This special issue of the International Journal of Hyperthermia presents clinical experiences and technological advances in the use of microwaves for thermal ablation of cancer. Thermal ablation has emerged as a prominent modality for curative and palliative treatment of tumours. During the procedure, an ablation applicator is inserted into the tumour, and the cancerous tissue is killed by heat (≫50 °C); typically, a few minutes above 50 °C or seconds above 60 °C are sufficient to kill cells completely.
Tumour ablation procedures are typically guided by intra-procedural imaging, and the ablation applicators may be inserted percutaneously (i.e. through a small incision in the skin), during open surgery, or during laparoscopy. A number of energy modalities are in clinical use for thermal tumour ablation including radiofrequency (RF) current, lasers, focussed ultrasound, cryoablation and microwaves (MW), with RF being the clinically dominant modality. Compared to these modalities, microwaves offer several advantages for ablation of large tumour volumes, particularly versus RF ablation. MW propagation within tissue is not limited by formation of char and water vapour, and thus high power levels can be applied for long durations yielding greater tissue temperatures and steeper thermal gradients. Consequently, direct MW heating is responsible for heating large tissue volumes. Microwaves are also less susceptible to blood vessel heat sinks compared to most other ablation modalities. Finally, the use of radiative MW antennas does not require the use of grounding pads (as necessary for RF), and eliminates associated complications (e.g. skin burns). MW ablation has emerged as potentially the most cost-effective ablation modality due to the large zone of volumetric heating, short treatment times and relatively small number of required applicators.
The use of microwaves for thermal ablation was first reported in the 1980s, when devices designed for hemostasis/coagulation were adapted for treatment of liver tumours. It has since been applied for ablation of tumours in the bone, kidney, lung and prostate, as well as for non-oncologic applications. There are a growing number of commercially available MW ablation systems with regulatory approval for treating oncologic indications. As MW technology becomes more widely available, it is anticipated that clinical application and optimisation of techniques for specific indications will grow. Early technical developments focussed on creating large, near-spherical ablation zones with coaxial antennas. Ongoing technical applications include the investigation and optimisation of energy delivery parameters, design of ablation devices with improved spatial control of energy deposition and modelling techniques for predictive planning of ablation procedures.
Ryan and Brace reviewed the development of MW ablation technology, as it evolved from systems previously developed for interstitial hyperthermia. Meloni et al provided an update of clinical experiences with MW ablation for treatment of liver tumours, illustrating how newer technologies have demonstrated some of the theoretical advantages of MW energy. Sidoff and Dupuy describe their centre’s extensive clinical experience with MW ablation of pulmonary malignancies, and provide an update on patient outcome following ablation. Amabile et al. comparatively assessed ablation profiles with a 2.45 GHz system in ex vivo bovine liver, in vivo porcine liver, and from clinical application in primary and secondary liver tumours. They found ablation profiles were dependent on disease type, and clinical ablation profiles were more similar to experimental ablation in ex vivo bovine liver, than ablations in porcine liver in vivo. Philips et al. comparative study demonstrates the safety and local control provided by combining MW ablation with surgical resection, for treating otherwise unresectable bipolar liver disease.
Sebek et al. report on a percutaneous 2.45 GHz applicator with angular control of energy deposition patterns, thereby providing a simplified means for targeting tumours adjacent to critical structures. Sawicki et al. investigated the effects of frequency between 2 and 26 GHz on ablation zone dimensions, and found that comparably sized ablations can be created at frequencies above the typically used 2.45 GHz, potentially opening up this frequency range for clinical devices. Saito et al. present a mathematical model to predict the ablation zone dimensions and demonstrates performance by comparing to ex vivo studies. Deshazer et al. present experimental measurements of antenna heating pattern including its change during ablation, as well as a comparison to computer model predictions. Lopresto et al. review current status and challenges in the area of computer aided MW ablation treatment planning. Finally, Amabile et al. present a mathematical model to predict tissue shrinkage during MW ablation, providing first steps towards clinical consideration of this important issue.
This journal issue thus provides a comprehensive overview of current status and future challenges of MW tumour ablation, both in terms of clinical applications as well as related to device technology.
Disclosure statement
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article.