Laser Lithotripsy Induces Dose-Dependent Temperature Elevation During Retrograde Intrarenal Surgery

Introduction

Retrograde intrarenal surgery (RIRS) using holmium:YAG (Ho:YAG) and thulium fiber laser (TFL) systems for lithotripsy has become the most common surgical treatment for kidney stones. ¹ Although advancements in laser technology, particularly the development of high-power modalities, have enhanced stone ablation efficiency, concerns have been raised regarding the potential for thermal injury to surrounding urothelial tissues. ²

Energy emitted by a laser used in ureteroscopy (URS) may dissipate, in the form of heat, into surrounding tissues, potentially causing thermal damage such as coagulation, carbonization, and protein denaturation.

Existing literature highlights the importance of modifiable surgical parameters, including irrigation temperature and flow rate, utilization of ureteral access sheaths (UASs), laser settings (power, pulse duration, and frequency), and operator duty cycle, in lowering intrarenal temperatures and thus mitigating potential thermal damage during URS.^3–5

However, no human studies, to date, have been done to measure both renal parenchymal and calyceal temperatures during RIRS with lithotripsy with both pulse-modulated Ho:YAG (Lumenis Pulse™ 120H) and SuperPulsed TFL (Olympus, Inc.) systems. The primary objective of this study was to measure human intracalyceal and renal parenchymal temperature changes in real time during endoscopic stone surgery with two high-power laser systems. Secondary objectives included correlating irrigation fluid temperatures with parenchymal temperatures in real time during endoscopic stone and determining which modifiable surgical factors influence renal temperature changes during endoscopic stone surgery with high-power laser systems. Another secondary objective included comparing calyceal and renal parenchymal temperature changes between the pulse-modulated Ho:YAG and SuperPulsed TFL systems.

Methods

Procedure

Stone burden was defined as the sum of the single longest diameters of all stones, as measured in the coronal dimension. For a more precise measurement, total stone volume was calculated by summing the volumes of individual stones. Maximal width and length were measured on axial views and height on coronal views, all under ≥8× magnification. Using these dimensions, individual volumes were determined via manual calculation with the European Association of Urology ellipsoid formula: V = 0.167 × π × L × W × H.

Patients were assigned to receive LLT with either a TFL (60 W Soltive™ SuperPulsed) or Ho:YAG (Lumenis Pulse™ 120H) laser system based on the availability of the laser system.

All patients were positioned in the modified Barts flank-free position followed by standard sterile preparation and draping. Cystoscopy was performed followed by flexible URS (Flex X2, Karl Storz) under direct vision into the renal collecting system. The procedure was performed without the use of a UAS. A comprehensive pyeloscopy was performed to identify and characterize the stone burden. As is our standard procedure with ECIRS, all stones were translocated using a basket to a favorable calyx for LLT followed by percutaneous evacuation of all stones, stone particles, and dust. All patients received gravity irrigation at room temperature with saline irrigant bags placed 80 cm above the patient’s abdomen (distance from top of the abdomen to the bottom of the irrigant bag). Although the precise volume of infused fluid was not measured, our standard protocol for URS involves using no more than a single 1-L saline bag.

After translocation of stones, a 1.5-mm multipoint thermal sensor (MTS) needle (Boston Scientific) was utilized to get percutaneous access to the favorable renal calyx in place of using a 18 g access needle (Fig. 1). The MTS needle was inserted into the collecting system under ultrasound and fluoroscopic guidance. Moreover, with the flexible ureteroscope in position, the needle penetration was observed in real time. After the needle was positioned properly with the distal most sensor in the calyx, it was calibrated according to protocol to ensure accurate functionality.

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FIG. 1.

Diagram of translocation of stones to a favorable calyx and percutaneous placement of multipoint thermal sensor with calyceal (5 mm) and parenchymal (15 mm [1 cm], 25 mm [2 cm], and 35 mm [3 cm]) marks.

