Theoretical and experimental investigation of interstitial ultrasound for ablation of tumors in and near the vertebrae and other bones

The goals of this work are to investigate the feasibility of interstitial ultrasound ablation of tumors in and near the spine and to develop treatment guidelines. Interstitial ultrasound ablation, which allows for directional control of heat deposition, could potentially take advantage of the preferential acoustic absorption and heating that occurs along sonicated bone surfaces in order to ablate targets adjacent to vertebrae without damaging nearby critical anatomy such as the spinal cord. A platform for modeling patient-specific temperature and thermal dose distributions during catheter-cooled interstitial ultrasound ablation involving bone was developed in Comsol. The 3D, transient, finite element models developed herein considered various approximations of the complex interactions between ultrasound and bone. Experiments in phantoms, ex vivo tissues, and in vivo tissues were used to validate the numerical models and to characterize the impact of bone on ablation performance. Temperature distributions were measured using both invasive thermometry and MR-based techniques. Comprehensive parametric and patient-specific simulation studies were performed to investigate the feasibility of interstitial ultrasound ablation of paraspinal tumors and osteolytic vertebral tumors and to develop treatment guidelines. High ultrasound absorption at bone/soft tissue interfaces increased the volumes of target tissue that could be ablated and/or reduced the necessary treatment times. Models using simplified approximations produced temperature and thermal dose profiles closely matching both experimental measurements and more comprehensive models. Patient-specific and parametric simulations indicated that tumors up to 44 mm in diameter and insulated from the spinal canal by at least 4-5 mm of bone could be completely ablated within 15 min. Critical anatomy closer to the tumor could be preserved by reducing the acoustic energy aimed towards these structures and/or through gaseous insulation. Preferential bone heating and the insulating quality of bone resulted in thermal lesions closely conforming to the shapes of osteolytic targets and fast treatment times. This work demonstrates that interstitial ultrasound ablation appears feasible for treatment of paraspinal and vertebral tumors. Models and experiments indicate that preferential bone heating may provide improved localization, faster treatment times, and larger treatment zones in unossified tumors bordering bone as compared to other heating modalities.