Ho: YAG and thulium fiber lasers exhibit comparable temperature distribution: an in vitro evaluation of thermal safety distances for endourological applications—section of EAU endourology

Introduction: The concept of thermal safety distance upon laser activation has been recently introduced. We aimed to compare the thermal safety distance of Ho: YAG and thulium fiber laser (TFL) activation at different setups using an in vitro model. Methods: The experimental setup used a glass pipette with an open roof bulb in a 37 °C water bath to simulate a renal calyx. We analyzed two scenarios: (1) Laser activation without irrigation; (2) Laser activation at the center of the calyx with irrigation (saline at 40 cmH2O) via a flexible ureteroscope with a 12–14 Fr access sheath. Thermal camera tests were conducted using a Ho: YAG and TFL lasers. A thermal camera monitored the temperature for 60 s of laser activation at 5, 10, and 20 W, with and without irrigation. Each test was repeated three times. The Dewey and Sapareto formula was used to calculate cumulative equivalent minutes at 43 °C (CEM43), with a safety threshold of 120 min. The safety distance was defined as the minimum distance required to overcome the safety threshold. Results: We observed no differences both in recorded temperature values and in their temporal/spatial distribution (all p > 0.05). The safety threshold was not exceeded for either Ho: YAG or TFL lasers at 5 W, 10 W, and 20 W when irrigation was applied, with no statistically significant difference in safety distance between the two lasers (p > 0.05). Similarly, without irrigation, only the 20 W (Ho: YAG: 0.93 ± 0.11 mm, TFL: 0.84 ± 0.09 mm) settings exceeded the safety threshold, and no significant difference in safety distance was found between the lasers (p > 0.05). Conclusions: No differences in safety distance were observed between TFL and Ho: YAG lasers, regardless of irrigation or power settings. It is recommended for both lasers to remain below 20 W in absence of proper irrigation.