Does Duty Cycle Modification Affect Temperature During Thulium Fiber Laser Lithotripsy?

Although the thulium fiber laser (TFL, 1940 nm) efficiently fragments stones, concerns have been raised regarding heat production. Recently, a TFL with a modified duty cycle (MDC) was designed to reduce heat generation. The purpose of this study was to compare heat generation between the MDC and conventional TFL.

Ten millimeter BegoStone phantoms were placed in the proximal ureter of a kidney and ureter model. The model was submerged in a 35°C saline bath, with continuous saline irrigation (22°C) maintained at 15 mL/min through a ureteroscope. Temperature was measured using a needle thermocouple. Five trials of 60 s continuous laser activation were performed for each power setting: 3 W (0.3 J/10 Hz), 10 W (1 J/10 Hz), 20 W (1 J/20 Hz) and 30 W (0.6 J/50 Hz). Thermal dose was calculated as cumulative equivalent minutes at 43°C (CEM₄₃; injury threshold> 20). Kruskal-Wallis and Mann-Whitney U tests were used for statistical analysis with p < 0.05 being significant.

Both lasers produced maximum temperatures (T_max) of 27°C at 3 W. The MDC TFL generated a significantly lower Tmax than the conventional TFL at 10 W (36.9°C vs 42.1°C, p < 0.001), 20 W (38.1°C vs 44.6°C, p < 0.001), and 30 W (52.5°C vs 63.6°C, p < 0.05). CEM₄₃ for the MDC and conventional TFL at 20 W were (0.00 vs 0.90, respectively), and at 30 W (425 vs 275,919, respectively).

The MDC TFL generated lower temperatures at 10, 20, and 30 W. Future studies are necessary to evaluate stone fragmentation efficiency of the MDC TFL.