PRODUCTOS DE LAS PRINCIPALES INSTITUCIONES FINANCIERAS

There are several techniques available for invasive temperature measurements during hyperthermia where the measurement technique used depend on heating technique employed. For a particular invasive temperature measurement technique for a particular heating modality it is necessary to understand the underlying physical principles of operation. Such information will help to evaluate the extent of compatibility with heating technique and the anatomical region of interests. Some of the major characteristics of the invasive thermometry techniques used in hyperthermia will now be described.

Thermocouples are the most common devices for measuring temperature in the clinical environment because of their low cost, ease of fabrication, small size, adequate accuracy, adequate linearity, adequate stability and the possibility of combining them into arrays. It is reported that in the clinical environment over 60% studies still rely on the use of thermocouples for temperature measurement (Dunscombe et al., 1989). However, the

22

major drawback of thermocouples is their interactions with strong electromagnetic fields. In addition, they suffer from thermal smearing due to heat conduction in their metallic leads. Moreover, depending on their coating and packaging they can also interact with ultrasonic field (Hynynen et al., 1983).

Thermistors are extremely sensitive and stable devices (Dunscombe et al., 1989). When thermistors are connected to the measurement electronics through high impedance leads, they will neither perturb nor be perturbed by the electromagnetic fields (Dunscombe et al., 1989). Multisensory probes of a micrometre size have been fabricated which are suitable for clinical use (Engler et al., 1987).

Fibre optic temperature measurements are also available in hyperthermia which involves various sensing materials including phosphors, semiconductors or liquid crystal and fibre optic links which offer environmental and the use of remote instrumentation (Wickersheim and Mei, 1987). These devices are minimally perturbing to the electromagnetic fields since there is no interaction with electromagnetic fields at radio frequencies. This type of thermometer can be fitted into the smallest catheters and measurements can be localised a small volume within the tissue (Dunscombe et al., 1989).

For mild hyperthermia where a temperature of 42°C is to be maintained, a high accuracy thermometry system is required. It is recommended that thermometry system for clinical use during local or regional mild hyperthermia should be calibrated to an accuracy of 0.2˚C with a precision of 0.1˚C (Dunscombe et al., 1989). The temperature accuracy of 0.2˚C implies effective thermal contact between the sensor and the tissue being monitored. Poor thermal contact can impair measurement accuracy particularly when temperatures are changing rapidly. Another intrinsic property of a thermometry system that should be considered is its stability. The frequency of calibration of the thermometry is chosen such that the stability, precision, and accuracy of the system are guaranteed. In addition for any types of invasive thermometry its susceptibility to humidity and method of sterilization should be considered.

There are several problems associated with the invasive temperature measurement techniques. Interference from RF fields with the sensitive electronics (which conditions and records the signals from the temperature sensor) has been a long recognized difficulty of making temperature measurements (Dunscombe et al., 1989). Many thermometry systems have been designed with shielding and filtering configurations aimed at eliminating this

23

problem (Horseman et al., 1983). In addition to the interference with metallic thermometers, there are problems associated with self-heating (Constable et al., 1987; Dunscombe et al., 1988).

A further effect can be significant when the temperature sensor assembly is thermally conducting. Smearing or distortion of the temperature distribution can occur due to the conduction along the sensor (Lyons et al., 1985).Such an effect is clearly more significant in temperature gradients such as found frequently in tissue close to large blood vessels; in the penumbra of heating field and near the edges of the heating volume (Lagendijk, 1982). Thermal lag due to the insulating properties of the encapsulating medium within which the temperature sensor is located can become clinically significant. Sometimes a tracking technique is employed to maximize the temperature information obtained per insertion (Engler et al., 1987). In this approach, sufficient time must be allowed at each location for thermal equilibrium to be attained which will depend on the time constant of the sensor within its encapsulation (Waterman, 1985).

An additional problem has been identified when performing temperature measurements in ultrasonic fields. The interaction of the field with plastic catheters has been observed (Waterman et al., 1990; Fessenden et al., 1984; Hynynen et al., 1983).

The aim of thermometry in hyperthermia is by measurement and/or predictive modelling to determine the distribution of temperature in 3 dimensions. Due to a lack of homogeneity of temperature achievable with almost all heating modalities, the positional accuracy of invasive thermometry is of vital importance.

With the exception of whole body hyperthermia, temperature gradients of between 1 and 10˚C cm-1 are characteristic of clinical hyperthermia heating technology (Samulski et al., 1987).The lower value is achievable with regional heating techniques while higher values are typical of gradients encountered during the application of interstitial techniques near the surface of externally heated sites and, in general, near tumour / normal tissue boundaries. The reproducibility of setup from treatment to treatment should ideally be better than 2mm and the position of the temperature sensors with respect to both patient’s anatomy and applicator must be considered (Dunscombe et al., 1989).

In the absence of other constraints, it is clear that thermometry will be more reproducible if the locations of measurements are in regions where large temperature gradients are not

24

expected. Large blood vessels which could constitute significant heat sinks may require special consideration if they lie within the intended therapeutic volume of heating field.

The use of linear arrays of sensors is a convenient technique for increasing the temperature information which can be obtained from the insertion of one catheter. An alternative method which employs only one thermometer to reach the same objective is a linear tracking technique (Gibbs, 1983). The recommended precision of positioning is unlikely to allow temperature measurements to be made at locations separated by more than about 5mm routinely. In addition to considering the precision of location of temperature measurement, the absolute accuracy must also be considered.

Finally, it is necessary to identify the locations of temperature measurement relative to the patient’s anatomy and the applicator position before clinical treatment commences. The specification of a temperature distribution to be achieved during clinical hyperthermia cannot be realized without a clear and accurate description of the location of measurement. Several techniques are used for identifying the location of a temperature measurement. Computed tomography (CT) in the presence of radio-opaque skin markers to indicate applicator position is universally applicable when available. Stereo or orthogonal X- ray films taken with the probe catheter filled with a radio opaque substance can be employed to identify the position of the catheter in 3 dimensions. Other imaging techniques such as ultrasound and MRI can also be used to help identify catheter location.