Letter to the Editor RE: Mekayten et al.,Will Stone Density Stop Being a Key Factor in Endourology? The Impact of Stone Density on Laser Time Using Lumenis Laser p120w and Standard 20w Laser: A Comparative Study (From: Mekayten M,Lorber A,Katafigiotis I,et al. J Endourol 2019;33:585–589;DOI: 10.1089/end.2019.0181)

Dear Editor:

It is still a matter of debate whether high-power and high-frequency Ho:YAG laser lithotripsy is clinically more effective than conventional low-power lithotripsy during flexible ureteroscopy. In vitro experiments reported faster lithotripsy than standard low-power devices, ¹ although experimental ideal conditions are likely not to be respected in the daily clinical practice. ² Mekayten and colleagues ³ recently reported data regarding the first clinical comparison between high-power (120 W) and low-power (20 W) Ho:YAG laser devices, showing shorter (50%) lasering time in procedures performed with the high-power system. No difference in terms of complications and stone-free rate was reported. Such clinical studies are of major importance, since they test in the real-life applicability of the preclinical data reported for 120 W Ho:YAG laser so far. However, we have some concerns to discuss.

First, current low-power lasers are represented nowadays by 30 to 35 W devices rather than 20 W devices, which according to technical data are usually limited in frequency up until 12 Hz. Newer low-power devices can be set up to 20 Hz, which means an almost double theoretical lasering speed compared with older devices, that is, what the authors reported using a 120 W machine. Where is this amount of extra energy and power going to? In experimental data, we observe a linear relationship between increase in frequency and reduced lithotripsy time. ¹ However, the authors demonstrated that with a mean frequency of 62 Hz, five times higher than that reported during low-power procedures, they were able to reduce the lasering time only by a half. Is this an issue of less effective delivery of laser energy? We better understand this discrepancy looking at the energy delivered per cubic millimeter of stone volume, which was significantly higher in the high-power setting. This suggests that a considerable amount of laser pulses did not hit the stone using the 120 W laser (10,593 vs 5449 total pulses), a loss of energy likely to cause intrarenal temperature rise. Increased temperature and renal damage caused by high-power settings have been demonstrated ⁴ and should be a concern. Moreover, we noticed that operative time was only one-third faster with high-power laser, despite lasering time was decreased by 50%. Assuming that ancillary operating theater times were comparable, could this time difference be attributed to decreased visual field quality related to the use of high frequencies, as previously described? ¹

In summary, the linear relationship between high-power and decreased lasering time is challenged by the real-life experience, and lasering with an ordinary low-power system (∼30 W) laser might achieve results similar to what was reported for high-power systems, but again this must be corroborated, because theory/laboratory is not always real life as previously commented. As a whole, we are not duly standing against the use of very high frequencies. We believe that they will be able to play a major role in achieving better dusting, provided that all the listed limitations will be worked out.