title: Interstitial ultrasound ablation of vertebral and paraspinal tumours: parametric and patient-specific simulations.
abstract: OBJECTIVE
  Theoretical parametric and patient-specific models are applied to assess the feasibility of interstitial ultrasound ablation of tumours in and near the spine and to identify potential treatment delivery strategies.
  
  
  METHODS
  3D patient-specific finite element models (n = 11) of interstitial ultrasound ablation of tumours associated with the spine were generated. Gaseous nerve insulation and various applicator configurations, frequencies (3 and 7 MHz), placement trajectories, and tumour locations were simulated. Parametric studies with multilayered models investigated the impacts of tumour attenuation, tumour dimension, and the thickness of bone insulating critical structures. Temperature and thermal dose were calculated to define ablation (>240 equivalent minutes at 43 °C (EM43 °C)) and safety margins (<45 °C and <6 EM43 °C), and to determine performance and required delivery parameters.
  
  
  RESULTS
  Osteolytic tumours (≤44 mm) encapsulated by bone could be successfully ablated with 7 MHz interstitial ultrasound (8.1-16.6 W/cm(2), 120-5900 J, 0.4-15 min). Ablation of tumours (94.6-100% volumetric) 0-14.5 mm from the spinal canal was achieved within 3-15 min without damaging critical nerves. 3 MHz devices provided faster ablation (390 versus 930 s) of an 18 mm diameter osteoblastic (high bone content) volume than 7 MHz devices. Critical anatomy in proximity to the tumour could be protected by selection of appropriate applicator configurations, active sectors, and applied power schemas, and through gaseous insulation. Preferential ultrasound absorption at bone surfaces facilitated faster, more effective ablations in osteolytic tumours and provided isolation of ablative energies and temperatures.
  
  
  CONCLUSIONS
  Parametric and patient-specific studies demonstrated the feasibility and potential advantages of interstitial ultrasound ablation treatment of paraspinal and osteolytic vertebral tumours.
authors.affiliation: Thermal Therapy Research Group, Department of Radiation Oncology, University of California , San Francisco , California .
authors.forename: Serena J
authors.initials: SJ
authors.lastname: Scott
authors.forename: Vasant
authors.initials: V
authors.lastname: Salgaonkar
authors.forename: Punit
authors.initials: P
authors.lastname: Prakash
authors.forename: E Clif
authors.initials: EC
authors.lastname: Burdette
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Diederich
mesh_terms: D000162:Acoustics; D020342:Finite Element Analysis; D006801:Humans; D006979:Hyperthermia, Induced / Q000379:methods*; D008962:Models, Theoretical; D013125:Spinal Neoplasms / Q000628:therapy*; D014464:Ultrasonic Therapy / Q000379:methods*
keywords: Bone; interstitial ultrasound; theoretical model; thermal ablation