Page 277 - Computational Modeling in Biomedical Engineering and Medical Physics

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8.3 Pin interstitial applicators for microwave hyperthermia
When the RF or MWs antenna applicator is designed, some efficiency criteria need to
be considered and control measures consequently applied (Morega et al., 2014):
Target control—the efficacy of the intervention is conditioned by precisely addressing
the whole target tissue, which has a finite volume; its position and dimensions are
determined by previous imaging analysis, during the fore-interventional stage, but
the interventional insertion of the applicator might require visual monitoring too.
Temperature control—the target tissue is heated in moderate hyperthermia to
42 C 45 C and the temperature is held for sessions of 20 30 min, while for abla-

tion the temperature could rise to 80 C 100 C in just seconds, for the burning of

the tissue. In all cases, the power of the radiation source and the duration of the
action need very careful monitoring.
Current technical solutions for designing interstitial applicators used to efficiently
heat small volumes of tissue are achieved through pin-applicators array antennae that
are inserted within the target volume or distributed around it.
Thin needle-shaped antennae (pin applicators) were introduced and tested for soft
tissue hyperthermia treatment in the early 2000s (Ito et al., 2001, 2002; Saito et al.,
2004). Their antenna is derived from a coaxial cable with short-circuited tip and a
ring-shaped radiating slot, placed at a short distance from the tip. The original design
was intensively tested and adapted for different application conditions, and the dimen-
sions were adapted for different operation frequencies. For example, Morega et al.
(2008) and Gas (2014) present optimized applicators models for the 2.45 GHz operat-
ing frequency; Gas and Czosnowski (2014) proposes a slightly modified design,
namely, the coaxial antenna with two or three radiating slots, while Bertram et al.
(2006) complements the design of the pin antenna with an electroconductive choke in
an attempt to finely tune radiation best features.
Figure 8.11 (far left) shows the pin antenna element with a particular design, opti-
mized for efficient operation at the Industrial, Scientific, and Medical (ISM) dedicated fre-
quency of 2.45 GHz, while the other pictograms represent arrays of similar pin-antennae.
When a single-pin is used, heated tissue is, as expected, concentrated near the
antenna-radiating slot, which leads to a highly nonuniform distribution of the temper-
ature. The assessment of the radiation and heating pattern for a single pin antenna is
the first step in the analysis of interstitial hyperthermia by MWs, like the studies pre-
sented in (Ito et al., 2002; Morega et al., 2008; Gas, 2014; Trujillo et al., 2018). An
array applicator made of several optimally distributed identical pins could provide
much uniform heating in the encompassed volume, by cumulative contributions of
the individual pin components of the array; the main goal of its design targets to find
the balance between the therapeutic efficiency and minimum invasiveness (Ito et al.,
2002). Array applicators with several pins (as shown in Fig. 8.11) and their