Application Research Of DTS In Radiotherapy Positioning

Recently, the morbidity rate and the death rate of the cancer tend to be going up, which have been a main killer to threaten to the life and health of human. Along with the development of cancer therapeutics, radiotherapy which is one of the most powerful tools for treating malignant tumors has been main means in treating malignant tumors with Surgical Oncology and Medical Oncology together. According to the statistics of the W. H.O, about 70% of cancer patients during the course of the different period need to accept radiation therapy. For instance:pure radiotherapy, preoperative and postoperative radiotherapy, radiation treatment in combination with chemotherapy and so on. Radiotherapy started latterly compared with other cancer tools. So far it is less than one hundred years, but it developed promptly especially when the computers came up. And the computers which made the radiotherapy developed toward the direction of quantitative, precision, stylized and intelligent.Local persistence or recurrence and distant metastasis are the main reasons for the failure of the tumor radiotherapy. Improving the local control rate will be of great significance to controlling the recurrence or metastasis of the tumor and also to improving the patients' survival rate. Precise radiotherapy, which most probable concentrate the ray on target zone in physics, make the normal tissue and organ around the tumor got little or no irradiated, kill cancer cells as maximum, and better protect normal tissue. It requires:accurate diagnosis, accurate positioning, precise plan, precise position, precise exposure, the high dose distribution of 3-D space must be consistent with the shape of the target area, and the dose distribution in the target area must meets the requirement.The accuracy and repeatability of the place are one of the key factors which influence the curative effect of tumor radiotherapy. The patients' position errors is mean that:relative to treatment machines, all the uncertain factors between the position of patients and radiation fields, and the difference between patients' actual irradiated part and planned ones during treatment. Set-up errors mainly come from random error and system error in the process of fractionated radiation. Random errors refer to the difference of the position when patients receive therapy each time; meantime it refers to the difference between different treatments during the therapy session. Another kind of position error is the error made within fractionated radiation. It refers to the difference produced by a single treatment during the fractionated radiation, which is mainly caused by the patients' random moving or cyclical mobile, including respiratory movement, heartbeat, and uncontrollable muscle contraction and so on.This research studies the methods of correcting position errors, reviews the application DTS in radiotherapy, and mainly focus on the study of DTS images reconstruction. Accelerated reconstruction of DTS images by the GPU based on CUDA framework is also implemented for set up error correction.Image-guided Radiotherapy (IGRT) is a technology proposed for the uncertain information of patients' anatomy and position. It can collect patients' image information and determine on the position and movement of target area and important structure. It can correct the position and dose distribution when necessary, so as to improve position's accuracy of radiation target area, improve the local control of tumors and reduce complications of the normal tissues. Measuring position errors should compare the real-time images of treatment position with the localized images of simulation. We can achieve the purpose of reducing position errors through the precise correction of position errors, without any change in conditions of treatment technology. It can correct the patients' position in the same fractionated radiation according to the position deviation of one or more measurements. It means on-line correction or off-line correction in the next fractionated radiation. Most of the current application is as follows:2D kV or MV X-ray, EPID, cone beam CT, Orthogonal X-ray system, CTVision and so on.Cone beam CT (CBCT) developed based on flat detector in recent years is characterized by small volume, light weight, and open architecture. It can integrate into linear accelerator directly and became one of the hot research topics of IGRT. The CBCT system uses complete isocentric motion. When X-ray tube and detector pivot at patients'head feet direction, the detector collects the projection data. After reconstructing CBCT images, it is necessary to register the CT images with CBCT images. So, we can get the set-up errors, thus achieve to position precisely. But obtaining the projection data for CBCT reconstruction needs 360°rotating. It needs longer scanning time (～1min) and results in patients' more dose.DTS is a new tomography imaging method based on the geometric principle of traditional tomography and combined with modern digital electronic technology, especially computer image processing technology. The main characteristics of DTS are small angle range projection data, retrospective reconstruction of arbitrary depth images, displaying 3D information and so on. Compared with traditional CT, DTS has the lower resolution, but only need simple equipment and can converge with X-ray equipment. The small angle range projection data greatly reduces the inspection fee and radiation dose. We can assert DTS will play a huge role in medical diagnosis in future. At present, DTS is mainly used in the inspection of mammary gland, thoracic cavity, joints and other fields. Its reconstruction algorithms can generally be divided into the two kinds of direct reconstruction method and the indirect reconstruction method. The direct reconstruction method includes analytic method and iterative method, the indirect reconstruction method includes back-projection method two-dimensional Fourier reconstruction method and convolution method. The advantage of analytic method is the fastest and the advantage of iterative method is it can reconstruct images with little noises.The denoising algorithms of DTS are divided into spatial frequency filtering, matrix inversion, filter back-projection method and the iterative denoising method, etc. None of them can be generally applied effectively in all cases of the reconstruction of DTS. Appropriate noise reduction method should be used when object structure characteristics is different.First of all, FDK-based algorithm is suggested and applied for DTS images reconstruction. Limited angles of projections are just needed for reconstruction. Experimental validation is implemented on the real patient projection images obtained by VARIAN 23EX linac. Definition of tissue and bone structure is greatly enhanced.We used the CBCT imaging equipment installed in the linear accelerator to get the projection data of the patient in order to reconstruct DTS images. Then, we register the DTS and reference images, thus get the set-up errors in order to position more accurately. Compared with CBCT, the strong points of DTS are lower dose(<1cGy), shorter scanning time(<10s) and littler mechanical constraints(10°～40°). Therefore, DTS is a good choose which ban be use in Radiotherapy Positioning for special patients and organs influenced by movement.Early, Computer Graphics is simple, can be completed by CPU. GPU is graphic processing unit. NVIDI company developed the first truly GPU-Geforce256 in 1999. Today, GPU has experienced the development by leaps and bounds. In the past ten years, the computing capacity of GPU has developed rapidly. About every six months, there will be a new product with performance doubled developed. Meanwhile, the development speed of the GPU computing performance greatly faster than the CPU. GPU is most suitable for parallel computing, and compared with the traditional maxicomputer is cheaper.CUDA is a new hardware and software architecture used for treating and managing GPU computing. CUDA thinks of GPU as data parallel computing equipment. It distributes and manages the all of the calculations. There is no need to take these calculations mapping to the graphics API. CUDA program is developed based on C language and extends the C language. Therefore the users who have learned any language can easily developed applications program based GPU.In the aspect of accelerated reconstruction, this research analysis the parallel computing principle of FDK algorithm, proposes the scheme using the GPU based on CUDA framework to realize the accelerated reconstruction. This scheme adopts the graphics card based on fresh hardware and software framework. We used the stream processor of the GPU, which has the special program mode, to accelerate the computing the filter of DTS.Then, this research discusses the application DTS in radiotherapy, summaries the theoretical of DRRs. DRRs images reconstruction is realized by projective algorithm. A registration program developed based on VC++6.0.At the end of this thesis, we also discuss some problems which haven't been completed. In addition, some works which will be done in the future are prospected. There are three major problems:first, because the restriction of the experimental condition, we haven't validated the usefulness of DTS with clinical data. It is uncertain of the number of angles being suitable for other acquisition condition and being the best number. Second, we should continue to improve the reconstruction of DTS, accomplish the Accelerated Computing of weighting and back projection. Third, it is necessary to develop a suit system for correcting position errors with our methods of DRRs and registration.