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RFA is the most used ablative modality worldwide, and its tissues. [29,30] MW can generate very high temperatures inside
effectiveness and safety have been fully proven. It is an the lesion in a very short time, potentially leading to improve
[3]
established therapeutic choice for non-surgical patients treatment efficacy and to obtain larger ablation volumes.
with early stage HCC, and some recent papers suggested
that RFA can be as effective as surgical resection in terms However, the use of MWA was limited for a long time
of overall survival and recurrence-free survival rates in because of technical limitations of some currently available
patients with small, centrally located HCC. [11,12] RFA has also MW systems. Major limitations included low power, shaft
been reported to be an effective treatment of LM ≤ 3 cm, in heating, large diameter probes (13-14 gauge), small ablation
particular from colorectal cancer, [8,9,19] and it may be indicated areas requiring multiple insertions, and non-spherical
in resectable lesions as an adjunct to resection, in inoperable ablation volumes, which have discouraged clinical application
lesions that demonstrate complete or partial response after of MWA in many western countries. Adding a cooling
chemotherapy, or in recurrent and progressive lesions. [18,20] jacket around the antenna was demonstrated to decrease
cable heating, thus increasing the amount of power that
The efficacy of RFA is influenced by tumor location and size. can be safely delivered. The introduction of a choke coil
[31]
RFA of HCC < 3 cm diameter achieves complete response in into the distal portion of the antenna was also proposed
over 90% of cases, whereas 50-70% of tumors 3-5 cm in size to decrease back heating effects, but this remedy caused
are completely ablated. RFA of HCC < 5 cm can achieve equal remarkable thickening of the antenna, making the devices
survival benefit compared with surgical resection whereas unsuitable for the percutaneous application. Recently, a
results for ablation of HCC larger than 5 cm are poor. [21-23] miniaturized device has been developed (the so-called Mini
®
Reported rate of mortality of percutaneous RFA is < 1%, and Choke ), which minimizes the back-heating effects using
major complication rate ranges from 0.6% to 8.9%. [24,25] slender MW antennas (14-16 gauge), and allowing for the
percutaneous application. [32]
Advantages of RFA
RFA is the best established and well experienced thermal Several prior studies demonstrated that early-generation
technique among all the thermal modalities available. Its MWA had equal effectiveness, safety, and survival when
efficacy, feasibility, and safety have been largely proved. compared with RFA, with shorter ablation time. [13,33-35] The
recent technical advances (in particular, the Mini Choke ) have
®
Limits of RFA been reported to achieve coagulation of areas larger than
Large lesions can require multiple overlapping ablations RFA. [36-38] Such a capability could result useful in the treatment
to create an adequate safety margin. Although it has of tumors ≥ 3 cm. A randomized prospective comparison of
recently been reported that the treatment of sub-capsular MWA and RFA in the treatment of HCC did not demonstrate
or high-risk located nodules does not increase the rate of any difference in the rates of residual or untreated disease,
[39]
treatment-related complications, the sub-capsular or and a local control rate up to 95% over a median follow-up
[26]
high-risk position of the nodules can represent a relative of 33 months was reported in the intraoperative treatment
contraindication to RFA. Tumors strictly close to large vessels of colorectal LM. Mortality and major complication rates
[40]
can be incompletely treated due to the heat-sink effect, using the most recent MWA devices are similar to RFA. [32]
in which thermal energy produced by ablation is partially
shunted away from the tumor by the cooler blood. [27,28] Advantages of MWA
MWA offers some advantages compared with RFA, including
MICROWAVE ABLATION greater intratumoral temperature, deeper penetration of
energy, propagation across the poorly conductive tissue, less
MWA utilizes MW frequency (typically at 900-2,500 MHz) sensitivity to the heat-sink effect, and larger ablation volume.
to cause oscillation of polar molecules in tissue (primarily Such peculiarities enable to treat larger tumors with adequate
water), increasing their kinetic energy and the temperature safety margin, and nodules closed to large vessels. In addition,
of the tissue. Although the final effect of MWA consists in MWA does not need the use of grounding pads.
coagulative necrosis of the lesion like RFA, the mechanism of
heating differs substantially, since MW energy radiates into Limits of MWA
the tissue through an interstitial antenna that determines Microwave energy is more difficult to distribute than RF
direct heating of the lesion. Whereas RF heating requires an energy. MW energy is carried in wavelengths, which are
electrically conductive path, MWs can propagate even through more cumbersome than the small wires used to feed energy
tissues with low electrical conductivity, high impedance, to RF electrodes, and are prone to heating when carrying
or low thermal conductivity, like charred or desiccated large amount of power. Consequently, MWA appears less

Hepatoma Research | Volume 1 | Issue 2 | July 15, 2015 53