Evaluation of Temperature Rise and Thermal Lesion Dimensions in Liver Laser Interstitial Thermotherapy

Introduction:  Laser  interstitial  thermotherapy  (LITT)  is  an  internal  ablation  therapy  method  consisting of a percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with  maximum diameter of 5 cm. In this treatment method, there isn’t any general information about the  relationship  between  increasing  the  exposure  power,  coagulation  and  carbonization  areas  with  the  changes  in  temperature.  In  this  study,  according  to  the  power  range  of  LITT  the  changes  in  the  temperature of liver tissue and the diameter of the necrotic area were measured.   Materials and Methods: In vitro LITT was performed on fleshly sheep liver tissue using a bare-tip  optical  fiber  from  a  Nd:Yag  laser.  A  power  setting  of  2,  2.4,  3,  3.4,  and  4  watt  were  used  for  an  exposure time of 300 sec. The temperature monitoring was performed during the heating and cooling  down by fixing micro thermocouples at 2.5 mm from the fiber tip. The thermal lesions which include  necrosis and carbonization areas were compared for each power.   Results: The result of the temperature monitoring was expressed as the mean value for each power.  The temperature charts show that at 2.5 mm from the fiber tip the max. tissue temperature is increased  from 276.20 ºC (for a power setting of 2 watt and a 308 sec of exposure time) to 728.2 ºC (for a power  setting of 3.4 watt and a 365 sec exposure time). At 6 mm from the fiber tip the max. temperature was  measured to be 86.4 ºC for  a power setting of 4 watt and 325 sec exposure time. For each power a non  linear  regression  analysis  was  performed  during  the  heating  and  cooling  down  for  the  dependent  (temperature) and independent (time) parameters. The max. value for the cubic equation is shown to be  R = 0.99 during the heating and for the exponential equation to be R = 0.89 during the cooling down.  A p value of 0.01 is considered significant. The diameter of the necrotic liver tissue increases from  12.95  mm  at  600  joules  to  16.15  mm  at  1200  joules  of  energy.  When  the  total  applied  energy  is  increased  from  600  to  1200  joules,  the  thermal  ablation  increased  by  25%  while  there  was  a  56%  increase  in  the  carbonization  area.    Increasing  the  carbonization  area  caused  a  decreases  in  the  penetrability of the laser beam.   Discussion and Conclusion: A useful treatment planning based on a non-linear regression analysis  could  be  prepared  for  the  treatment  of  hepatocellular  carcinoma.  In  this  analysis,  the  temperature  changes in the necrotic area are monitored as a function of power setting in the range of 2-4 watts in  LITT.