The Propagation of Laser Light in Skin by Monte Carlo- Diffusion Method: A Fast and Accurate Method to Simulate Photon Migration in Biological Tissues

INTRODUCTION: Due to the importance of laser light penetration and propagation in biological tissues, many researchers have proposed several numerical methods such as Monte Carlo, finite element and green function methods. Among them, the Monte Carlo method is an accurate method which can be applied for different tissues. However, because of its statistical nature, Monte Carlo simulation requires a large number of photon pockets to be traced, so it is computationally expensive and time- consuming. Although other numerical methods based on the diffusion method are fast, they have two important limitations: first, they are not valid near the bounder of sample and source, and second, their accuracy is less than Monte Carlo method.METHODS: In this study, we combine the accuracy of Monte Carlo method and speed of the diffusion method. This hybrid method is faster than Monte Carlo Method and its accuracy is higher than the diffusion method.RESULTS: We first evaluate this hybrid model and the reflectance of a biological phantom is calculated by Monte Carlo method and this hybrid model. Then the propagation of laser light in the skin tissue has been studied.CONCLUSION: In this study, a combined method based on the Monte Carlo method and the diffuse equation is introduced. This hybrid method is five times faster than Monte Carlo Method, and its accuracy is higher than the diffusion method. The propagation of laser light in skin has also been studied by this hybrid method and its accuracy shows that it can be applied for laser penetration in biological tissues. It seems that this method is good for photo dynamic therapy (PDT) and optical imagin

Intraoperative use of c-arm cone beam CT for quality assurance of low dose rate prostate brachytherapy dose delivery

Prostate cancer is the most diagnosed cancer among men. Many patients with localized prostate cancer are treated with brachytherapy, one form of which involves permanent implantation of approximately 100 radioactive sources into and sometimes immediately around the prostate while the patient is anesthetized. During the procedure, transrectal ultrasound (TRUS) and fluoroscopic images (acquired using a mobile C-arm fluoroscopic X-ray system) are used to guide and visually assess implant quality, but do not provide accurate quantitative dosimetry. Thus, the patient undergoes a CT scan after the implantation is completed for dosimetric evaluation. However, this practice is not ideal as it occurs after the patient has left the operating room, when there is no longer any opportunity to modify the implant, if required. In this research project, a workflow was developed to assess the feasibility of performing intraoperative dosimetry using two routinely available imaging systems (a cone beam CT (CBCT) capable C-arm, and an ultrasound machine) for intraoperative dosimetric assessment of permanent implant prostate brachytherapy. In the proposed methods, the locations of all implanted sources were obtained from either 3D reconstructions of multiple planar radiographs, or from CBCT images. They were then registered to prostate contours delineated on the TRUS images, based on a common subset of sources identified on both image sets. In this process, prostate contours were deformed, using a finite element model, to take into account the effect of probe pressure in the TRUS images. Prostate dosimetric parameters obtained using this method were in agreement with postimplant CT dosimetry results, considering the uncertainty associated with each of these methods. An algorithm for automatic detection of seeds on TRUS images using a convolutional neural network was also developed during the course of this work. The model was trained to detect the needle track first and then the individual sources within the needle track. This automated approach outperformed a human observer in precision. The results of the work described in this thesis support the conclusion that the proposed dosimetry methods are feasible for real-time intraoperative dosimetric analysis of the implant and can potentially also replace postimplant CT dosimetry.Science, Faculty ofPhysics and Astronomy, Department ofGraduat