Thulium Fiber Laser: Long Pulse Width as an Independent Risk Factor for Excessive Heat Generation

Introduction: Laser dosimetry selection is utilized to reduce stone retropulsion. Diode-pumped thulium fiber lasers (TFLs) allow for increased pulse energy by lengthening the pulse width. As pulse width lengthens, thermal diffusion has the potential to increase temperature generation outside of an irradiated target. We test the hypothesis that extending the TFL pulse duration longer than the thermal relaxation time poses an independent risk for increased nonspecific heat generation. Methods: A two-dimensional numerical simulation of TFL thermal confinement was performed. Energy output and pulse duration of the SOLTIVE Premium-SuperPulsed TFL were measured. Pulse energies (1.5 J) were selected for pulse durations shorter and longer than thermal relaxation time. Temperature was measured by thermocouple positioned 1 cm lateral to the fiber tip. We compared temperature increases in vitro for identical dosimetries (1.5 J at 20 Hz for 300 seconds) for short (3.1 ms) vs long (11.8 ms) pulse durations. Results: TFL thermal confinement time was calculated at 11.4 ms. For any given preset (short or long), pulse width (duration) increased as pulse energy increased. For any given pulse energy, pulse duration was 3 to 4 times longer comparing short vs long presets. For both short and long pulse settings, temperature increased as total energy increased. The maximum temperature achieved for the long pulse width was 79.6°C vs 71.8°C for the short-width arm (p = 0.002). Conclusion: TFL pulse duration longer than the thermal relaxation time poses an independent risk for increased nonspecific heat generation.