KTP High Power Laser-Tissue Interactions: In Vitro Experiment and Simulation

Abstract

Benign prostatic hyperplasia (BPH) is one of the most common disorders and the most common cause of lower urinary tract symptoms (LUTS) in elderly men. Transurethral resection of the prostate (TURP) is considered the gold standard treatment for BPH; however, laser prostatectomy has several advantages over TURP with regard to reduced catheterization and morbidity particularly in high-risk patients. Of the many lasers that can be used in the laser prostatectomy, the GreenLight potassium-titanyl-phosphate (KTP) laser is one of the most commonly used. The optimum outcome of this laser procedure is ablation with minimal coagulation. Our objective was to experimentally and theoretically characterize the KTP laser-tissue interactions in order to understand the laser and tissue parameters that lead to an optimal outcome. The rat hind limb muscle was used as a model for the in vitro study. Q-switched 532 nm laser (GreenLight PV, American Medical Systems, MN) was used and the laser was allowed to scan over the tissue with controlled speed and working distance, distance from the tip of the optical fiber to the tissue's surface. Injury was assessed by digital images of the tissue's cross section in terms of ablation, zone of removed tissue, and coagulation, zone of thermally denatured protein, zones. Ablation simulation was done in Matlab R2008a and the ablation was assumed to be a vaporization process. Thermal properties of water were assumed for the basic first run, where the ablation energy of tissue was assumed to be the vaporization energy of water. Parametric study involving changing the ablation energy, thermal conductivity, and absorption (attenuation) coefficient of tissue was performed. Ablated tissue volume increased with decreased scanning speed and trended to saturate at higher values of working distance and lower scanning speed. The optimal therapeutic outcome (minimal coagulation volume and maximal ablation volume) was achieved at 0 mm working distance and 1 mm/s scanning speed. The simulation was able to capture and predict the effect of the tested parameters (ablation energy, thermal conductivity, and absorption coefficient) on the therapeutic outcome. Notably, decreased ablation energy and thermal conductivity, and increased absorption coefficient were predicted to enhance the KTP laser ablation therapeutic outcome (i.e., reduce coagulation volume and increase ablation volume). This will open avenues to improve KTP laser prostatectomy by modulating optical, thermal, and mechanical properties of prostatic tissue.