The Soltive 60 W SuperPulsed TFL (Olympus, Inc.) or Lumenis P120 Ho:YAG laser with MOSES™ technology (Boston Scientific) systems was used for LLT with a 200-µm laser fiber. Temperature data were collected from each patient during LLT using three commonly used laser settings. Fragmentation was performed with the following settings: 1 J/2 Hz (TFL) and 0.5 J/5 Hz (Ho:YAG), 2–2.5 W with the aim to break stones into large pieces. Dusting was performed with the following settings: 0.3 J/50 Hz, 15 W with the aim to pulverize stone into fine powder in a continuous “painting” motion over the stone’s surface. Pop-dusting was performed with the following settings: 0.5 J/80 Hz, 40 W with the aim to rapidly fragment kidney stones into extremely fine, powder-like pieces. Each laser mode was used for 2 minutes, with intracalyceal and parenchymal tissue temperatures measured in real time. If tissue temperature exceeded 43°C, lasering was paused. Before commencing a new laser setting, the temperature of the caliceal fluid and renal parenchyma was allowed to return to baseline body temperature prior to initiating a new laser setting.

During LLT, temperature measurements were recorded using the MTS needle at 5 mm (calyceal), 15 mm (1 cm), 25 mm (2 cm), and 35 mm (3 cm) from the probe’s tip. The 5-mm sensor was located just within the collecting system, whereas the other sensors measured temperatures along the parenchyma (medulla and cortex) in real time. At each site along the MTS needle, we monitored for temperatures over 43°C and recorded the duration for which this threshold was surpassed. The operating surgeon assessed and graded any laser-induced damage intraoperatively, immediately following the completion of lithotripsy. The laser damage was graded with the following criteria: Grade 0: none, Grade 1: mucosal, Grade 2: submucosal, and Grade 3: full thickness. ⁶

Following temperature measurements, a fascial dilator was placed over the MTS needle under direct vision, to facilitate the placement of a guidewire and to perform balloon dilation of the tract to 24F followed by PCNL of all stones and fragments and dust in standard fashion. At the conclusion of the case, the surgeon decided as usual if a ureteral stent or nephrostomy tube was necessary. Categorical data were compared using chi-square or Fisher’s exact tests, whereas continuous variables were analyzed via t-tests. Normality of distributions for all comparisons was assessed via Kolmogorov–Smirnov and Shapiro–Wilk tests of normality. Given that the majority of comparisons/distributions in our study did not violate the assumption of normality, we opted to compare data between our groups using means/t-tests and standard deviations.

Results

Between December 2023 and July 2024, a total of 26 patients met the inclusion criteria for ECIRS. Six patients were preoperatively excluded according to exclusion criteria, resulting in a final analysis of 20 patients, with 10 in each treatment arm (Fig. 2).

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FIG. 2.

Flow diagram.

There were no differences in demographics, stone characteristics, and clinical characteristics between both groups (Table 1). Total operative time, operator duty cycle, and frequency of visible laser damage were similar between both groups. Nine out of 20 (45%) were done totally tubeless, without stents or nephrostomy tubes. The remaining 11 had stents but no nephrostomy tubes.

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Table 1.

Baseline Characteristics and Operative Information Between Ho:YAG and TFL Groups

	Ho:YAG	TFL	p Value
Age [STD]	54.8 [16.3]	58.6 [11.6]	0.556
BMI [STD]	28.4 [3.5]	26.6 [7.4]	0.512
Gender, n (%)			0.170
Male	2 (80.0)	4 (40.0)
Female	2 (20.0)	6 (60.0)
ASA score, n (%)			0.211
2	10 (100.0)	7 (70.0)
3	0 (0.0)	3 (30.0)
Diabetes, n (%)	1 (10.0)	0 (0.0)	1.000
Diuretics, n (%)	0 (0.0)	0 (0.0)	—
ARB or ACE inhibitor, n (%)	1 (10.0)	3 (30.0)	0.582
Anticoagulants, n (%)	0 (0.0)	1 (10.0)	1.000
Antiplatelets, n (%)	1 (10.0)	0 (0.0)	1.000
History of recurrent UTI, n (%)	0 (0.0)	2 (20.0)	0.474
Positive preop urine culture, n (%)	1 (10.0)	0 (0.0)	1.000
Preoperative creatinine [STD]	1.0 [0.2]	1.0 [0.3]	0.979
Preoperative EGFR [STD]	88.2 [23.7]	79.2 [20.4]	0.376
Previous stone surgery, n (%)	7 (70.0)	8 (80.0)	1.000
ESWL	0 (0.0)	1 (10.0)	1.000
URS	4 (40.0)	3 (30.0)	1.000
PCNL	2 (20.0)	4 (40.0)	0.628
Laterality, n (%)			1.000
Left	6 (60.0)	6 (60.0)
Right	4 (40.0)	4 (40.0)
Renal pelvis AP diameter, cm [STD]	2.2 [0.8]	2.0 [0.7]	0.503
Total stone burden, mm [STD]	21.7 [10.0]	22.2 [10.6]	0.906
Stone volume, mm3 [STD]	3292.0 [3140.1]	2157.6 [2607.6]	0.391
HU [STD]	876.9 [370.5]	801.9 [173.6]	0.572
Size of laser fiber, nm, n (%)			0.057
150	1 (10.0)	6 (60.0)
200	9 (90.0)	4 (40.0)
Visible laser damage, n (%)			0.141
Grade 0: none	5 (50.0)	9 (90.0)
Grade 1: mucosal	5 (50.0)	1 (10.0)
Grade 2: submucosal	0 (0.0)	0 (0.0)
Grade 3: full thickness	0 (0.0)	0 (0.0)
Total laser energy used, kJ [STD]	3.8 [1.5]	5.2 [1.3]	0.038*
Operator duty cycle, minutes [STD]	16.02 [2.02]	16.33 [3.09]	0.790
Pedal time, minutes [STD]	4.81 [0.61]	4.90 [0.93]	0.801
Total operative time, minutes [STD]	76.7 [15.9]	70.7 [13.9]	0.390

