Hyperthermia Therapy - Controlling Temperatures

Controlling Temperatures

One of the challenges in thermal therapy is delivering the appropriate amount of heat to the correct part of the patient's body. For this technique to be effective, the temperatures must be high enough, and the temperatures must be sustained long enough, to damage or kill the cancer cells. However, if the temperatures are too high, or if they are kept elevated for too long, then serious side effects, including death, can result. The smaller the place that is heated, and the shorter the treatment time, the lower the side effects.

To minimize damage to healthy tissue and other adverse effects, physicians carefully monitor the temperature of the affected area. The goal is to keep local temperatures under 44 °C (111 °F) to avoid damage to surrounding tissues, and the whole body temperatures under 42 °C (108 °F), which is the upper limit compatible with life. These temperatures compare to the normal human body temperature, taken internally, of about 37.6 °C (99.6 °F).

A great deal of current research focuses on precisely positioning heat-delivery devices (catheters, microwave and ultrasound applicators, etc.) using ultrasound or magnetic resonance imaging, as well as developing new types of nanoparticles that make them particularly efficient absorbers while offering little or no concerns about toxicity to other tissues. Clinicians also hope to use advanced imaging techniques to monitor heat treatments in real time; heat-induced changes in tissue are sometimes perceptible using these imaging instruments.

The thermoacoustic (TA) effect refers to the generation of acoustic waves by electromagnetic (EM) irradiation, such as optical or microwave/radio frequency waves. In the past ten years, thermoacoustic tomography (TAT) using pulsed EM excitation has undergone tremendous growth. Energy deposition inside biological tissue through the absorption of incident EM pulses will create a transient temperature rise on the order of 10 mK. In the thermoelastic mechanism of acoustic generation, a sound or stress wave is produced as a consequence of the expansion induced by the temperature variation. Thermoacoustic signals are temperature dependent, which is an ideal characteristic for use in monitoring biological tissue temperature. The thermoacoustic pressure has the following expression

P = µ_aHβc2/c_p,

where µ_a is the microwave absorption coefficient, H is the heating function and can be written as the product of a spatial absorption function and a temporal illumination function, β is the isobaric volume expansion coefficient, c₀ is the speed of sound, c_p is the heat capacity. The thermal expansion coefficient defines the fractional changes in the volume of a material with temperature; normally, its value increases almost linearly with temperature except for the lowest temperatures. Thus, the thermoacoustic pressure can be written in the following form:

P = (A+BT)*P₀

where A and B is a constant, which can be gotten by the linearship between temperature and thermal expansion coefficient. T is the temperature, P₀ is the thermoacoustic pressure at baseline temperature. The equation demonstrates that the thermoacoustic pressure is directly proportional to temperature where its variation is the reaction of sample thermodynamic parameter changes with heat.

This characteristic of thermoacoustic signals that give us a new method to monitor thermotherapy temperature, has the potential to be developed into a viable alternative to current clinical temperature monitoring device for microwave thermotherapy.