Biomedical Engineering Reference
In-Depth Information
successful. Highly relevant for the future development of hyper-
thermia technology is the recent finding of a thermal-dose effect
relationship in 420 patients with locally advanced cervical can-
cer (LACC) treated with RT + HT [14]. Even after adjustment for
other correlating factors in the multivariate analysis (RT dose,
tumor stage, size, performance status), the intraluminally mea-
sured thermal dose parameter remains significantly correlated
with response and survival.
All these findings on the impact of quality of the hyper-
thermia treatment on clinical outcome clearly indicate that
there is only one direction to go in hyperthermia! To obtain
the highest probability of tumor control and enhance our
ability to verify whether a specific biological mechanism of
hyperthermia indeed is active, we must increase our ability
to deliver a specified, highly controlled, and quantitatively
plus objectively documented quality of the hyperthermia
treatment.
references are made to quantitative values of the desired require-
ments. Also, a number of associated practical limitations (“les-
sons learned”), which have to be solved in order to achieve the
required level of control in hyperthermia treatment quality, will
be addressed.
8.4 Historical perspective of the
Development of External
Electromagnetic Devices
Since the early years of clinical hyperthermia, research has been
directed at the development of either quasi-static or radiative
techniques to apply and control hyperthermia using electromag-
netic energy. Around 1975, electromagnetic fields were used only
in physical therapy to increase local tissue perfusion in order
to stimulate faster recovery of muscle or ligament injuries. For
practical reasons, the first devices evaluated for hyperthermia
came from the departments of physical therapy. Characteristics
of the physical therapy devices were that they operated only at the
ISM (Industry Science and Medicine) frequencies, had a single
radiofrequency power source, one applicator, and a limited size
of the treatment field. The applicators used both the magnetic
and electric field component to transfer electromagnetic energy
from the antenna/electrode to the patient. For physical therapy
an essential advantage of their applicator design was that a direct
contact between the applicator and the patient was not needed as
energy transfer goes through air, the latter being highly conve-
nient for the patient. Applying the devices for physical therapy
for hyperthermia treatment quickly showed their limitations in
the degree of freedom to adapt the energy distribution in tissue.
Also skin cooling by air proved to be heterogeneous, thus inef-
fective for hyperthermia, and was abandoned in favor of cooling
using a water bolus.
The experienced shortcomings in the existing equipment for
physical therapy constituted the starting point for the develop-
ment of more advanced heating systems for hyperthermia. It
must be noted that, as advanced computer models were lacking,
all research efforts were based on translating fundamental and
experimental knowledge of electromagnetic fields into an empir-
ical design of a hyperthermia applicator. For a good understand-
ing of why the Hyperthermia Society has arrived at the currently
applied technology (see Section 8.5), it helps to briefly review the
early experiences with the different approaches.
8.3 requirements of a Modern External
Electromagnetic Heating Device
The clinical and biological need for enhanced control and uni-
form quantitative documentation of the quality of the applied
hyperthermia treatment as defined in the previous para-
graph provides a clear direction for engineers, physicists, and
companies designing and selling hyperthermia equipment. If
hyperthermia is to be accepted as an integral part of the onco-
logical treatment pallet, care must be taken that the involved
clinicians (radiation and medical oncologists) have control
of the prescription of the thermal dose and are accurately
informed of the thermal dose actually delivered. Furthermore,
hyperthermia devices must be designed such that they provide
the hyperthermia technologists with an easy-to-use instru-
ment, making it a joy to deliver the prescribed thermal dose in
a reliable manner. Consequently, innovation of hyperthermia
technology should focus on improving the quality of treatment
through enhanced control of targeting the RF energy to the
tumor.
In principle the ideal hyperthermia system consists of an
applicator holding multiple antenna elements to transfer the
electromagnetic energy into the tissue, allowing excellent spatial
and temporal control, an integrated water bolus to control skin
temperature, and a temperature measuring system that provides
real-time, 3D information on the temperature distribution. Of
course the whole system has a computer-controlled feedback
loop in combination with an intelligent graphical user interface,
watchdog functions, and automatic data analysis. In reality a
number of practical limitations is prohibiting the construction
of such an ideal system, although the systems currently in use
are slowly converging to these highly advanced levels. You can
only shape the future if you know your history. Therefore, the
next sections will briefly summarize the historical developments
and experiences followed by discussions concerning the main
parts identifiable in a hyperthermia system. When possible,
8.4.1 radiofrequency Inductive Heating
Transferring electromagnetic energy using the magnetic field
component initially appeared attractive for the earlier men-
tioned possibility of avoiding direct contact between the
applicator and skin. The most basic applicator is an inductive
concentric coil that is placed around the patient. However,
inductive concentric coil devices have zero power deposition
at the center of the patient and thereby restrict the clinical use
to eccentric tumors [15,16,17]. As a solution to this problem, a
