Different modalities for ablation
“We are looking to move beyond the limitations of radiofrequency ablation, both in the volumes it can achieve and its oncologic efficacy. I think that the two standout modalities at the moment are microwave and cryoablation.
With cryoablation we can see the therapeutic or lethal ice evolving during the procedure and subsuming the tumour very accurately.
Microwave has matured into a time- and volume-effective tool. It appears to give us faster, more reliable volumes of treatment and is less given to heat sump effects than radiofrequency ablation. In our practice with iterative microwave ablation, we are now routinely getting effective treatment diameter of 6–7cm when necessary.
We use cryoablation for renal and bone tumours and microwave in the liver and lung.
Cryoablation works exceedingly well, especially in tumours that are of smaller volume although we are seeing very reliable results, with diligent technique, even in larger renal tumours of 4–7cm.
We have looked retrospectively at our outcomes for the same size of tumours in the liver using optimal radiofrequency ablation and microwave technique and are undoubtedly achieving faster and more predicatble treatment volumes with microwave ablation. We will be presenting these data in due course.
I think that the two technologies stand side-by-side very well in a fully equipped interventional radiology department. With cryoablation you need to place multiple probes and as such it is a bit more expensive and I think that has made some European centres hesitate. You need to place on average about 3–4 probes, depending on needle calibre and tumour size, but the strength is that you see the lethal ice evolving in minutes to subsume the tumour.
With microwave we are seeing significantly lower subtotal treatment rates for difficult tumours such as colorectal metastases. This should translate into diminished recurrence rates, but it takes time to accrue this data.
Technological developments in cryoablation and microwave ablation
With microwave, we have the newer 2.45 GHz probes. With these, we are getting effective 3–4cm diameter ablation zones within as many minutes (3–4 minutes) and by means of overlapping ablations, we are getting very effective treatment volumes. Radiofrequency ablation was not time-practical in this respect. Microwave has reduced the time for the procedure and allows us to achieve much more effective, multistationed treatments.
In the last few years cryoablation with thin second- and third-generation argon cryoprobes has undoubtedly become more practical and scaleable. In Europe, we have been slow to take up cryoablation in interventional radiology. It is striking how a number of North American interventional radiology centres have appreciated the benefits of relatively painless and scaleable cryoablation, yet barring one or two centres, we have simply not seen that appreciation in European centres, as yet. Yes, it is more expensive, but we are achieving excellent radiological outcomes for sub-4cm renal tumours with accruing evidence that we are now in effective equipoise with surgical resection for this tumour group.
New data on cryoablation for solid renal tumours
In our recently assessed experience of 139 renal tumours treated by cryoablation in 118 patients, we have achieved complete treatments in 97.8% with a mean radiological follow-up of 15 months and only one late local recurrence. Although further follow-up is needed for this indolent disease, these data, for a mean tumour size of 31mm suggest equivalent oncological outcomes to surgical resection.
Interestingly, our results mirror those of the Mayo Clinic group in Rochester and suggest what can be achieved with diligent technique.
With regard to microwave, we have some in-house post-ablation, CT volumetry, showing very effective and routinely achievable ablation zone volumes of 65ml which in our experience is a considerable improvement over our previous optimised, iterative radiofrequency ablation technique.
Limitations of cryoablation and microwave ablation
The limitations of cryoablation are that it is undoubtedly more expensive and slightly more time consuming than radiofrequency ablation. With cryoablation there are also still some anxieties about larger volume treatment in the liver.
Accurate dosimetry remains an issue with microwave ablation. In the act of treating a tumour, microwave, just like radiofrequency, obscures the tumour and many workers are currently seeking to improve treatment dosimetry. Microwave and potentially irreversible electroporation are looking to evolve into dominant technologies in the liver and in the lung and it may be accurate treatment dosimetry that picks them apart.
For colorectal metastases beyond 25mm, radiofrequency ablation is just not proving reliable
With monopolar radiofrequency ablation, the problem is that we have just seen too many subtotal treatments because of inadequate temperatures at the disease margin. Although it has proved in its time to be a major step forward for small volume metastases, say colorectal metastases up to 25mm, beyond that, the marginal recurrence rate has proved unacceptable.
When you go up to 35mm, you just cannot get oncologically adequate outcomes with radiofrequency and we must look to alternatives.
Advice to young interventional oncologists
Young interventional oncologists need to understand the overall management of the cancer patient and where interventional oncology best fits alongside other currently standard therapies.
We need to develop an understanding of how interventional oncology techniques relate to other standard treatment modalities at present. Where do we fit in? That is the key question.
We need a formal understanding of the disease and all available treatment options so that we can sit in a multidisciplinary group, and understand how interventional oncology fits in and contributes to the management of patients.”
