Clinical experience with external local hyperthermia in treatment of superficial malignant tumors

The distribution of power and heat produced by interstitial microwave antenna arrays: II. The role of antenna spacing and insertion depth

The distribution of power and temperature generated by 915 MHz interstitial microwave antenna arrays was studied in static muscle-equivalent phantoms and both perfused and non-perfused canine thigh muscle. These arrays, which would form the geometric basis of larger volume implants, consisted of four parallel antennas oriented such that transverse to their long axes they formed the corners of a square. Arrays with 2 and 3 cm sides were compared at various depths of insertion where the nodes for all four antennas were coincident at the same depth. The position relative to the antenna nodes of the maximum power and highest temperature within the array volume varied with the depth of insertion of the antennas. Though power dropped rapidly distal to the nodes at all depths, a shift in the location of the maximum power proximal to the nodes resulted in an increase in the effective heating volume at certain insertion depths. For 2 cm array spacing the highest power and temperature were measured along the central axis of the array at all insertion depths. However, arrays using 3 cm spacing generated their maximum power adjacent to the antennas with only 50% of this level occurring along the central axis. When the temperature produced by 3 cm arrays was measured in phantoms midway through simulated 30-minute hyperthermia treatments, the effect of thermal conduction on the temperature distribution was evident. Though power was only 50% centrally, the highest temperatures occurred there. This same pattern of central heating occurred in perfused canine muscle demonstrating the importance of conductive and convective heat redistribution in reducing thermal gradients within the array volume.

The distribution of power and heat produced by interstitial microwave antenna arrays: I. Comparative phantom and canine studies

To adequately plan and administer localized hyperthermia with interstitial microwave antennas, the thermal distribution patterns generated by such antennas must be characterized. This study evaluated the performance of single node 915 MHz antennas operating either alone or as a 2 cm square array of four parallel antennas using both muscle-equivalent phantoms and canine thigh muscle. Two types of measurements were compared. Specific absorption rate (SAR), where temperature increases resulting from short duration microwave pulses were used to define power distribution, and temperature gradients during simulated hyperthermia treatments. SAR measurements in phantoms were comparable to those obtained in non-perfused canine muscle demonstrating the usefulness of the phantom for these measurements. For a single antenna there was a rapid decrease in power radially which resulted in a steep thermal gradient at distances within 0.5 cm. However, the power generated by a four-antenna array was highest along its central axis and declined to approximately 50% near the antennas at the array periphery. Along the central axis of the array power decreased most rapidly distal to the antenna nodes. The distribution of temperature measured during simulated hyperthermia treatments in phantoms paralleled the SAR distribution and was comparable to the temperature gradient observed in perfused canine muscle, suggesting that phantoms could be used to predict temperature distributions in resting muscle tissue.