Document Type : Original Paper
M.Sc. in Medical Physics, Medical Physics Dept., Tarbiat Modarres University, Tehran, Iran.
Associate Professor, Medical Physics Dept., Tarbiat Modarres University, Tehran, Iran.
Associate Professor, Radiology Dept., Tehran Medical Sciences University, Tehran, Iran.
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