Biomedical Engineering Reference
In-Depth Information
only way to fulfill this criterion is by using a multi-antenna phased
array system. Such a system generates a circumferential electri-
cal field distribution around the patient with the ability to move
the focus point through the body by selecting for all antennas the
appropriate setting of amplitude and phase of the RF-signal. SAR
steering is preferentially supported by extensive hyperthermia
treatment planning [66,67,68]. To obtain a sufficient penetration
depth, the current systems operate at frequencies ranging from
70 to 120 MHz. A physical consequence of the low frequencies
selected is that the focal spot size in homogenous muscle or abdo-
men tissue will be large (10−15 cm in diameter), and the tissue is
always located in the near field of the antennae.
Extensive electromagnetic modeling studies have indicated
a need for higher frequencies and more degrees of freedom in
order to better focus the energy to the target region and thus to
increase tumor temperature [69,70]. The only exception might
be cervical cancer [71,72]. Seebass et al. [71] showed for a patient
model with cervical carcinoma a substantially better hyperther-
mia quality for an applicator of three antenna rings, with four
paired dipoles per ring (= Sigma Eye) than for a single ring with
four paired dipoles (= Sigma-60). Note, however, that they also
showed that for three rings, each with each up to 12 independent
antennas, increasing the frequency to 150 or 200 MHz did not
further improve hyperthermia quality.
At present, three “radiative” devices are used for the clinical
application of loco-regional hyperthermia—two academic and
one commercial device.
FIGURE 8.11
BSD-500. 24 dual-armed Archimedean spiral antenna
flexible applicator.
patient anatomy than larger waveguide applicators. Important
features of the applicator are its ability to generate uniform heat-
ing of 12 cm diameter regions or to fit the SAR distribution to
irregularly shaped tumors by utilizing appropriate power com-
binations for the individual antennas [64].
The 24-element flexible array applicator of BSD Medical
Corporation is the commercial version of the “microwave
blanket” as earlier developed by Lee et al. [65]. The applica-
tor consists of an array of 24 dual-armed Archimedean spiral
antennas mounted on a rectangular, flexible silicone carrier
that provides the required flexibility for the applicator to follow
the tissue surface, and also a sufficient frame to keep a constant
water bolus thickness (Figure 8.11). The applicator was specially
developed for treatment of chest wall recurrence and can either
be placed free at the target region or strapped to the body to
enhance secure positioning. The spiral antennas are connected
in pairs of three to one of the eight RF power amplifiers of the
BSD-500 system. By controlling the output of each RF-power
amplifier, the SAR pattern can be modified to adapt the SAR dis-
tribution to the contours of the tumor and to provide spatial SAR
control to respond to patient complaints. Based on the operating
frequency of 915 MHz, both multi-element array applicators are
recommended for hyperthermia treatment of superficial tissue
disease <2 cm deep.
8.5.2.1 the “aMC” Loco-regional
Hyperthermia System
The AMC-4 [73] and AMC-8 systems consist of four and eight
70 MHz waveguides organized in one and two rings, respec-
tively (Figure 8.12). Each waveguide has an aperture size of 20.2
× 34.3 cm
2
and can be translated independently in the direc-
tion normal to its aperture [74]. In the clinic, the position of the
waveguides is chosen such that a gap of 5 cm exists between the
waveguides and the patient. Water boluses are placed between
these gaps to ensure adequate coupling of the incident electro-
magnetic field. Furthermore, these water boluses circulated with
distilled water provide superficial cooling that is essential to pre-
vent overheating. For the AMC-8 system the distance between
the two rings can be varied. The volume of the patient that is
heated with both the AMC-4 and the AMC-8 system is relatively
large compared to the target volume, however this is common
for all loco-regional hyperthermia devices.
Loco-regional hyperthermia treatment with the 3D AMC-8
system can lead to a clinically relevant increase (plus 0.5°C in
T90 and T50) of the target temperature compared to treatment
with the 2D AMC-4 system. However, patient variability is high.
The increase in temperature is associated with a substantial (36%
to 71%) increase in applied RF-power for the AMC-4 system.
Clearly, this can be explained by the fact that the heated volume
is significantly larger for the AMC-8 system. This in turn leads
to the advice to keep track of the increase in systemic tempera-
ture [75], which is common during loco-regional hyperthermia.
8.5.2 Loco-regional Hyperthermia Devices
Loco-regional hyperthermia is most commonly applied for
advanced tumors located in the lower pelvis or in the abdomen.
Children and young adolescents represent a special group of
patients for loco-regional hyperthermia with special demands on
the equipment. In recent years new equipment has been devel-
oped to extend loco-regional hyperthermia also to tumors in the
head and neck regions. Considering the demonstrated thermal
dose−effect relationships, the ability to control the 3-dimensional
energy distribution is a mandatory requirement of a loco-regional
hyperthermia system. For electromagnetic heating devices, the
