The first yttrium aluminum garnet (Y₃ Al₅ O₁₂) laser was developed by Bell Laboratories in 1962 [12]. While other solid phase lasers have been created, for the purposes of BPH laser surgeries, the YAG laser is the base model for all of the subsequent solid phase lasers, because of its efficiency, optical quality, and high thermal conductivity, which permits high rates of repetition.

Doping the YAG crystal with the rare earth element holmium instead of neodymium changes the physical characteristics of the laser and causes a 2100 nm beam to be emitted, near the absorption peak of water. The effect is a laser that superheats water, creating a vaporization bubble at the tip of the delivery fiber. The bubble expands rapidly, destabilizing molecules in the tissue it contacts, tearing the tissue apart in a photomechanical fashion, followed by tissue evaporation. The absorption depth in tissue is 0.4 mm, allowing for a more precise incision than the prior Ng:YAG based lasers. The holmium: YAG laser provides excellent hemostasis, especially if delivered in a pulsed mode [13]. It was first introduced in experimental urology in 1990 and the first human use of the holmium:YAG laser was described in 1992 [17]. With time the holmium:YAG lasers have been offered in sequentially more powerful versions, originally in 20 W, now available from 50 to 120 W.

The thulium laser was developed with the intent to more precisely match the water absorption peak in soft tissue. When the YAG crystal is doped with thulium it emits around 2000 nm wavelength laser, with a 0.25 mm depth of penetration. The result is a laser with similar hemostasis as the holmium laser, with minimal collateral tissue damage. It is administered at a higher energy setting and a continuous rather than a pulsed mode, arguing for more efficient tissue vaporization [13]. The first reported use of the thulium: YAG laser in human prostates was in 2005 [18]. Similar to the other lasers, subsequent higher energy thulium lasers have been introduced, now available in up to 150 W.

Distinctly mechanically different from the YAG based solid lasers are the semiconductor diode lasers. Laser light is produced using light-emitting diodes (LEDs) between reflecting mirrors in a resonator tube. They are smaller, more energy efficient, and less expensive than most other lasers now in use. The semiconductor laser wavelength can be tuned by various modifications. In general, their depth of penetration (0.5–5 mm) is more than of the 532 nm, holmium and thulium lasers, although the exact penetration depth is dependent on the wave length [13]. In 1996, interstitial laser coagulation of the prostate was performed using an 830 nm diode laser [19]. In 2007 a 980 nm diode laser was used for BPH [20]. Initially offered in 120 W, the diode laser became available in 200 W in 2009. In 2013 a 1318 nm diode lasers (Eraser) was eventually introduced for BPH [21].

Interstitial laser coagulation with Nd:YAG laser was an early development in the treatment of BPH. First described in 1993, the main feature of this method was preservation of the prostatic urethra and its urothelium [22]. The procedure is performed by placing laser-diffusing fibers directly into the prostatic adenoma, either via the transurethral cystoscopic approach, or the perineal approach. Laser energy then produces coagulation necrosis within the adenoma, which subsequently undergoes atrophy [23]. This method is safe in anticoagulated patients, but substantial tissue edema occurs with this method resulting in prolonged (7–21 days) postoperative catheterization. Retreatment rates were as high as 20% at 2 years, and 50% at 5 years. Due to the recognized limitations of the interstitial laser coagulation technique several authors have concluded that this modality should probably be restricted to selected, high-risk patients [24]. Ablations and vaporizations.

While the actual physiological mechanism differs, based on the physical properties by which the laser cause tissues injury as described above, the core idea with ablative and vaporizing techniques is the same. With ablation or vaporization dissolution of the tissue from the urethra and the adenoma occurs, thus shrinking the prostatic volume. Initially described in the 1980s, only a handful of ablative/vaporizing cases were reported until the invention of the side firing laser fiber in 1990 after which its use increased exponentially [25].

One of the first ablative techniques to take advantage of the side firing laser was the transurethral ultrasound-guided laser-induced prostatectomy (TULIP) , described in 1991 [26] with first clinical results reported in 1993 [27–29]. With the TULIP technique, ultrasound was used to guide a side firing fiber with a Nd:YAG laser . By lasing the prostatic adenoma transurethrally an area of heat-induced coagulative necrosis is created, which extends approximately 1 cm into the tissue. To unobstruct the prostate, TULIP relies on coagulation necrosis of the BPH with subsequent tissue sloughing. While it had reasonable outcomes, TULIP was eventually abandoned as it was and both costly and cumbersome to execute.

Another early technique, was visual laser ablation of the prostate (VLAP ) using Ng:YAG laser, first described in 1993 by Norris et al. Long term results were reported in 1995 [30]. Unlike TULIP , which utilized ultrasound guidance, in VLAP the laser was visually guided through a transurethral scope, but otherwise both VLAP and TULIP utilized the same tissue and treatment principles. Furthermore, VLAP was easier to learn and perform than TULIP , but it was limited to prostates 40 g and smaller. Delayed sloughing and edema of the tissue caused by the VLAP procedure lead to irritative lower urinary tract syndrome (LUTS) and urinary retention requiring catheterization in up to 30% of cases, extending in some patients to 3 months [30]. Thus, despite the benefits and ease of use, VLAP fell out of clinical utilization.

