Most urologists have heard the buzz around the thulium laser for lithotripsy. With numerous advancements in laser technology and several new platforms hitting the market, the options can be overwhelming. Is thulium worth the hype? This guide will summarize the key features of thulium laser technology and review the initial clinical data.

Thulium Laser Mechanism and Advantages

To understand the advantages of the thulium laser, a quick physics lesson is helpful. The optical LASER (for Light Amplification by Stimulated Emission of Radiation) begins by passing white light through an amplification medium, where electrons within the medium become excited. During relaxation, photons of a particular wavelength are released and reflect within a mirrored chamber to stimulate emission of more photons and generate a laser beam.

For over 20 years, the gold standard of laser lithotripsy has been the holmium:yttrium-aluminum-garnet (Ho:YAG) laser, which uses an optical laser core. In contrast, newly developed diode lasers pump electrons into charged semiconductors to produce photons, which resonate within the semiconductor junctions to perpetuate the conversion of electrons to photons. Diode lasers are utilized in the solid-state design with thulium:YAG (Tm:YAG) crystals and in the thulium fiber laser (TFL) design, where the laser energy is generated within a thulium ion doped fiber. These changes have allowed lower-energy lasers to produce a wider range of parameters for laser lithotripsy and confer several advantages. The parameters of currently available thulium laser platforms compared to Ho:YAG are summarized in the Table.

Table. Comparison of laser parameters²

Thulium lasers offer wavelength with higher water absorption and a wider range of pulse energy, pulse duration, frequency and fiber diameter.
*Currently not available in the U.S.

Power requirements

The first major advantage of the thulium laser is its lower power requirement. The Ho:YAG laser requires a flash lamp light source to excite electrons of holmium ions in the optical laser core. More than 90% of the energy emitted by this flashlamp is not absorbed by holmium ions and is lost as heat.¹ This loss of heat necessitates a water cooling system, which increases the size and mass of the laser device. In contrast, thulium diode lasers are powered by a voltage differential. The minimal energy lost as heat can be cooled with a fan cooling system.² With these lower power requirements, thulium lasers are smaller, quieter and can function using a standard power outlet (120–240V).

Optical wavelength and water absorption

In general, laser lithotripsy utilizes the photothermal expansion of water molecules within a stone’s pores and fissures to cause fragmentation. Ho:YAG lasers emit light at 2,120 nm, which when absorbed by water results in a penetration depth of about 0.4 mm.³ Thulium lasers, by contrast, can more closely match the water absorption peak of 1,940 nm and achieve up to 4 times greater absorption (see Table). These higher absorption coefficients allow for a lower fragmentation threshold and a shorter depth of beam penetrance as the energy is quickly absorbed. Therefore, stone fragments can be smaller with finer dust produced.

Pulse energy, duration and frequency

The energy released with every pulse is directly related to the mass of stone loss.⁴ However, high-energy pulses can cause retropulsion and fiber tip degradation, thereby limiting the miniaturization of fiber diameter.^1,5 A counteracting parameter is the pulse duration, or the length of time the laser is on. Longer pulses result in less fiber tip degradation, up to 50% less retropulsion and up to 60% more effective ablation of softer stones.^4,5 Solid-state optical lasers such as holmium are limited in their ability to produce low energy, long-duration pulses due to the flashlamp mechanism of recharging the laser core. Use of diode lasers like thulium allows a wider range of pulse energy and duration. To achieve efficient stone ablation, newer models can achieve much higher frequencies (number of pulses per second), which may further improve dusting performance. While thulium lasers can reach frequencies of up to 2,400 Hz, frequencies of less than 300 Hz are often recommended to avoid thermal buildup.⁶

Figure. Absorption coefficients of water (1,800–2,400 nm).^3,14,19 Thulium lasers utilize wavelengths that closely match the peak of water absorption within the range of light transmitted in silica optical fibers (<2,700 nm).

