The relationship of temperature profiles to frequency during interstitial hyperthermia

Regional hyperthermia is currently being investigated as a potential adjuvant to radiation therapy treatment of malignant disease. Since tumor response is directly related to treatment temperature, thermal distributions within tumors and surrounding normal tissue must be predictable under various conditions. Normal canine muscle was heated to approximately 42 degrees C with radiofrequency current fields over the frequency range of 500 kHz to 300 MHz. With two rows of four interstitially implanted needles acting as electrodes, thermal profiles show that temperature increases occurred between the driving and ground plane electrodes. Temperature increases throughout the tissue were generally greatest at the center of the volume treated; however, the temperature profiles within the tissue were dependent upon selection of generator frequency. Temperature measurements with thermocouples placed within an RF field are difficult at frequencies over 500 kHz. At 500 kHz, induced RF current flow in microthermocouples is low enough to provide sensitive temperature measurement during periods of heating. This observation is significant because it allows treatment temperatures to be measured during the period of heating and subsequent control of heat deposition within the treated volume.