209 What Happens to Brain Outside the Thermal Ablation Zones? An Assessment of Needle-Based Therapeutic Ultrasound in Survival Swine

Abstract

INTRODUCTION: For stereotactic radiosurgery, isodense lines must be considered to determine how surrounding tissue is affected. In thermal ablative therapy, such as laser interstitial thermal therapy (LITT) transcranial MR-guided focused ultrasound (tcMRgFUS) and needle-based therapeutic ultrasound (NBTU) how the surrounding area is effected has not been well studies in large animal models. METHODS: Four swine underwent MRgRA NBTU using varying treatment doses. Neurological evaluation of swine was performed up to 30 days post-procedure. Serial MRI images were obtained, and the most representative were overlaid with isodose lines and compared to brain tissue acquired post-mortem which underwent histopathologic analysis. These results were also compared to predicted volumes using a finite element analysis (FEA). RESULTS: NBTU applicator orientation at 360° induced smaller lesion volumes (33.19 mm3; 120 s, 3 W; 30.05 mm3, 180 s, 4 W) versus 180o (77.20 mm3, 120 s, 3 W; 109.29 mm3; 180 s; 4 W). The ablation-zone was characterized by dense macrophage infiltration and glial/neuronal loss as demonstrated by glial fibrillary acidic protein (GFAP) and neurofilament (NF) absence and avid CD163 staining. The transition-zone between lesion and normal brain demonstrated decreased macrophage infiltration and measured ∼345 microns. None of the experimental swine suffered any long-term neurologic or behavioral complications. MRTI/histology overlay demonstrated the boundary of ablation at ∼200 (cumulative equivalent minutes) CEM43 isodose boundary. CONCLUSIONS: We successfully performed MRgRA NBTU ablation in swine and demonstrated minimal histologic changes extended past the ablation-zone. The lesion was characterized by macrophage infiltration and glial/neuronal loss which decreased through the transition-zone. Whether NBTU offers advantage over tcMRgFUS and LITT in this regard warrants further study.