Fiber Diameter

Thulium lasers offer fibers as small as 150 μm. Smaller fiber diameters allow for greater ureteroscope flexibility and improved visibility due to increased irrigation through the working channel.² An additional advantage of thinner fibers is greater energy density for efficient fragmentation and less retropulsion.¹ Prior laser models were limited in small fiber diameter by the large pulse energy. Newer lasers can utilize thinner fibers with lower pulse energy and longer pulse duration to optimize stone ablation and reduce retropulsion.

Clinical Outcomes of Thulium

Pre-clinical studies corroborate the dusting capabilities of thulium lasers. In a direct comparison of Tm:YAG and Ho:YAG at similar frequency, energy and pulse duration settings, Ho:YAG outperformed Tm:YAG by 14%. However, when utilizing Tm:YAG’s capabilities for higher frequencies and pulse duration, there was 32% advantage in stone ablation over Ho:YAG.⁷ These findings suggested an efficient and customizable alternative to Ho:YAG.

Hands-on experiences with thulium lasers also support its theoretical advantages over holmium. TFL has been shown to dust all urinary stone types with maximal width of stone dust not exceeding 0.254 mm.⁸ TFL is associated with up to 4 times deeper ablation craters for equivalent pulse energy and frequency, up to twice as fast in fragmentation, up to 10 times as fast dusting and less user-reported retropulsion.^2,5,9,10 Stone-free rates with TFL in recent clinical experiences range between 85%–100%.⁹

Two randomized clinical trials published this year have compared thulium to holmium. The first study from Russia of 174 patients with ureteral stones showed that TFL was associated with significantly faster operative time (25 vs 32 minutes), lasering time (8 vs 16 minutes), less retropulsion (4% vs 69%), clearer view (87% vs 64%) and decreased length of hospital stay (2.4 vs 3.2 days).¹¹ The second study from Norway of 119 patients with ureteral and/or renal stones showed that TFL had shorter operative time (49 vs 57 minutes), renal stone stone-free rate (86% vs 49%) and decreased bleeding that obscured the endoscopic view (5% vs 22%).¹² Experts with clinical experience in TFL for laser lithotripsy have touted the efficiency of TFL in dusting, but may prefer Ho:YAG for fragmentation with the current techniques and settings.¹³ Some harder stones such as cysteine and calcium oxalate monohydrate can appear to char or even melt when performing thulium laser lithotripsy.

A major limitation of thulium lasers is the temperature safety concern. At frequencies below 300 Hz, water temperature rise is comparable to that of holmium and there is low risk of thermal damage.^6,14 Nevertheless, 2 studies have shown ureteric thermal damage in porcine models, especially at higher frequencies where temperatures in the renal pelvis exceeded 44C.^15,16 Concerns about thermal damage led to an Food and Drug Administration recall of the Olympus Soltive^® TFL laser in June 2021 that issued a software update to include a low power (8 W) ureteral preset and to warn about use in sensitive tissues (eg ureter) when selecting settings greater than 20 W.¹⁷ For now, clinical data show low complication rates (5%–6%), few cases of severe complications and no published reports of severe tissue thermal damage during surgery.^2,9,18

Thulium: Worth the Hype?

Holmium laser lithotripsy has been the gold standard due to high efficacy, favorable safety profile and ubiquitous surgeon experience. However, holmium has been challenged by thulium lasers in recent years. Thulium laser systems (Tm:YAG and TFL) offer low-power, compact and versatile alternatives. As experience with thulium grows and clinical data emerge, laser settings and safety parameters will continue to be optimized. It is important that urologists become familiar with these laser systems in order to deliver customized care on a case-by-case basis. Highly efficient lithotripsy with thulium lasers may allow endourologists to push the boundaries for dusting techniques in ureteroscopy and mini percutaneous nephrolithotomy. Future studies will further refine optimal clinical indications, device settings and cost-effectiveness.