title: Considerations for theoretical modelling of thermal ablation with catheter-based ultrasonic sources: implications for treatment planning, monitoring and control.
abstract: OBJECTIVE
  To determine the impact of including dynamic changes in tissue physical properties during heating on feedback controlled thermal ablation with catheter-based ultrasound. Additionally, we compared the impact of several indicators of thermal damage on predicted extents of ablation zones for planning and monitoring ablations with this modality.
  
  
  METHODS
  A 3D model of ultrasound ablation with interstitial and transurethral applicators incorporating temperature-based feedback control was used to simulate thermal ablations in prostate and liver tissue. We investigated five coupled models of heat dependent changes in tissue acoustic attenuation/absorption and blood perfusion of varying degrees of complexity. Dimensions of the ablation zone were computed using temperature, thermal dose, and Arrhenius thermal damage indicators of coagulative necrosis. A comparison of the predictions by each of these models was illustrated on a patient-specific anatomy in the treatment planning setting.
  
  
  RESULTS
  Models including dynamic changes in blood perfusion and acoustic attenuation as a function of thermal dose/damage predicted near-identical ablation zone volumes (maximum variation < 2.5%). Accounting for dynamic acoustic attenuation appeared to play a critical role in estimating ablation zone size, as models using constant values for acoustic attenuation predicted ablation zone volumes up to 50% larger or 47% smaller in liver and prostate tissue, respectively. Thermal dose (t(43) ≥ 240 min) and thermal damage (Ω ≥ 4.6) thresholds for coagulative necrosis are in good agreement for all heating durations, temperature thresholds in the range of 54°C for short (<5 min) duration ablations and 50°C for long (15 min) ablations may serve as surrogates for determination of the outer treatment boundary.
  
  
  CONCLUSIONS
  Accounting for dynamic changes in acoustic attenuation/absorption appeared to play a critical role in predicted extents of ablation zones. For typical 5-15 min ablations with this modality, thermal dose and Arrhenius damage measures of ablation zone dimensions are in good agreement, while appropriately selected temperature thresholds provide a computationally cheaper surrogate.
authors.affiliation: Thermal Therapy Research Group, Department of Radiation Oncology, University of California-San Francisco, CA 94143, USA. prakashp@radonc.ucsf.edu
authors.forename: Punit
authors.initials: P
authors.lastname: Prakash
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Diederich
mesh_terms: D000162:Acoustics; D017115:Catheter Ablation / Q000379:methods*; D020342:Finite Element Analysis; D006801:Humans; D008099:Liver / Q000601:surgery*; D008297:Male; D008954:Models, Biological; D011467:Prostate / Q000601:surgery*; D014464:Ultrasonic Therapy / Q000379:methods*