Laser technology in urology is currently used for both stone lithotripsy and prostate enucleation. Thulium fiber laser (TFL) is a novel laser, with initial studies showing potential benefits over other lasers both in terms of its effectiveness and safety profile.
In the first part of this review, a descriptive analysis of the theoretical concepts behind TFL was performed. This part focused on the physics and laser parameters as applied to the clinical practice. These were interpreted in the context of other lasers, namely, the Holmium:YAG laser to highlight the theoretical advantages as well as potential pitfalls offered by the TFL. In the second part of the review, a narrative synthesis of in-vitro studies regarding TFL and its modifications is performed assessing stone-related parameters, namely, ablation rate, operative time retropulsion, and safety.
TFL achieved high ablation rates in most studies and performed better than Holmium:YAG laser across a range of different settings and ablation modes when the two lasers were compared. Moreover, its ability to use low pulse energy ensures minimal stone retropulsion with the retropulsion threshold estimated to be 2–4 times higher than that of Holmium:YAG laser. From a safety viewpoint, TFL poses no additional risks than other lasers, although it does potentially lead to slightly higher temperatures in the surrounding tissues during lithotripsy.
The unique properties of TFL have made it an attractive alternative to conventional laser techniques currently used in urology. Clinical studies are required before its application can become more widespread.
Keywords: BPH, fiber laser, fragmentation, laser, stones, thulium, urolithiasisThe first use of lasers in urology was reported in 1976 by Staehler et al. [1] whereby an argon laser beam was experimentally tested on the bladder wall. Advancements of fiber-optic technology, newer digital ureteroscopes, and novel laser techniques have enabled modern lasers to be used for various indications, most notably for lithotripsy and prostate enucleation.
Holmium:YAG laser remains the most commonly used laser technique in urology, which was first investigated 30 years ago and subsequently introduced in clinical practice in 1993. [2] Its ability to cut and coagulate tissue, multipurpose usability, and suitability to be used with modern endoscopes meant that Holmium:YAG laser technology quickly gained popularity. [2]
As technology improves, the quest for the better laser technology with safety and efficacy in mind has led to the evolution of thulium fiber laser (TFL), which has started to achieve due recognition with the first experimental lithotripsy studies conducted in 2005. [3] Initial results from ex vivo studies have been quite promising with improved and quicker stone ablation and reduced retropulsion [3] among its numerous advantages over more conventional lasers, although these findings are yet to be replicated in clinical studies.
The aim of this review was to examine the theoretical and technological aspects behind the TFL and how these correlate with its use in clinical practice, thereby assessing available evidence from in-vitro and clinical studies. A systematic review of data regarding TFL lithotripsy will be conducted at the end of this article.
We look at the theoretical aspects of the TFL as well as the laser parameters and their application in the clinical setting. The review article was based on a search of various bibliographic databases including MEDLINE, EMBASE, Cochrane Controlled Registered of Trials, and Google Scholar. They were searched for relevant English language studies published anytime. The keywords “thulium fiber laser,” “TFL,” “lasertripsy,” and “lithotripsy” were used. Boolean operators (AND, OR) were used to refine the search. Chain searching of references of all included articles was performed to identify further relevant articles. Because of heterogeneity of published data, a meta-analysis of the various clinical and technological parameters was not possible; thus, a narrative synthesis has been carried out.
Thulium is a rare-earth element, which exists in the trivalent state (Tm 3+ ). [4] It is silvery grey in color and rather soft and malleable. [4] It undergoes slow oxidation in air and melts at a temperature of 1550°C to form Thulium oxide (Tm2O3). [4]
The infrared light emitted by the TFL has a typical wavelength of 1940 nm, although it can vary between 1810 nm and 2100 nm, depending on the design of the TFL. [5] This is in contrast with other lasers used in urology, namely, the FREDDY laser, which has a wavelength of 532–1064 nm and the more popular Holmium:YAG laser with a wavelength of 2100 nm. [5] The water absorption coefficient (WAC) determines how well the infrared radiation emitted by a fiber laser is absorbed by water and consequently the efficiency of stone ablation. [6] TFL has a WAC μa=129.2 cm −1 , whereas its wavelength closely matches the water absorption peak. [7] This is potentially clinically advantageous for two reasons, namely, that it theoretically leads to more efficient lithotripsy while keeping photothermal damage to surrounding tissue to a minimum. To put this into context, Holmium:YAG laser’s WAC at a wavelength of 2100 nm is μa=28 cm −1[8] , which means that water absorbs TFL energy approximately four times higher than it does with Holmium:YAG laser energy ( Figure 1 ). Like any other laser, TFL’s WAC is temperature dependent, with an experimental study showing a linear decrease in WAC as the temperature is increased from 20°C to 80°C. [9] The opposite effect was observed at a wavelength of 1920 nm, [9] which raises the prospect of whether manipulating laser wavelength can further optimize TFL’s ablation efficiency as tissue is heated toward vaporization.
