title: Radiofrequency ablation: the effect of distance and baseline temperature on thermal dose required for coagulation.
abstract: OBJECTIVE
  To determine the effects of applied current, distance from an RF electrode and baseline tissue temperature upon thermal dosimetry requirements to induce coagulation in ex vivo bovine liver and in vivo porcine muscle models.
  
  
  METHODS
  RF ablation was performed in ex vivo liver at varying baseline temperatures-19-21 degrees C (n = 114), 8-10 degrees C (n = 27), and 27-28 degrees C (n = 27)-using a 3-cm tip electrode and systematically varied current 400-1,300 mA, to achieve defined diameters of coagulation (20, 30 and 40 +/- 2 mm), and in in vivo muscle (n = 18) to achieve 35 mm +/- 2 mm of coagulation. Thermal dose required for coagulation was calculated as the area under the curve and cumulative equivalent minutes at 43 degrees C.
  
  
  RESULTS
  Thermal dose correlated with current in a negative exponential fashion for all three diameters of coagulation in ex vivo experiments (p < 0.001). The temperatures at the end of RF heating at the ablation margin were not reproducible, but varied 38 degrees C-74.7 degrees C, for 30 mm coagulation in ex vivo liver, and 59.8 degrees C-68.4 degrees C in the in vivo experiment. CEM(43) correlated with current as a family of positive exponential functions (r(2) = 0.76). However, a very wide range of CEM(43) values (on the order of 10(15)) was noted. Although baseline temperatures in the ex vivo experiment did not change required thermal dose, the relationships between end temperature at the ablation margin and RF current were statistically different (p < 0.001) as analysed at the 400 mA intercept.
  
  
  CONCLUSIONS
  In both models, thermal dosimetry required to achieve coagulation was not constant, but current and distance dependent. Hence, other formulas for thermal dose equivalence may be needed to predict conditions for thermal ablation.
authors.affiliation: Laboratory for Minimally Invasive Tumor Therapies, Department of Radiology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA.
authors.forename: Pawel
authors.initials: P
authors.lastname: Mertyna
authors.forename: Mark W
authors.initials: MW
authors.lastname: Dewhirst
authors.forename: Elkan
authors.initials: E
authors.lastname: Halpern
authors.forename: Wallace
authors.initials: W
authors.lastname: Goldberg
authors.forename: S Nahum
authors.initials: SN
authors.lastname: Goldberg
mesh_terms: D000818:Animals; D017115:Catheter Ablation / Q000379:methods*; D002417:Cattle; D006358:Hot Temperature / Q000627:therapeutic use*; D006979:Hyperthermia, Induced / Q000379:methods; D008099:Liver / Q000601:surgery; D018482:Muscle, Skeletal / Q000601:surgery; D013552:Swine; D013817:Thermography