The European association of urology (EAU) guidelines recommended PNL as the first line treatment option for staghorn stones as it provides adequately sized tract to the kidney to retrieve large stone fragments with less morbidity and faster recovery when compared to open surgery. Nonetheless, in the setting of staghorn calculi, PNL is associated with high rate of high-grade complications including sepsis, severe bleeding necessitating blood transfusion, injury to the surrounding organs. In addition, multiple tracts and auxiliary procedures are usually required which is associated with increased morbidity [12]. Recently, there has been an emerging role of FURS with laser lithotripsy in treatment of large size renal sones because of its less invasive nature compared to PNL and utilization of high-power lasers with different pulse modulation that allow dusting of the stones into fragments without need of basketing or retrieval of the stone fragments. A recent meta-analysis evaluating outcomes of FURS for renal stones > 2 cm, the SFR ranged between 77 and 96.7% after completion of treatment, with an average of 1.6 procedures per patient [6].

In this study we investigated the feasibility, safety and efficacy of FURS with TFL lithotripsy for treating partial and complete staghorn stones with a median stone volume of 7339 mm3 and the initial results were very promising. On one hand, the true SFR (no RF) was 53% after the initial treatment which is close to that reported after standard PNL that ranged between 49 and 56% [3, 4, 13]. Taking into consideration that spiral CT was used in all cases to assess SFR. Park et al. found that X-ray and CT controlled SFRs can differ dramatically (62.3 vs. 20.8%) [14]. We considered only patients with zero fragments to be stone free excluding those with insignificant RF < 2 mm [15].

On the other hand, the rate of postoperative complications was 25% and all were of minor grades (Grade I, II). It’s noteworthy that median hemoglobin deficit was 0.2 gm/dl and no patient received blood transfusion. In addition, 93% of cases were discharged home at the same day of surgery and only 2 cases were admitted because of febrile UTI and were treated with antibiotics. The overall complication rate after PNL for staghorn stones ranged between 18 and 32%, blood transfusion was required in 6–16% of cases while high grade complications (Grade III-V) ranged between 8 and 10% and the median length of hospital stay ranged between 2 and 6 days in recently published series [3, 4, 13, 16, 17].

Thulium fiber laser (TFL) has recently been introduced in our endourological armamentarium provoking a great interest because of its potential advantages over the standard Ho: YAG laser in terms of higher absorption coefficient, less retropulsion, the ability to work at a very high frequency with low energy levels resulting in very efficient dusting and shorter operative time [3, 10, 18, 19]. It has been observed that ablation speed for TFL is 2 to 5 times higher than Ho: YAG even with similar pulse energy and frequency settings [20]. High frequency modes result in higher ablation speed and TFL can attain higher frequency of up to 1000 Hz while keeping energy low [21]. TFL produces uniform pulse energy similar to that of Moses technology resulting in formation of bubbles within a single laser pulse typically smaller than Ho: YAG leading to minimal retropulsion [10, 22].

In this study, we evaluated the performance of TFL lithotripsy by the analysis of laser efficacy (energy needed to ablate 1 mm3 of stone volume, Joules/mm3) and ablation speed (the stone volume divided by the laser active time, mm3/s) as a surrogate of lithotripsy efficacy [11]. We found that use of TFL for effective treatment of staghorn stones, with a mean volume of 7339 mm3, required a mean 7.5 J/mm3 of stone volume with average ablation speed of 1.3 mm3/sec and active laser emission 85% of the total operative time. In addition, a strong positive correlation between stone volume and ablation speed (r = 0.86) and a moderate negative correlation between stone volume and laser efficacy (r = − 0.6) so the higher the stone volume, the less energy required for ablation (lesser J/mm3 required). In contrast, stone density had weak correlation with laser time and laser energy denoting the efficacy of TFL lithotripsy even in hard stones. These results were consistent with previously published studies evaluating performance of TFL in large stones [23]. Enikeev et al. also noted no correlation between laser time and stone density while using TFL during PNL [18].

Regarding Ho: YAG laser, Ventimiglia et al. reported a median laser efficacy of 19 J/mm3 and ablation speed of 0.7 mm3/s found that for a median stone volume of 1599 mm3 using 35 W machine [11]. Majdalany et al. assessed efficiency measures for Ho: YAG with Moses technology and found that for a mean stone volume of 290 mm3, mean laser efficacy was 38.2 J/mm3 and the mean ablation speed was 0.9 mm3/s [24]. In a RCT comparing Ho: YAG and TFL during mini-PNL, the authors noted shorter stone fragmentation time and shorter operative time in favour of TFL [25].

Another issue that is usually blown out is the higher temperature rise with the use of TFL because of higher absorption of TFL energy. Taratkin et al. compared thermal effects between TFL and Ho: YAG laser in an in vitro model by measuring water temperature. Energy settings for both lasers were adjusted at 0.2 J and 40 Hz with laser firing for 60 s and different irrigation rates were used. The authors documented that no significant difference in temperature rise between both lasers [26]. Theoretically the increased heat production may cause thermal injury to the renal parenchyma especially with prolonged use. In our study, there was no significant change in the estimated GFR before and after the procedure. In addition, renal DMSA scan that was available to 13 patients didn’t demonstrate any cortical scarring or areas of reduced perfusion after the procedure denoting the safety of utilization of TFL for prolonged time to disintegrate large volume renal stones.

To the best of our knowledge, this is the first prospective study documenting feasibility, efficacy and safety of FURS with TFL lithotripsy for treating exclusively staghorn stones with high success rate and minimal morbidity. Moreover, the vast majority of cases (93%) were discharged home safely at the same day of surgery with significantly less morbidity and shorter hospital stay. We also, reported efficiency measures of TFL to provide an arm for comparison of different laser types in future studies. Our study isn’t devoid of limitations, first is the small sample size but we report our initial experience and the feasibility to effectively manage large complex stones with TFL. Second, lack of comparison and randomization between FURS and the gold standard PNL for management of staghorn stones, however this issue should be discussed in future multicenter RCT. Lastly, long-term assessment of renal function wasn’t done to investigate whether the thermal effect of TFL especially when used for long time will affect renal function or not.