Graph showing relationship between wavelength (nm) and water absorption coefficient (mm −1 ) for TFL and Holmium:YAG laser [10]
In a traditional TFL, the pump energy is provided by the laser diodes as compared with a flash lamp pump used in a Holmium:YAG laser. These semiconductor devices work by converting electrical energy to optical energy, which is then used for the excitation of Thulium ions. [10] Laser diodes have several advantages over flash lamps, namely, that it can operate at a lower power and is smaller in size. Moreover, the presence of an air cooling system, as will be explained below, overcomes any thermal issues associated with laser diodes. [10] The main pitfall with such energy source is that it has a critical heating problem, which can result in thermal stress to the diode, although this is more likely to become an issue with high-powered industrial devices. [11]
Most of the high power fiber laser consists of a rare-earth-doped optical fiber coated with a low-index polymer as shown in Figure 2 . Multimode laser diodes (pump) are launched at one end of the active fiber. The pump is guided by the cladding and subsequently absorbed by the rare-earth ions doped in the core. The emission of rare-earth ions, spatial distribution of light in core, and fiber Bragg gratings ensure a stable laser output beam. Fiber laser can be considered as a device that converts low brightness of laser diodes into a high brightness source, defined by the waveguiding properties of the core. [12]
Schematic representation of a cladding pumped fiber laser
In the case of TFL, the core of the optical fiber is doped with Tm2O3 and additional dopants such as Al2O3 in a silica matrix to avoid quenching of rare-earth ions. Both Al2O3 and Tm2O3 are refractive index increasing components; hence, they constitute a high refractive index core of fiber. In contrast, cladding is normally pure silica and the outermost cladding is a polymer coating. The fiber is cladding pumped at 793 nm, which results in the excitation of thulium ions within the core region, which is typically 10–20 μm in diameter fiber. Consequently, a laser beam is generated with a wavelength of ~1940 nm. Furthermore, a small cross-sectional area of the silica fiber is also very important as it allows extreme deflection of flexible ureteroscope to perform difficult lithotripsy procedures such as in the case of lower pole stones and also allows optimal irrigation through the miniature working channel. [13]
The thermal distortion of the fiber laser output beam is negligible as compared with the solid-state lasers, thanks to the large surface-area-to-volume ratio offered by fiber. [14] Furthermore, a small core size in thulium-doped silica fiber ensures a near-diffraction limited output beam profile of the TFL unlike the Holmium:YAG laser ( Figure 3 ). In practical terms, this means that a laser beam can be focused onto a small spot, resulting in more efficient tissue ablation or lithotripsy. Moreover, the beam profile offered by the TFL would allow delivery of high power laser beams through a very small core fiber, 50–200 μm in diameter. [15]
Spatial collimated beam profile of 30-W TFL in continuous wave operation. Adapted with permission from Kneis et al. [14] © The Optical Society.
The initial spike in the temporal beam profile of TFL is less steep and shorter than that for Holmium:YAG laser. [15] This initial energy spike produces a laser beam, which is directly related to the size of the vapor bubble produced, which, in turn, translates to the pressure exerted on the stone and subsequently its retropulsion. The lower energy spike produced by TFL ensures a more uniform temporal beam profile and, consequently, a more even energy distribution. [15] In a study of bubble dynamics, Hardy et al. [16] and Gonzalez et al. [17] showed that the vapor bubble produced by TFL was four times smaller than that of Holmium:YAG laser owing to lower peak power and smaller core fibers.
