Temperature profile during endourological laser activation: introducing the thermal safety distance concept

To examine temporal-spatial distribution of heat generated upon laser activation in a bench model of renal calyx. To establish reference values for a safety distance between the laser fiber and healthy tissue during laser lithotripsy.

We developed an in-vitro experimental setup employing a glass pipette and laser activation under various intra-operative parameters, such as power and presence of irrigation. A thermal camera was used to monitor both temporal and spatial temperature changes during uninterrupted 60-second laser activation. We computed the thermal dose according to Sapareto and Dewey's formula at different distances from the laser fiber tip, in order to determine a safety distance.

A positive correlation was observed between average power and the highest recorded temperature (Spearman's coefficient 0.94, p < 0.001). Irrigation was found to reduce the highest recorded temperature, with a maximum average reduction of 9.4 °C at 40 W (p = 0.002). A positive correlation existed between average power and safety distance values (Spearman's coefficient 0.86, p = 0.001). A thermal dose indicative of tissue damage was observed at 20 W without irrigation (safety distance 0.93±0.11 mm). While at 40 W, irrigation led to slight reduction in mean safety distance (4.47±0.85 vs. 5.22±0.09 mm, p = 0.08).

Laser settings with an average power greater than 10 W deliver a thermal dose indicative of tissue damage, which increases with higher average power values. According to safety distance values from this study, a maximum of 10 W should be used in the ureter, and a maximum of 20 W should be used in kidney in presence of irrigation.