Theoretical study of convergent ultrasound hyperthermia for treating bone tumors

This study investigates the optimal external parameters for using an ultras ound applicator for treating bone tumors. This system utilized spherically arranged applicators such as scanned focused ultrasound, and spherically fo cused multielement applicators. The power deposition pattern is modeled as geometric gain with exponential attenuation. The specific absorption rate r atio (SARR) criteria have been used to determine the proper heating domain of ultrasound driving frequency and therapeutic tumor diameter. The results demonstrate that the optimal driving frequency depends on tumor depth, ult rasound absorption of bone marrow, and diameter of bone, but it is independ ent of the acoustic window area and SARR. The treatable diameter of bone tu mor increased when the absorption ratio of bone marrow to tumor, acoustic w indow of surface skin, and diameter of bone were elevated. However, the tre atable diameter of bone tumor decreased when muscle thickness, SARR of bone tumor site to the surface skin, bone marrow, and bone declined. To deliver the ultrasound energy into the tumor site and to avoid the potential damag e to the normal tissue as much as possible, the specific absorption rate (S AR) in the bone tumor site has to be three times higher than that in the su rface skin, tumor/marrow, and marrow/bone interfaces. The temperature distr ibutions can verify the SARR criteria in this model. This study provides th e information for choosing the optimal operating frequency of the ultrasoun d transducer and the acoustic window on the skin surface, and for designing the ultrasound applicator for clinical implementation. (C) 2000 IPEM. Publ ished by Elsevier Science Ltd. All rights reserved.