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XVII. Basic Principles: Laser Settings
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Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Electrons gain energy from an external source of energy; this energy can then be released as photons. The emitted photons have significant temporal and spatial coherence, delivered in a narrow beam. This allows a high level of energy to be delivered onto an object—eg. a stone or mass. The medium used to create the specific wavelengths is how lasers are named: holmium, neodymium, KTP, etc. In urology, lasers are used for the treatment of nephrolithiasis, benign prostatic hypertrophy, penile cancer, penile condyloma, urothelial carcinoma, and urethral stricture disease. This article reviews the different lasers employed in urologic surgery and the typical settings for the most common uses of each laser.
The Holmium: YAG (Yttrium-Aluminum-Garnet) laser uses holmium put into a YAG crystal. It emits a wavelength of 2140 nm to deliver energy in a pulsatile fashion. This laser uses a thermomechanical mechanism of action, creating a bubble of vaporization at the tip of the laser fiber. This bubble expands and dissipates energy onto the object directly in front of the bubble. It can be used for BPH surgery, stones, urethral strictures, bladder neck contractures, and urothelial carcinoma (Figure 1). Holmium has a penetration depth of <0.5 mm.
Recommended settings are (Figure 2):
• Bladder tumors: 0.6 to 1.2 joules at a frequency of 8-10 hz
• Stones: Begin at 0.6 – 0.8 joules and a pulse rate of 6-8 hertz. The pulse rate can be increased for quicker fragmentation.
• Bladder neck contractures: 2 joules at 40 hertz
• Prostate ablation: 2 joules at 40 hertz
Figure 1: Holmium laser fiber, safety wire seen above stone, red light is to aim laser.
Figure 2: Holmium Laser Settings
The Neodymium:YAG laser uses neodymium put into a YAG crystal. This laser emits a wavelength of 1064 nm. It has a deeper penetration than and better coagulation compared to the holmium laser. It isn’t well absorbed by body pigments and works by coagulative necrosis with tumor sloughing rather than ablation. The Nd:YAG laser is best suited for the treatment of urothelial carcinoma. Of note, the most dangerous complication from using this laser thermal damage to structures on the other side of the tumor (eg. bowel). Nd:YAG has a penetration depth of 5-6 mm.
Recommended settings are:
• Bladder tumors: 20-30 watts for 2 seconds
• Renal pelvis tumors: 15-20 watts for 3 seconds
The KTP laser (potassium-titanyl phosphate) uses a Nd:YAG laser passed through a KTP crystal to create a green beam with a wavelength of 532 nm. This doubles the frequency and, therefore, halves the wavelength. It penetrates half the depth of the Neodymium but otherwise has characteristics of the Nd:YAG. The KTP laser can be used for BPH surgery, incision of urethral strictures, bladder neck contractures, and superficial penile cancer. The Nd:YAG remains better for urothelial carcinoma given its increased depth of penetration.
Recommended settings for the KTP laser settings are as follows:
• Photoselective vaporization of the prostate: 80 watts to 180 watts for vaporization; 20-30 watts for coagulation (Figures 3 and 4). Start at 80 watts to allow the fiber to warm up, then progress higher.
Figure 3: KTP laser with red light showing direction of beam.
Figure 4: KTP laser with beam activated.
The CO2 laser was one of the first lasers developed. The CO2 laser emits an invisible beam and has a wavelength of 10,600 nm. This wavelength is highly absorbed by water making the depth of penetration minimal (0.1 mm). However, the heat from this laser penetrates to 0.5 mm and can induce thermal coagulation. Given its significant absorption it is capable of incising and removing small superficial lesions. The CO2 laser is mainly used for superficial penile lesions such as condyloma and carcinoma.
Recommended settings are:
• Skin (eg. penile) lesions: setting: 5-10 watts. The laser is applied until the tissue turns white, signifying adequate vaporization.
Andrew Harris, MD
Cheif Resident, University of Pennsylvania