By Christopher L Brace
Thermal ablation has become a key player in the treatment of not only liver cancer, but tumours in the kidney, lung and bone as well. Radiofrequency and cryoablation devices were first to market and remain the most widely used tools today. Ablation can provide overall survival rates similar to surgical resection for small hepatic tumour but in spite of this, it has also been associated with higher local recurrence rates. Why? The lack of an appropriate ablative margin (5–10mm) is the greatest predictor of local recurrence. Radiofrequency and cryoablations are typically 20–30mm in diameter, which can theoretically treat a 10–20mm tumour and a margin. However, the average tumour treated with ablation is around 25mm, just at the edge of what is practical. Proximity to a large blood vessel, which can protect adjacent tumour cells from thermal damage, has also been implicated in recurrence.
It seems that increasing ablation zone size is the logical step to reducing local recurrence. However, there are technical obstacles for many current ablation systems. While radiofrequency current can be delivered through small, relatively simple devices, water vapour and dehydration in the ablation zone preclude further energy delivery. Microwave energy does not have this limitation and has been shown to create larger ablations with less susceptibility to vascular heat sinks, but early systems were relatively invasive, underpowered and inconsistent. Laser ablation is MRI-compatible and can be applied using multiple applicators but is also limited by tissue charring. High-intensity focused ultrasound (HIFU) is non-invasive and relatively precise but is restricted by long treatment times, patient motion and poor ultrasound windows. Cryoablation is highly visible on imaging but grows by passive thermal diffusion, making it difficult to achieve large ablations from a single device. Therefore, larger tumours have historically been approached with sequential overlapping ablations, a deployable device, or multiple devices in concert.
So here we are: there are dozens of thermal ablation systems available worldwide. Where do we go now? For starters, more evidence. Few of the available systems, especially those recently emerged, have been evaluated scientifically. In many cases the decisions of how much power to deliver, for how long and in what manner have been determined by vendor-supplied ex vivo studies, not independent characterisations. I do not disregard the vendor as an information source, but each vendor uses different techniques, different parameters, and seeks different goals when assessing their own devices. Clinical use of these devices should become more educated by appropriate preclinical and clinical data.
There is room for system optimisation too. Ongoing research is detangling the complicated physical interactions that produce a thermal ablation. Our improved understanding of how ablations develop is producing more optimal device designs, methods of power delivery, and system architectures. For example, early microwave ablation systems could overheat tissues along the proximal antenna shaft and countermeasures like shorter, lower power treatments limited ablation size. Effective shaft cooling and automatic power control algorithms have nearly eliminated this problem in the new generation of high power microwave ablation systems. Power delivery is now only limited by regulatory restrictions. Large and reproducible ablations are possible.
We are also making treatments more precise. Multiple-applicator systems can help shape the ablation zone to the anatomy and improve the ablative margin. Matching systems to clinical indications is also likely to improve results. For example, while cryoablation may not be suited for primary tumours in cirrhotic livers, it can be effective against renal tumours and near structures susceptible to thermal damage. Some microwave devices appear to perform better in lung tissue than others. Ultrasound devices can provide some measure of directional control. Increasing precision will help users tailor treatments to the anatomy or biology.
Systems are also becoming more ergonomic. Many of the bulky, cumbersome systems of yesterday are being supplanted by spatially efficient systems with more intuitive controls and features designed specifically for interventional oncology procedures.
But devices are not the whole story. Cancer is obstinate. I believe that operator expertise and tumour biology are more important factors than the treatment device itself. We engineers can help improve processes for treatment planning for less experienced users, refine techniques like hydro-dissection to allow more aggressive treatments, generate new methods of imaging feedback, or enhance visualisation software. And more effective regional and systemic therapies will confront cancer biology. Only continued co-operation between physicians, scientists, engineers and vendors will get us where we need to be.
Christopher L Brace is an assistant professor in Radiology and Biomedical Engineering at Wisconsin-Madison University, USA