The pulse energy for current TFL ranges between 0.025 and 6 J. The provision for low energy setting allows the production of finer particles during dusting and reduced stone retropulsion as the stone vibrates rather than recoils during ablation. [17]
TFL can reach pulse frequencies of 2100 Hz, which is about 21 times more than the maximum frequency achieved by a Holmium:YAG laser. [7,18] This high pulse frequency is probably unnecessary, and the highest repetition rate reported in medical literature currently is 500 Hz. [19] The advantage of such high frequencies is that it can be used in combination with low pulse energies in a highly efficient stone dusting mode. [19]
TFL pulse duration settings range from 200 μs to 1100 μs, thereby allowing the operator to use the TFL in both short and long pulse modes. Studies have shown that pulse width does not affect ablation volume, although a difference in crater size is observed between the short and long pulse mode. [20] The short pulse mode will, however, lead to more stone retropulsion and more fiber-tip degradation, which is one advantage of TFL over Holmium:YAG laser as the latter is restricted to the short pulse mode. [21] Furthermore, this reduction in retropulsion decreases the non-contact lithotripsy phase by 3.5 times, improving the fragmentation efficiency in the popcorn mode and decreases the overall operative time. [22]
TFL can operate at a high peak power, with newer models reaching up to 500 W (superpulse mode), far exceeding those that can be achieved with the Holmium:YAG laser. This is possible because of less heat dissipation with TFL as compared with Holmium:YAG laser. [22] Nevertheless, the clinical utility of such high power is debatable as lithotripsy settings rarely exceed powers of 30W and the risk of collateral damage to surrounding tissues, at high powers, must also be acknowledged.
TFL uses an air cooling system as compared with Holmium:YAG laser, which uses a water cooling system. [25] This is possible because the TFL is more energy-efficient and, therefore, an air cooling system is sufficient to dissipate any residual heat energy. [20,26] Moreover, additional benefits of using a fan ventilation cooling system are that it allows the TFL to operate at a higher power and also reduces the size of the generator required when compared with the Holmium:YAG refrigeration system. It is worth noting that for power generators >50 W, an external water cooling system is still required. [26]
In an era of cost-cutting and sensible spending within health care, the TFL appears to fit the bill for a variety of reasons. Unlike Holmium:YAG laser, TFL can operate using the main power supply and does not require a high amperage power outlet. [28] The lifetime of a laser diode is estimated to be around 70,000 h, [29] and it is less susceptible to wear and tear unlike flash lamps in Holmium:YAG lasers. The uniform laser beam profile and certain modifications to the silica core fiber such as a hollow steel tip reduce fiber-tip degradation secondary to burn-back, which makes TFL fibers reusable, thereby also highlighting its environmental credentials. [30] Although theoretical expenditure for both production and maintenance of TFL appears to be lower than that of other lasers, detailed cost-effectiveness analysis within clinical studies will be required before its use becomes more mainstream.
Ocular injury remains the primary concern with the use of lasers. This potential hazard has been extensively investigated with regard to the use of Holmium:YAG lasers, with studies showing that eye injury is rare and three conditions need to be satisfied for eye injury to occur, namely, high energy settings, short distance (1400 nm, most of the absorbed radiation is restricted to the cornea and not the more sensitive retina, with cataracts and corneal abrasions, therefore, being the more likely sequelae. [31]
Another potential hazard is the risk of the thermal injury such as skin burns. It is unclear whether TFL poses any additional risks to those already specific to laser fibers. It is likely that any potential hazard is due to factors unrelated to TFL’s intrinsic characteristics, such as the absorption and scattering coefficients of the skin, irradiance of the laser beam, duration of the exposure, and size of the area irradiated. [31]
Multiple in-vitro analytic studies, assessing the characteristics of TFL for lithotripsy have been published ( Table 1 ). It was not possible to combine the results of the different studies because of variable laser settings, experimental setup, and different core fibers, and therefore, a narrative synthesis and analysis regarding the different outcomes can be found below.
List of studies on thulium fiber laser lithotripsy including laser settings and outcomes (Continue)
➢ Ablation rate: TFL had higher ablation rate than Holmium:YAG at all combinations of pulse energy and frequency