Continuous variables reported as medians. p Values derived from chi-square tests, Fisher’s exact tests, or t-tests, with significance set to p < 0.05 (bolded and * for reference).

AP = anterior posterior; ESWL = extracorporeal shock wave lithotripsy; Ho:YAG = holmium:YAG; PCNL = percutaneous nephrolithotomy; STD = standard deviation; TFL = thulium fiber laser; URS = ureteroscopy.

Mean baseline calyceal temperatures (T_calyx) for Ho:YAG and TFL were below 33°C across all time points for both groups (Table 2). During fragmentation, 1 J and 2 Hz, 2 W, mean calyceal temperatures for Ho:YAG and TFL were 31.8°C vs 32.5°C at 0 seconds, 33.4°C vs 33.3°C at 60 seconds, and 34.0°C vs 33.4°C at 120 seconds (p = 0.404, p = 0.941, p = 0.699; Fig. 3). During dusting, 0.3 J and 50 Hz, 15 W, mean T_calyx for Ho:YAG and TFL was 32.5°C vs 31.9°C at 0 seconds, 39.1°C vs 42.1°C at 60 seconds, and 37.6°C vs 42.6°C at 120 seconds (p = 0.526, p = 0.289, p = 0.082). During dusting, T_calyx exceeded 43°C in four TFL patients and four Ho:YAG patients (mean T_max: 44.3°C vs 40.4°C, p = 0.175). Maximum calyceal temperature (T_max) in TFL and Ho:YAG patients was 51°C and 56°C (p = 0.18), respectively. During pop-dusting, 0.5 J and 80 Hz, mean T_calyx for Ho:YAG and TFL was 33.0°C vs 32.7°C at 0 seconds, 43.0°C vs 47.3°C at 60 seconds, and 40.0°C vs 51.2°C at 90 seconds (p = 0.754, p = 0.264, p = 0.071). The T_calyx exceeded 43°C in 10 TFL and 8 Ho:YAG patients with a significantly higher mean T_max in the TFL arm (TFL: 52.9°C, Ho:YAG: 45.1°C, p = 0.027).

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FIG. 3.

Mean calyceal temperature by laser and setting over time. Error bars represent standard error of the mean.

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Table 2.

