Research On Tissue Inhomogeneity Correction And Dose Model For Irregular Fields In Radiation Therapy

In this thesis,we focused on two key problems of the three-dimensional radiotherapy planning research:human tissue inhomogeneity dose correction and extraction of irregular field pencil beam convolution kernels,which are essential to increase the accuracy of dose calculation for radiotherapy planning.First of all,the history of radiotherapy planning system is systematically reviewed,and the characteristics of fundamental techniques are briefly introduced. All of those are important for the research in the thesis.Until now the conventional radiotherapy planning system(RTPS) mostly utilizes a homogenous human body model for dose calculation. But human tissue inhomogeneity has a nontrivial influence on the actual dose profile and directly affects the accuracy of dose computation. Methods of photon inhomogeneity correction were generally examined for their photon transport approximations. A fast algorithm performing tissue inhomogeneity corrections using a modified form of generalized equation of power law tissue-air ratio method(BATHO) is proposed. The correction is performed using the quantitative anatomical data produced by a modem CT scanner. The algorithm is fast and easy to be added to the conventional RTFS,and can fulfill the requirements of inhomogeneity correction in clinical radiotherapy.The dose calculation model used for irregular irradiation field is based on two-dimensional convolution principle. The core of this model is the extraction of pencil beam convolution kernels. In this thesis,the methods for extracting convolution kernels are analyzed in details. A new scatter dose calculation model is created;on the basis of differential reconstruction model we presented a method to derive the pencil beam kernel from measured beam data,including central axis depth doses,phantom scatter factors and output factors. Comparisons with measurements showed that the calculated dose distributions fitted well with the measurements.