New for January 2015: the Unknown Editor wants to share his experience with RF ablation, read about it here.
Click here to go to a daughter page on VSWR problems encountered in medical applications.
The Varian boys out of Stanford (Microwave Hall of Famers!) pretty much made Medical Linear Accelerators the mainstay of cancer treatment with their research in the 1940's, eclipsing the use of active Cobalt 60 radiation sources with a much more controllable and "power-off" safe radiation source. From the early 1970's to today the Medical LINAC (portmanteau for linear particle accelerator) has been the work horse of the medical cancer treatment industry.
Just yesterday (euphemistically speaking) we had Thyratrons, triggering Klystrons, modulating outputs of electron guns, with outputs running down waveguides, through bunching and steering coils, pulling 270 degree turns with bending magnets to precisely "nail" a target to output a selection of as many as six different electron energies and maybe 4 photon energies from anywhere in a 360 degree rotation. Yes, with a waveguide rotational coupling.
Today (literally) Computed Tomography (radiological CT) and Linear Accelerator technology have been married together into a single system with a common source to deliver the most precisely controlled radiation dose that has ever been delivered.
In addition to radiation, another important use of microwave energy in medicine is for the thermal ablation of tissue. In this application microwave energy is used to create localised dielectric heating (diathermy) resulting in controlled destruction of tissue. Microwave ablation (MW ablation) is the next evolution of diathermy treatment and being a radiating technology overcomes many issues such as current conduction problems with grounding pads as used in high frequency and radio frequency diathermy.
Watch a video on RF ablation of varicose veins
Microwave ablation also provides desiccation of tissue without the excessive charring and nerve damage associated with RF ablation. Various applications include treatment of large tumours or removal of unwanted tissue masses, for example liver tumours, lung tumours and prostate ablation. Microwaves can also be used to coagulate bleeding in highly vascular organs such as the liver and spleen.
As microwaves have shorter wavelengths the choice of frequency can benefit the application, for example large volume ablations can typically be made at 915 MHz and 2.45 GHz and use of higher frequencies in the range 5.8 GHz - 10 GHz can create shallow penetration of energy resulting in very precise ablations suitable for treatments such as skin cancer, ablation of the heart to treat arrhythmia, uterine fibroids, multiple small liver metastases, corneal ablation (vision correction), spinal nerve ablation (back pain), varicose vein treatment, verrucae treatment and many other specific treatments.
A few common misconceptions about microwave ablation include the use of frequencies chosen to align with ISM frequency bands. The IEC standard 60601-2-6 "Particular requirements for the safety of microwave therapy equipment" is applicable to treatments operating from 300 MHz but not exceeding 30 GHz. Typically ablation treatments are intended not to radiate into air and therefore shouldn't create interference with non-ISM frequency bands.
Another common preconception about using microwaves in surgery is that they are uncontrollable. This has arisen as a result of using standard industrial magnetrons and basing measurements such as reflected power in microwave medical equipment on ideal 50 ohm microwave components. Modern microwave generators may employ stable reliable solid state sources however the dielectric properties of tissue varies considerably during treatments therefore microwave applicators (antennas) are not always optimally matched to an ideal 50 ohms which can result in significant mismatch. This can result in measurement uncertainty and VSWR problems which accounts for the perception of an uncontrollable treatment. Recent techniques, such as those developed by Emblation Limited, overcome this problem in medical microwave applications to create a mismatch tolerant controllable user experience that enhances patient safety and treatment reliability for the next generation of microwave ablation treatments.
Leading research in the use of microwave in medicine is being carried out at University of Wisconsin, Dartmouth College, Duke University, University of Bath, Bangor University and at many other leading universities. The technology has successfully been commercialised into treatments offered by a number of companies including Covidien, BSD medical, H.S. Hospital Service, Neuwave and an ever increasing number of other organisations.
In the field of oncology MW ablation now offers a new tool in the arsenal of weapons to fight cancer, providing new opportunities to save many lives.
Microwaves made it happen.
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