Temperature Outcomes Between Ho:YAG and TFL Groups

	Mean calyceal temperature, °C			Mean 15 mm temperature, °C			Mean 25 mm temperature, °C			Mean 35 mm temperature, °C
	Ho:YAG	TFL	p Value	Ho:YAG	TFL	p Value	Ho:YAG	TFL	p Value	Ho:YAG	TFL	p Value
Baseline [STD]
0 seconds	32.2 [1.8]	31.6 [3.4]	0.628	34.7 [1.6]	34.9 [1.4]	0.776	35.0 [1.6]	35.8 [0.8]	0.172	34.8 [1.7]	35.5 [1.0]	0.274
30 seconds	32.4 [1.8]	31.9 [2.6]	0.626	34.9 [1.5]	35.1 [1.1]	0.741	35.1 [1.5]	35.8 [0.8]	0.219	35.0 [1.6]	35.4 [1.0]	0.515
60 seconds	32.6 [1.8]	32.2 [2.4]	0.684	34.9 [1.5]	35.1 [1.1]	0.741	35.0 [1.6]	35.8 [0.8]	0.172	34.8 [1.8]	35.4 [1.0]	0.359
90 seconds	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—
120 seconds	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—	0.0 [0.0]	0.0 [0.0]	—
Fragmentation [STD]
0 seconds	31.8 [1.5]	32.5 [2.1]	0.404	34.2 [1.4]	35.2 [1.0]	0.087	34.4 [1.5]	35.3 [1.1]	0.141	35.0 [1.5]	35.3 [1.3]	0.632
30 seconds	32.6 [2.5]	33.6 [3.0]	0.425	34.5 [1.9]	35.3 [1.2]	0.274	34.6 [1.6]	35.1 [1.0]	0.424	34.8 [1.8]	35.1 [1.3]	0.675
60 seconds	33.4 [3.1]	33.3 [2.8]	0.941	34.7 [2.3]	35.3 [1.2]	0.468	34.7 [1.8]	35.1 [1.0]	0.543	34.8 [1.9]	35.0 [1.2]	0.782
90 seconds	33.5 [3.1]	33.1 [3.0]	0.771	34.8 [2.3]	35.4 [1.3]	0.482	35.0 [1.8]	35.2 [0.9]	0.755	35.0 [1.9]	34.9 [1.4]	0.896
120 seconds	34.0 [3.4]	33.4 [3.4]	0.699	34.8 [2.3]	32.3 [9.3]	0.430	34.8 [1.9]	35.2 [1.0]	0.564	35.0 [1.9]	34.9 [1.4]	0.896
Dusting [STD]
0 seconds	32.5 [1.8]	31.9 [2.3]	0.526	35.0 [1.6]	35.0 [1.1]	1.000	35.1 [1.4]	35.2 [1.0]	0.861	35.3 [1.4]	35.3 [1.3]	1.000
30 seconds	36.4 [4.2]	39.0 [4.2]	0.182	35.3 [2.0]	35.7 [1.4]	0.613	35.4 [1.5]	35.4 [1.4]	1.000	35.5 [1.7]	35.4 [1.6]	0.894
60 seconds	39.1 [6.4]	42.1 [5.9]	0.289	36.9 [4.5]	36.5 [2.3]	0.805	36.1 [2.4]	36.1 [2.3]	1.000	35.4 [1.6]	35.9 [2.6]	0.635
90 seconds	37.5 [3.4]	42.1 [5.6]	0.069	35.5 [1.5]	36.1 [1.6]	0.428	35.6 [1.2]	35.3 [1.0]	0.512	35.4 [1.7]	35.0 [1.4]	0.647
120 seconds	37.6 [3.2]	42.6 [6.6]	0.082	36.3 [1.2]	36.6 [1.3]	0.554	35.9 [1.1]	35.6 [0.9]	0.634	35.6 [1.5]	35.1 [1.2]	0.505
Pop-dusting [STD]
0 seconds	33.0 [2.1]	32.7 [2.2]	0.754	34.6 [2.7]	35.1 [1.4]	0.607	34.7 [2.6]	35.4 [0.7]	0.434	34.7 [2.5]	35.3 [0.8]	0.492
30 seconds	41.2 [6.8]	43.4 [6.8]	0.479	37.9 [4.1]	37.6 [3.2]	0.858	36.7 [1.9]	36.0 [1.5]	0.370	36.0 [1.8]	35.8 [1.2]	0.772
60 seconds	43.0 [5.3]	47.3 [10.1]	0.264	38.4 [2.8]	38.3 [2.5]	0.954	36.8 [2.1]	36.1 [1.4]	0.463	36.1 [2.1]	35.9 [1.3]	0.795
90 seconds	40.0 [4.2]	51.2 [6.9]	0.071	39.0 [4.2]	39.5 [4.0]	0.899	36.0 [0.0]	36.8 [1.6]	0.259	34.5 [0.7]	36.3 [1.5]	0.077
120 seconds	38.0 [.]	53.5 [9.2]	—	37.0 [.]	40.0 [0.0]	—	36.0 [.]	36.5 [0.7]	—	35.0 [.]	35.0 [1.4]	—

p Values derived from t-tests, with significance set to p < 0.05 (bolded and * for reference).

