Study On Some Key Issues Of Image Guided Radiotherapy

Intensity modulated radiotherapy can deliver highly conformal radiation dose to astationary target, while protecting the surrounding organs at risk from excessiveradiation. However, the inter-fraction and the intra-fraction motion of the tumor andsurrounding organs at risk can greatly compromise the effectiveness of the technique.Four-dimensional CT and cone beam CT is widely used to obtain the intra-fraction andinter-fraction motion of tumor and organs at risk. The development of four-dimensionalradiotherapy planning and on-line adaptive radiotherapy methods can reduce the dosedeviation induced by the tumor motion, to achieve the purpose of precise radiotherapy.First, the intra-fraction motion of the tumor and surrounding organs at risk wasanalyzed through four-dimensional CT. Making use of the spatial continuity betweenadjacent CT images, the artifacts in four-dimensional CT can be detected and correctedto get a clear four-dimensional CT dataset.Second, based on the four-dimensional CT, a concept of "optimum deformation"of aperture was proposed. The "optimum deformation" of aperture makes the aperture atthe reference phase can follow the movement of the tumor and organs at risk andproduce a corresponding tracking aperture. Then the shape and weight of all aperturesare separately optimized by the simulated annealing and conjugate gradient method togenerate the optimal multi-leaf collimator based four-dimensional radiation treatmentplanning.Last, the paper analyzed the inter-fraction motion of tumor and organs at risk byonline cone-beam CT images. For the poor electron density accuracy of cone-beam CTimages, a gradient based deformable registration algorithm was developed to improveregistration accuracy between cone-beam CT and fan-beam CT images. Because of thetime-critical features of online clinical applications, we further implemented thedeformable registration algorithm in parallel on the GPU. It dramatically shortenedregistration time. Based on the deformation field output by registration algorithm, anautomatic mapping algorithm can automatically map contours of tumor and organs from the planning CT to the cone-beam CT. Based on the deformation field output byregistration algorithm, a direct aperture modification algorithm has been used to modifythe radiation treatment plan, so that the new plan can adapt to the position and shapechanges of the tumor and organs at risk.In summary,4D-CT,4D tracking treatment plan and on-line modified treatmentplan can more accurately deliver dose to the tumor while further reducing the dosereceived by the surrounding organs at risk. It can reduce the incidence of radiationcomplications, improve patient quality of life.