled treatments. Consider “no touch” and “wedge” techniques to minimise recurrences, ensure that you have formal anaesthetic assistance and critically, attend multidisciplinary team meetings and engage in proceedings.”
“I have been involved with radiofrequency ablation since 1994 and I think that microwave ablation brings a lot of potential to the table. We have got to improve outcomes. The truth is that there is a lot of ablation practice still incurring significant recurrence rates. The surgeon has the discipline of a pathologist telling him whether his resection margins are negative or positive, we act as our own judge, jury and executioner [because we decide for ourselves whether the ablation zone really is adequate to take out that metastasis?],” he said. “This confers a responsibility on the interventional oncologist to optimise technique”.
Breen’s first tip was to “understand the physics of the device”. Radiofrequency ablation has a trailing thermal profile which relies on conductive heating beyond 2mm which can result in irregular ablation volumes. “Microwave ablation invokes active heating through a 2cm sphere which gives it a much better thermal profile and fewer vagaries to conduction and convection.”
Tip number two was “do the procedure right”. He told delegates that there was increasing evidence that general anaesthesia with CT guidance enhances the outcome of the procedure. “There should be absolutely diligent technique—less conscious sedation and isolated, 2D-ultrasound. “Patient positioning is critical, as is freezing the respiration, so that you can get truly accurate probe positioning (placement tolerance should be ±3mm). There should be a clear pre-planning of approach and cases, adequately performed, can take 1.5–2 hrs,” he said.
He also told delegates that it was vital to optimise outcomes, appreciate treatment dosimetry – ie understand how the probe behaves – and predict the dose response. “In our own practice, we have found that the treatment curve begins to plateau at about 4–5 minutes. In other words, we should start to think about restationing our microwave probe to another site to achieve optimal outcomes. Restationing the probe or multiprobe techniques are very important, that is how you get optimised and much more robust treatment volumes,” he said.
While presenting on radiofrequency ablation vs. microwave ablation in practice, Breen said that CT volumetry data from Southampton of mature post-ablation volumes achieved in clinical cases with optimised and state of the art radiofrequency ablation and Microsulis microwave ablation (MWA) had been compared. It was possible to achieve a mean ablation zone of 35.1cm3 (which corresponds to a 4cm sphere) with RFA, and a mean 64.8cm3 ablation zone which corresponds to a 5cm sphere with MWA. (This averaged ablation volume data related to a recent cohort of 36 tumours (HCC and mCRC) with equivalent mean tumour sizes, treated by either RFA or MWA).
He also told delegates that Southampton ex vivo bovine dosimetry studies indicate a predictable dose-response curve and practical plateau at four-five minutes. “Sphericity also starts to plateau at about four-five minutes”.
In keeping with new advances in the technology, Breen told delegates that he advocated a “no touch technique” in which the tumour was barely broached, instead the margins around the tumour were targetted, in order to routinely achieve substantial ablation zones. He also stated that it was important to consider “the movable and immovable aspects of the procedure.” Taking time to displace adjacent structures, particularly with microwave, is very important. Consideration should also be given to fiducials, particularly in targeting high segment eight disease, he said.
Breen highlighted participation in multidisciplinary team meetings as he concluded his talk. “Unless interventional radiologists attend tumour boards or multidisciplinary team meetings, we will be left simply receiving pre-selected cases. It is important to understand and participate in the therapeutic debate. Be cautious as early referrals are often amongst the most difficult cases. It is only when your clinical colleagues become more confident of the results of your technique that you start to get some of the easier and better quality cases. Choose the first few cases very carefully,” he said.