Mean baseline parenchymal temperatures (T_tissue) for both groups were also comparable and remained below 36°C across all time points. During fragmentation, mean T_tissue at the 15-mm probe tip for Ho:YAG and TFL was 34.2°C vs 35.2°C at 0 seconds, 34.7°C vs 35.3°C at 60 seconds, and 34.8°C vs 32.3°C at 120 seconds (p = 0.087, p = 0.468, p = 0.430). Maximum calyceal and parenchymal temperatures averaged below 36°C and never exceeded 43°C during fragmentation (Table 3).

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Table 3.

Temperature Outcomes Between Ho:YAG and TFL Groups

	Ho:YAG	TFL	p Value
Maximum calyceal temperature, °C [STD]
Baseline	32.7 [1.9]	32.5 [2.6]	0.846
Fragmentation	34.1 [3.4]	34.4 [3.0]	0.837
Dusting	40.4 [5.8]	44.3 [6.5]	0.175
Pop-dusting	45.1 [5.2]	52.9 [8.6]	0.027*
Maximum 15 mm temperature, °C [STD]
Baseline	34.9 [1.5]	35.1 [1.1]	0.741
Fragmentation	34.9 [2.2]	35.6 [1.2]	0.387
Dusting	37.6 [4.1]	37.2 [2.0]	0.788
Pop-dusting	39.8 [4.2]	39.9 [4.1]	0.958
Maximum 25 mm temperature, °C [STD]
Baseline	35.1 [1.5]	35.8 [0.8]	0.219
Fragmentation	35.0 [1.8]	35.5 [1.0]	0.445
Dusting	36.6 [2.2]	36.3 [2.2]	0.760
Pop-dusting	37.0 [1.9]	36.7 [1.3]	0.687
Maximum 35 mm temperature, °C [STD]
Baseline	35.0 [1.6]	35.5 [1.0]	0.419
Fragmentation	35.3 [1.6]	35.3 [1.3]	1.000
Dusting	35.8 [1.4]	36.1 [2.4]	0.736
Pop-dusting	36.4 [1.5]	36.2 [1.2]	0.749
Time to maximum temperature, seconds [STD]
Baseline	—	—	—
Fragmentation	60.0 [.]	67.5 [45.0]	—
Dusting	76.7 [37.1]	82.5 [21.2]	0.693
Pop-dusting	49.5 [31.5]	75.0 [35.9]	0.136
Time to 43 °C (if reached), seconds [STD]
Baseline (n = 0)	—	—	—
Fragmentation (n = 0)	—	—	—
Dusting (H: n = 3; T: n = 6)	60.0 [30.0]	50.0 [24.5]	0.648
Pop-dusting (H: n = 7; T: n = 9)	45.0 [22.5]	43.3 [15.8]	0.871

p Values derived from t-tests, with significance set to p < 0.05 (bolded and * for reference).

During dusting, mean T_tissue at the 15-mm probe tip for Ho:YAG and TFL was 35.0°C vs 35.0°C at 0 seconds, 36.9°C vs 36.5°C at 60 seconds, and 36.3°C vs 36.6°C at 120 seconds (p = 1.000, p = 0.805, p = 0.554). The mean parenchymal T_max at 15 mm was 37.6°C and 37.2°C during dusting in the Ho:YAG and TFL groups, respectively (p = 0.788; Fig. 4). T_tissue during dusting only exceeded 43°C in one patient in the Ho:YAG cohort (49°C after 60 seconds at the 15 mm probe; Fig.5). At the 25 mm and 35 mm markers, the mean T_max was <37°C for both Ho:YAG and TFL with no significant statistical differences between groups. During pop-dusting, mean T_tissue at the 15-mm probe tip for Ho:YAG and TFL was 34.6°C vs 35.1°C at 0 seconds, 38.4°C vs 38.3°C at 60 seconds, and 39.0°C vs 39.5°C at 90 seconds (p = 0.607, p = 0.954, p = 0.899). The mean parenchymal T_max at 15 mm was 39.8°C and 39.9°C during pop-dusting in the Ho:YAG and TFL groups, respectively (p = 0.958). T_tissue during pop-dusting exceeded 43°C two times in the Ho:YAG cohort and three in the TFL cohort, which all occurred at the 15 mm probe (Fig. 6). At the 25-mm and 35-mm probes, mean T_max was comparable between groups and averaged below 37°C during pop-dusting.

