Microwave applicators are becoming more prevalent in cancer ablation therapy due to factors of penetration, high
power, and shortened treatment time. These applicators create the largest zones of necrosis of available energy sources.
Progress has been made both with interstitial applicators for surgical, laparoscopic, or radiological approaches, as well as
surface applicators that provide hemostasis or precoagulation prior to resection. Most commonly, the applicators operate
at 915 MHz or 2450 MHz, and are well matched to tissue. Surgical applicators are as large as 5.6 mm and have the
capability to operate at 100-200 W. With smaller applicators, internal cooling may be required to avoid heating sensitive
skin surfaces if used percutaneously or laparoscopically. With the interstitial applicators, animal studies have shown a
strong relationship between power and ablation volume, including reaching a steady-state plateau in performance based
more on power level and less on time. As shown in-vivo, MW surface applicators are very efficient in surface
coagulation for hemostasis or precoagulation and in the treatment of surface breaking lesions. These applicators are also
capable of deep penetration as applied from the surface. Characteristic treatment times for interstitial applicators are
four minutes and for surface applicators, one minute or less is sufficient. Examples will be shown of multi-organ results
with surface coagulation using high-power microwaves. Finally, future trends will be discussed that include treatment
planning, multiple applicators, and navigation.
Microsulis, in conjunction with the University of Bath have developed a set of novel microwave applicators for the ablation of soft tissues. These interstitial applicators have been designed for use in open surgical, laparoscopic and percutaneous settings and range in diameter from 2.4 to 7 mm. A 20 mm diameter flat faced interface applicator was developed as an adjunct to the open surgical interstitial applicator and has been applied to the treatment of surface breaking lesions in hepatobiliary surgery. Taken as a complete tool set the applicators are capable of treating a wide range of conditions in a safe and efficacious manner.
The modality employs a radiated electromagnetic field at the allocated medical frequency of 2.45 GHz and powers between 30 and 150 Watts. Computer simulations, bench testing, safety and efficacy testing, ex-vivo and in-vivo work plus clinical trials have demonstrated that these systems are capable of generating large volumes of ablation in short times with favourable ablation geometries. Clinical studies have shown very low complication rates with minimal local recurrence. It is considered that this modality offers major advantages over currently marketed products.
The technique is considered to be particularly safe as it is quick and there is no passage of current obviating the requirement for grounding pads. Since the microwave field operates primarily on water and all soft tissues with the exception of fat are made up of approximately 70% water the heating pattern is highly predictable making repeatability a key factor for this modality.
Microwave Endometrial Ablation (MEA) is a technique that can be used for the treatment of abnormal uterine bleeding. The procedure involves sweeping a specially designed microwave applicator throughout the uterine cavity to achieve an ideally uniform depth of tissue necrosis of between 5 and 6mm. We have performed a computer analysis of the MEA procedure in which finite element analysis was used to determine the SAR pattern around the applicator. This was followed by a Green Function based solution of the Bioheat equation to determine the resulting induced temperatures. The method developed is applicable to situations involving a moving microwave source, as used in MEA. The validity of the simulation was verified by measurements in a tissue phantom material using a purpose built applicator and a calibrated pulling device. From the calculated temperatures the depth of necrosis was assessed through integration of the resulting rates of cell death estimated using the Arrhenius equation. The Arrhenius parameters used were derived from published data on BHK cells. Good agreement was seen between the calculated depths of cell necrosis and those found in human in-vivo testing.