The KTP laser was initially introduced in BPH as an adjunct to VLAP, where it was used to make a bladder neck incision at the end of the case [31]. The first pure KTP vaporization procedure, also known as photoselective vaporization of the prostate (PVP ) , using a 60 W laser and a side firing laser fiber was described in 1998, [32], with the same authors reporting 2 year outcomes in 2000. An 80 W laser was described in 2003 with 2 year results in 2005 [33]. A year later the LBO 532 nm laser (GreenLight HPS) was introduced and the 2 year data was presented in 2010 [34]. In 2011, a case was described using the most recent version, the 180 W LBO laser (Green Light XPS). With the 180 W LBO laser, a new design of a side firing laser fiber (MoXy) was introduced. It included inbuilt saline circulation for cooling and laser fiber preservation, increasing fiber longevity. The combination of the 180 W LBO laser with the fiber was reported to vaporize tissue at twice the speed that could be achieved with the 120 W laser. The GOLIATH study, comparing the 180 W LBO laser vaporization to TURP at 2 years in a randomized trial, was published in 2016, exhibiting efficacy and safety outcomes to be similar between the two procedures [35].

The holmium:YAG laser entered the BPH world in the same fashion as the KTP, as an adjunct to Ng:YAG VLAP. The next step was prostate vaporization in the same ‘painting’ fashion as the Nd:YAG and KTP lasers , reported in 1996. Although the procedure (called HoLAP, holmium laser ablation of the prostate) which utilizes straight rather than a side firing fiber was easy to learn and effective, it was too time consuming when dealing with larger prostates [36]. For glands smaller than 60 mL it has been shown to have similar long term outcomes compared to PVP [37]. Of note, no new data has been published on HoLAP since 2013.

Thulium lasers have been more utilized in resection and enucleation and limited reports exist on thulium abalation/vaporization.

Diode laser vaporization of the prostate (DiVAP ) , has been reported with surgical techniques comparable to other vaporization procedures [20]. Although tissue incision is feasible with diode lasers, given the lasers depth of penetration, avoiding deep coagulation can be challenging. Unfortunately, the available evidence on diode lasers is mostly based on low-quality studies with small patient cohorts, making comparison to other laser ablative techniques difficult [38]. However, in a prospective comparison with 120 W GreenLight PVP it was found have better hemostasis, but higher re-treatment rates and complications [39]. In order to address the limitations of DiVAP, a new straight firing quarts coated fiber (Twister fiber) was introduced that does not project a laser beam, but concentrates the energy at the fibers tip. A randomized trial showed that the quartz tipped head used for DiVAP showed similar efficacy as the old fiber, with decreased over-all complications, dysuria and tissue sloughing [40]. Long term outcomes are currently pending.

Prior to the introduction of the more powerful lasers, ablations and vaporizations were frustratingly slow and inefficient. Therefor this led to the development of prostate laser resections , most notably HoLRP (holmium laser resection of the prostate) which basically simulates traditional TURP [41]. Thulium laser skipped the ablation technique and was first presented in BPH as the thulium laser resection of the prostate tangerine technique [18], followed by simultaneous resection of TURP-like chips and vaporization of tissue, which was proven to be safe and effective [42]. While these technique have acceptable long term outcomes, they have been compared to and found to have inferior results to enucleations with the same lasers [43].

Morbidity aside, simple open prostatectomy has superior long term outcomes when it comes to BPH, especially with large prostate glands. Laser enucleation essentially follows the anatomic principal of a simple prostatectomy via a transurethral approach and without a cystotomy or skin incision. Fraundorfer and Gilling developed the first of these procedures, the Holmium Laser Enucleation of the Prostate (HoLEP) in 1998 [44]. Long term follow up of up to 10 years is available with durable decrease in International Prostate Symptom Score, flow rates and post void residuals. Long term risk of a bladder neck contracture or urethral stricture is 2–7% and retreat rates for adenoma regrowth <1% [45, 46]. Additionally, long term outcomes of a randomized trial comparing HoLEP with TURP demonstrated HoLEP to be at least equivalent to TURP in the with fewer re-operations being necessary [47]. One of the main criticisms of the HoLEP procedure is a long learning curve, limiting its dissemination to the community setting.

Enucleations using similar technique but different lasers have since been described. Enucleation using a thulium laser was described in 2009 [48], 980 nm diode laser in 2010 [49], KPT in 2013 [50] and 1318 nm diode laser in 2013 [51]. Of these, only the thulium vapo enucleation has published 2-year outcomes data, revealing similar data to those reported for HoLEP at that time point [52].