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FIG. 4.

Mean T_max at each renal landmark by laser and setting. Error bars represent standard error of the mean.

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FIG. 5.

Individual maximum temperature readings at 15-mm probe for Ho:YAG and TFL across laser settings. Ho:YAG = holmium:YAG; TFL = thulium fiber laser.

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FIG. 6.

Heat map representation of calyceal and parenchymal temperatures for both pulse-modulated Ho:YAG and SuperPulsed TFL for all settings.

Discussion

High-powered lasers have demonstrated increased efficiency in LLT; however, they also pose a risk of elevating intrarenal temperatures, potentially leading to thermal injury to the kidney. This study evaluates calyceal and parenchymal temperature elevations up to 120 seconds following the activation of high-powered lasers. Additionally, we analyzed the minimum, maximum, and mean calyceal and parenchymal temperatures during laser activation using both pulse-modulated Ho:YAG and SuperPulsed TFL systems.

With the advent of high-powered lasers, several modifiable risk factors have been investigated to mitigate the risk of thermal injury, one of which is laser settings. In this study, we observed that with low-powered settings (2–2.5 W), renal temperatures—including both calyceal and parenchymal measurements—remained below 43°C for both TFL and Ho:YAG systems. In contrast, high-powered settings (>15 W) led to temperatures exceeding 43°C within as little as 15 seconds. Similarly, a study that measured calyceal fluid temperature utilizing a Type K thermocouple during retrograde LLT with a TFL laser at incremental settings reported that a temperature of 43°C was reached in 3 out of 10 patients at 20 W and in 8 out of 10 patients at 30 W, whereas it remained below this threshold at 5 W and 10 W. ⁷ In an in vivo porcine model, all dusting and most fragmentation settings (dusting: 0.5 J, 80 Hz, 40 W; low-power fragmentation: 1 J, 10 Hz, 10 W; high-power fragmentation: 1.5 J, 20 Hz, 30 W) resulted in calyceal temperatures reaching or exceeding 44°C. ⁸ All the settings >5 W in our study and in prior in vivo studies have resulted in calyceal temperatures reaching or exceeding 43°C. These findings underscore the direct relationship between laser power settings and peak calyceal fluid temperatures. During pop-dusting, the average maximum temperature (avgT_max) was significantly higher for TFL than for pulse-modulated Ho:YAG. This finding is consistent with a previous in vitro URS study using an impacted ureteral stone model with no outflow where they found that TFL produced significantly higher temperatures than Dornier and Empower HL lasers across most power settings (p < 0.001). ⁹ The sole exception occurred at 20 W, where no significant difference was observed between TFL and Empower HL. However, the study’s methodology presents a key limitation. Temperature was measured with a thermocouple placed in the ureteral fluid, just 2 mm from the laser fiber. When treating an impacted stone, this method is known to yield falsely elevated readings, which may not accurately reflect the true temperature of the ureteral tissue. Calyceal fluid temperatures do not necessarily indicate that renal tissue temperatures increase, and parenchymal temperature elevations may be more indicative of renal injury because of high-powered lasers.

Given the importance of parenchymal temperatures in evaluating potential tissue injury, our study measured medullary and cortical temperatures at approximately 1, 2, and 3 cm from the tip of the renal papilla. According to the thermal dose model described by Sapareto and Dewey, tissue maintained at 43°C for 120 minutes experiences an equivalent thermal dose to tissue maintained at 50°C for 56 seconds or 56°C for 1 second. This concept has been validated across multiple tissue types, including renal tissue.^10,11 Thermal cell injury is known to occur progressively at sustained temperatures above 43°C. ¹² Our study demonstrated that under high-powered laser settings, calyceal and parenchymal temperatures exceeded 43°C in as little as 15 seconds. Specifically, during dusting (0.3 J, 50 Hz, 15 W), parenchymal temperatures exceeded 43°C in 1/20 patients (Ho:YAG patient), reaching 49°C after 60 seconds at the 15-mm marker. At the 25-mm and 35-mm markers, the mean peak temperature remained below 37°C for both Ho:YAG and TFL, with no significant differences between the two groups. However, during pop-dusting (0.5 J, 80 Hz), parenchymal temperatures exceeded 43°C in 5/20 patients (2 Ho:YAG patients and 3 TFL patients), all occurring at the 15-mm marker. At the 25-mm and 35-mm markers, mean peak temperatures remained below 37°C across both groups. Notably, with pop-dusting, parenchymal temperatures exceeded 43°C in as little as 20 seconds.

