Physical predictors of adequate hyperthermia with the annular phased array

Evaluation of the Sigma 60 applicator for regional hyperthermia in terms of scattering parameters

Scattering parameters adequately describe the interference between ports of a multiportal electromagnetic device when the device dimensions are comparable with the wavelength of the electromagnetic waves within the device. Since the Sigma 60 applicator is a four-port electromagnetic device, the interference between ports (quadrants) is described by a 4 x 4 scattering matrix. The load and frequency dependence of the scattering parameters were studied. The exact values of the parameters depends on the load within the applicator, but typically have minima at frequencies around 80 MHz and sometimes at 100-110 MHz. The effects of the coupling between quadrants can be substantial. Marked changes in the heating pattern can occur, particularly if the phase of the coupling element and the phase between quadrants both approach 90 degrees. Examples are shown in which the effects of coupling can qualitatively alter the intended SAR pattern. Simple steps which can be taken to minimize this phenomenon are demonstrated. Recommendations for clinical practice are discussed. Scattering parameters obtained with a non-absorbing phantom can be used for the quality assurance evaluation of the device.

Phantom studies and preliminary clinical experience with the BSD 2000

The BSD 2000 system is an array of microwave antennas operating in the 60-120 MHz range. It is a four-quadrant regional hyperthermia device with phase control permitting the operator to choose the point of constructive interference. A computer preplanning system is provided. We have compared the computer predictions with measured relative specific absorption rate (SAR) distributions in muscle equivalent phantoms and pig cadaver. The predicted SAR distribution is in qualitative agreement with observation, although differences of approximately 3-5 cm in the location of iso- SAR contours were noted. The longitudinal dependence of relative of distance cephalad to the central plane of the power distribution was measured, and it was found that a length of 15-20 cm was covered by the 50% iso-SAR. SAR measurements in the spinal canal of a pig cadaver showed that, even at resonance frequency, the measured SAR in the cervical spine was small (0.07 of central axis SAR). However, the spinal canal SAR as measured in the central plane of an evenly balanced configuration was 0.6 of the peak SAR. Marked reduction of the spinal canal SAR could be obtained with steering techniques and was in good agreement with the computer preplan. This paper reviews preliminary clinical experience with 17 patients. All but 2 patients were treated with steering techniques to permit partial sparing of normal tissues. The goal of maintaining central tumour temperature at or above 42 degrees C for at least 30 min was maintained in 41 of 67 sessions (61%) and in at least one session for all but 2 patients. Seventy-seven per cent of monitored tumor points attained at least 42 degrees C in patients for whom thermal mapping was performed. Significant cardiovascular stress developed during hyperthermia in two patients. There have been five complete responses and seven partial responses in the 17 patients.