Studies On The Application Of Electron Hybrid Pencil-beam Model To 3D Dose Calculation

It has become more important to research radiotherapy because tumor has been a major factor to harm the public health and radiotherapy is a major method in cancer treatment.Radiotherapy may be used to treat localized solid tumors. The prerequisite, which can be acquired by CT, NMR or PET technology, is to localize the tumor and determine its region. After determining the CTV, OARs and PTV, we will work out a treatment plan with a TPS program.Nowadays, the electron beam has become a widely used tool in the radiation treatment of cancer. However, one of the major difficulties in routine clinical procedure is to accurately calculate the electron dose distribution for a reasonable calculation time. At present, most treatment planning calculations use the pencil-beam model, which is based on Fermi-Eyges theory and assumes that broad beams are composed of a finite number of pencil beams. Because the limitation results from the approximation of Fermi-Eyges theory, the Fermi-Eyges theory is not used in pencil-beam models for clinical treatment planning without modification. In order to get more accurate results, a number ofinvestigators have improved the Fermi-Fyges theory and pencil-beam models. These models constitute improvements over the original pencil-beam model resulting from direct application of the Fermi-Eyges theory, while at the same time using this small-angle scattering theory as a starting point.Electron transport Bipartition Model is provided by Luo Zhengming in 1985. By introducing a partition condition, the difference fJuence of electron was divided into two pans: forward-direction electron and diffusion electron. By solving the boltzmeinn equation for forward-direction and diffusion electron, the all transport parameters can be obtained.The hybrid electron pencil-beam model (HPBM) embeds the bipartition model into Fermi-Eyges multiple-scattering theory by using the fact that away from the edges of a large field, the electron distribution function exactly equals that for an infinitely wide electron beam. The bipartition model can calculate various electron transport quantities in a homogeneous or horizontally layered medium with high accuracy, for an infinite] y broad beam. Because the accuracy of the HPBM is dependent upon the accuracy of the bipartition model itself and the HPBM utilizes less measured data, the HPBM exhibits definite potential for use in clinical treatment.By discussing the effects from the accelerator head, which contain the angular distribution and energy spectrum of incident electron, the bremsstrahlung, and the effects from the beam divergence, SSD variation and small field, this work has improved the HPBM. Because we arc mainly interested in clinical applications, this work focuses on dose calculation and calculation efficiency, rather than on other transport quantities such as fluence and angular distribution. This preliminarystudy has demonstrated the potential for use of the model in the clinical therapy.