Conversely, findings from an in vivo porcine study indicated that temperatures surpassing 43°C did not penetrate deeper within the kidney. ⁴ Even under the most unfavorable conditions—including prewarmed irrigation, absence of a UAS, and the use of a single-lumen ureteroscope—renal temperatures remained below 43°C. This phenomenon is likely attributable to increased ureteral peristalsis and the heat-sinking properties of porcine renal tissue. Our findings from the first in vivo assessment of human renal parenchymal temperature during RIRS with LLT highlight the importance of optimizing laser settings to minimize the risk of thermal injury while maintaining procedural efficacy. In addition, our findings of elevated parenchymal temperatures during lithotripsy, coupled with the advent of new high-powered lasers, underscore the urgent need to study the associated risk of thermal damage.

Calyceal and parenchymal temperatures are influenced not only by transmitted energy but also by the surrounding medium and flow dynamics, including renal pelvis anteroposterior diameter (APD) and irrigation/outflow characteristics. In vitro studies have demonstrated that decreasing irrigation flow rate leads to elevated calyceal temperatures. ¹³ Specifically, Wollin et al. reported that with no flow, all maximum temperatures exceeded 43°C, surpassing those observed with lower-energy settings of the Lumenis Pulse 100 W Ho:YAG at 0.8 J/8 Hz, 1 J/10 Hz, and 1 J/20 Hz (p < 0.05). The mechanism by which irrigation lowers calyceal temperature is attributed to heat transfer to the irrigant as it passes over the lasered stone. ¹⁴ In a similar in vitro study, the highest temperature recorded was 70.3°C, occurring with no irrigation after 60 seconds of continuous laser firing (pulse 120 W Ho:YAG, 1.0 J × 40 Hz). Notably, fluid temperature in the test tube was measured via a thermocouple positioned 5 mm from the tube’s bottom. ¹⁵ These in vitro studies employed a thermocouple within a static fluid environment (tube or water bath), lacking outflow. When extrapolating in vitro findings to in vivo conditions, the interplay between irrigation and outflow becomes critical. ⁸ An in vivo porcine study investigating various irrigation and outflow scenarios, including single- vs dual-lumen ureteroscopes, warmed vs room-temperature irrigation, and with/without a 14 F UAS with the SuperPulsed TFL, identified the use of a UAS as the most significant factor affecting intrarenal temperature. Temperatures reached or exceeded 44°C across all laser and irrigation settings when a UAS was not employed. ⁴ The present study, conducted without UAS, using room-temperature irrigation and single-lumen ureteroscopes, represents a “worst-case scenario.” Although previous research suggests an inverse correlation between intrarenal temperature and APD, this association was not observed in the current study. Although APD can reflect renal pelvic volume, it does not consistently predict renal outflow. This discrepancy may stem from variations in temperature measurement methodologies.

We acknowledge several limitations in our study. To ensure a standardized methodology across laser types, we performed RIRS without a sheath and with gravity irrigation via a single-lumen ureteroscope—which is our standard method. We acknowledge that this is a “worse-case scenario,” but RIRS is commonly done in this manner, including at our own center. Although this approach enhances comparability, it may reduce the generalizability of our findings. Despite the limited sample size, the qualitative data provide compelling evidence of clinical relevance. Despite these limitations, we believe our study is well designed and provides valuable insights into an underexplored topic such as temperature dynamics during URS, particularly within the renal parenchyma.

Conclusions

High-powered settings in both Ho:YAG and TFL raise greater concerns regarding calyceal and parenchymal heating (>43°C) and the risk of renal injury compared with low-powered settings. The duration of high-powered energy activation should be carefully considered. Our study highlights the need for further research to explore parenchymal temperatures and their possible association with renal injury.

Authors’ Contributions

K.G.: Conceptualization and study design. C.C., V.D., and K.G.: Data collection. C.C., K.G., and D.L.: Data analysis. K.G., K.G., D.L., and Z.S.: Writing. R.K., W.M.A., B.G., K.G., and M.G.: Review and editing.