Modeling photocoagulation of the prostate during exposure to laser radiation

Laser prostatectomy is an alternative to conventional surgical resection for the treatment of benign prostatic hyperplasia. This work involves the development of a three-dimensional finite element model to predict temperature distributions, and tissue damage in laser irradiated prostate tissue. The model solves Penne's bio-heat transfer equation with the addition of a heat source term due to the absorption of light from a scanning Gaussian profile laser beam. Experiments were conducted to measure the optical and thermal properties of canine prostate tissue, and to determine the effects of thermal coagulation of the tissue on these properties. The optical absorption and anisotropy coefficients were found to decrease slightly with increasing exposure temperature up to 65{dollar}spcirc{dollar}C, then increased sharply. Scattering coefficients increased slowly from baseline temperatures to 60{dollar}spcirc,{dollar} followed by a sharp increase between 60{dollar}spcirc{dollar} and 70{dollar}spcirc{dollar}C. The thermal conductivity, density, and heat capacity of canine prostate tissue did not change significantly during tissue coagulation. Quantitative measurements of prostatic blood flow were obtained before, during, and after laser irradiation using the radiolabeled microsphere technique. An increase in total prostatic blood flow (ml{dollar}cdot{dollar}min{dollar}sp{lcub}-1{rcub}{lcub}cdot{rcub}gsp{lcub}-1{rcub}){dollar} from 0.17 {dollar}pm{dollar} 0.03 to 0.38 {dollar}pm{dollar} 0.03 occurred during laser irradiation, then returned to 0.24 {dollar}pm{dollar} 0.04 twenty minutes later. Heterogeneity of flow during laser exposure was evident in the formation of three distinct radial regions; a zero flow coagulated zone, a high flow hyperemic ring, and a normal flow undamaged region. A finite element model was then used to investigate the effects that dynamic optical properties and blood perfusion have on the resulting temperature distributions. Char formation and tissue ablation were not accounted for in the simulations. Results indicated that rapid changes in optical scattering decrease the volume of tissue coagulated by laser exposure, and that blood perfusion has little to no effect on temperature distributions in the prostate. Comparison of the theoretical model with experimental data demonstrated that depth of coagulation along the central axis of the beam could be estimated accurately. However, simulated temperature distributions were inaccurate due to the inability to simulate the beam profile of the laser fiber used in the experiments. Further development of the model may aid in the analysis of commercial laser delivery devices, as well as in the evaluation of various strategies for the treatment of BPH.