How do we assess the efficacy of Ho:YAG low-power laser lithotripsy for the treatment of upper tract urinary stones? Introducing the Joules/mm3and laser activity concepts

Objectives To estimate the total energy needed to ablate 1mm(3)of stone volume (Joules/mm(3)) during flexible ureteroscopic lithotripsy using a low-power Ho:YAG laser device, as a proxy of lithotripsy efficacy. Patients and methods We selected 30 patients submitted to flexible ureteroscopy for renal stones whose volume was bigger than 500 mm(3). A 35 W Ho:YAG laser (Dornier Medilas H Solvo 35, Germany) was used for every procedure with a 272 mu m laser fiber. We recorded laser parameters, the total energy delivered by the laser fiber, the time from the first laser pulse until the last one (lithotripsy time), and the active laser time as provided by the machine. We then estimated J/mm(3)values and determinants, along with ablation speed (mm(3)/s), and laser activity (ratio between laser active time and lithotripsy time). Results Median (IQR) stone volume and stone density were respectively 1599 (630-3502) mm(3)and 1040 (753-1275) Hounsfield units (HU). In terms of laser parameters, median (IQR) energy and frequency were 0.6 (0.4-0.8) J and 15 (15-18) Hz. Median (IQR) total delivered energy and lithotripsy time were 37,050 (13,375-57,680) J and 68 (36-88) min, respectively. Median (IQR) J/mm(3)and ablation speed were, respectively, 19 (14-24) J/mm(3)and 0.7 (0.4-0.9) mm(3)/s. The laser was active during 84% (70-95%) of the total lithotripsy time. HU density > 1000 was associated with reduced efficacy. Conclusions It is possible to perform laser lithotripsy using a low-power laser device with a virtually continuous laser activity. The estimation of the pre-operative parameters as well as the J/mm(3)values are fundamental for a proper pre-operatory planning.