Page 77 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

54 BIOMECHANICS OF THE HUMAN BODY

produce cold tracts in the heated volume. Thus, to heat the tissue and tumor volume effectively and
safely, it is critical to experimentally or theoretically monitor the temporal and spatial temperature
gradient during the hyperthermia treatment.

2.5.2 Temperature Monitoring during Thermal Treatment
One of the reasons hyperthermia has not yet become widely accepted as a mode of therapy is the
lack of noninvasive and inexpensive temperature measurement technology for routine use. Invasive
temperature devices have a number of restrictions when applied to temperature monitoring during
hyperthermia. These restrictions include small representative tissue sample of the entire tissue and
tumor regions, difficulty in inserting the sensor into a deep-seated tumor, and discomfort to patients
during the insertion. Because of the problems associated with invasive temperature measurement
techniques, there has been a strong demand for noninvasive temperature feedback techniques such
as ultrasonic imaging and microwave radiometry imaging (MRI). In addition to their focusing and
real-time capabilities, ultrasound-based techniques are capable of providing satisfactory temperature
resolution as well as hot-spot localization in soft tissue. MRI was applied as a noninvasive ther-
mometry method, but it has limited temperature resolution (~0.5°C) and spatial resolution (~1 cm)
and, therefore, can provide only an estimate of the average temperature over a certain tissue volume.
Further, MRI is a costly technique, and therefore, it does not comply with the clinical requirements
of treatment monitoring for tissue temperature distribution.

2.5.3 Heating Pattern Induced by Hyperthermia Applicators

Ideal Treatment Volume and Temperature Distribution. Heating pattern or specific absorption
rate (SAR) induced by external devices is defined as the heat energy deposited in the tissue or tumor
per second per unit mass or volume of tissue. In optimal treatment planning, it is the temperature
rather than the SAR distribution that is optimized in the treatment plan. The maximum temperature
generally occurs in the tissue region with heat deposition. However, one should note that SAR and
temperature distribution may not have the same profile, since temperature distribution can also be
affected by the environment or imposed boundary conditions.
The thermal goal of a clinically practical hyperthermia treatment is to maximize the volume of
tumor tissue that is raised to the therapeutic temperature. This maximization should be accomplished
while keeping the volume of normal tissue at or below some clinically specific temperature level.
There are difficulties to reaching the optimal temperature distribution with the presently available
heating devices. Most clinical heating systems have had such fixed power deposition patterns that
optimization was limited. In recent years, the cooperation between engineers and clinicians has
resulted in a new generation of heating equipment. These heating devices have considerably more
flexibility in their ability to deposit power in different patterns that help reach the treatment goal.
Further, the ideal temperature distribution may be achieved by manipulating the geometrical consid-
eration or regional blood flow. In most of the transurethral microwave catheters, circulated cold
water is installed in the catheter to provide protection to the sensitive prostatic urethra. Manipulation
of the flow rate and temperature of the water have been demonstrated to facilitate the achievement
of high temperature penetrating deep in the transition zone (Liu et al., 2000). Preheating the large
arterial blood to some extent before it enters the treatment region has also been shown to improve
the temperature homogeneity in that area.

Currently Used Heating Approaches. It is known that the size and location of the tumor have a
significant impact on applicator design and type of heating. In most of the heating devices, heat is
deposited in the tissue via electromagnetic wave absorption (microwave or radio frequency), electric
conductive heating, ultrasound absorption, laser, and magnetic particles, etc. In this section, different
heating devices are introduced and their advantages and limitations are described.