Effects of hyperthermia on bone. I. Heating rate patterns induced by microwave irradiation in bone and muscle phantoms

We describe the initial heating rate patterns generated by microwave irradiation of 915 MHz, with constant power output, in muscle-equivalent phantoms containing a freshly excised bone, and compared with those in phantoms consisting of muscle-equivalent gel only. At 1 cm depth the muscle was cooler in the centre of the field when bone was present underneath. Also, the orientation of the bone in the field had a pronounced effect on the heating rate profiles in the overlying muscle: when the long axis of the bone was parallel to E field, a hot area in the centre of the field was observed; after rotation of the applicator by 90° so that the long axis of the bone was perpendicular to the E field, more homogeneous heating was obtained along most of the field. In contrast, the heating patterns obtained in the cortex of the bone at similar depth (1.3 cm) were not substantially influenced by its orientation in the field. Depending on field location, the heating rate of the cortical bone closest to the applicator was within 50-75% of the SAR in muscle at the same depth. We believe that these data may be useful for the extension of such measurements in vivo, to permit the effective application of hyperthermia, with or without radiation, in the treatment of bone lesions.