Gou Chengjun

The L-shell ionization of Ho and Os induced by electron impact

The electron-induced L-shell X-ray partial production cross sections, total production cross sections and mean ionization cross sections for Ho and Os have been measured as functions of electron energies from near threshold up to 36 keV by using a thin target with thick substrate technique. The influence of the electrons reflected from the substrate was corrected by using the electron transport bipartition model. Also, the corrected measured results were compared with theoretical predictions proposed by Gryzinski and McGuire.

Three-dimensional electron dose calculation using an improved hybrid pencil beam model

An improved hybrid-pencil beam model (HPBM) for electron-beam three-dimensional dose calculation has been studied. The model is based on the fact that away from the edges of a large field, the electron distribution function exactly equals that for an infinitely wide electron beam. In the present model, we use the bipartition model to calculate the longitudinal part of the pencil-beam distribution function, and Fermi-Eyges multiple-scattering theory to calculate its transverse part. In order to describe the electron beam characteristics accurately, we introduce a new parameter, which is extracted from measured profile data near the surface of a water phantom, to correct the transverse distribution determined by the Fermi-Eyges theory. Furthermore, we introduce an effective energy spectrum to describe the effect on the collimated electron beam of the accelerator head. The dose distributions calculated with the improved HPBM were compared with the experimental data, and the agreement was within 1% in most of cases. This preliminary study has demonstrated the potential for use of the model in the clinical therapy.

L-Shell Ionization Cross Section Measurements of Dysprosium and Samarium by Low-Energy Electron Impact

The Lα, Lβ and Lγ x-ray production cross sections of Dy and Sm by electron impact are measured at energies from near threshold to tens of keV. In the experiments, thin targets with thick substrates are used. Meanwhile, the electron transport bipartition model is used to eliminate the influence of electrons reflected from the thick substrates on measurements. The measured x-ray production cross sections are also compared with the theoretical predictions by Gryzinski and McGuire.

The penetration, diffusion and energy deposition of high-energy photon

This paper presents a new theory for calculating the transport of high-energy photons and their secondary charged particles. We call this new algorithm characteristic line method, which is completely analytic. Using this new method we cannot only accurately calculate the transport behaviour of energetic photons, but also precisely describes the transport behaviour and energy deposition of secondary electrons, photoelectrons, Compton recoil electrons and positron-electron pairs. Its calculation efficiency is much higher than that of the Monte Carlo method. The theory can be directly applied to layered media situation and obtain a pencil-beam-modelled solution. Therefore, it may be applied to clinical applications for radiation therapy.

Monte Carlo dosimetry of a new (90)Y brachytherapy source.

Purpose In this study, we attempted to obtain full dosimetric data for a new 90Y brachytherapy source developed by the College of Chemistry (Sichuan University) for use in high-dose-rate after-loading systems. Material and methods The dosimetric data for this new source were used as required by the dose calculation formalisms proposed by the AAPM Task Group 60 and Task Group 149. The active core length of the new 90Y source was increased to 4.7 mm compared to the value of 2.5 mm for the old 90Sr/90Y source. The Monte Carlo simulation toolkit Geant4 was used to calculate these parameters. The source was located in a 30-cm-radius theoretical sphere water phantom. Results The dosimetric data included the reference absorbed dose rate, the radial dose function in the range of 1.0 to 8.0 mm in the longitudinal axis, and the anisotropy function with a θ in the range of 0° to 90° at 5° intervals and an r in the range of 1.0 to 8.0 mm in 0.2-mm intervals. The reference absorbed dose rate for the new 90Y source was determined to be equal to 1.6608 ± 0.0008 cGy s–1 mCi–1, compared to the values of 0.9063 ± 0.0005 cGy s–1 mCi–1 that were calculated for the old 90Sr/90Y source. A polynomial function was also obtained for the radial dose function by curve fitting. Conclusions Dosimetric data are provided for the new 90Y brachytherapy source. These data are meant to be used commercially in after-loading system.

Off-axis dose distribution for rectangle proton beam

This paper modifies an analytical algorithm originally developed for electron dose calculations to evaluate the off-axis dose distribution of rectangle proton beam. This spatial distribution could be described by Fermi–Eyges theory since a proton undergoes small-angle scattering when it passes through medium. Predictions of the algorithm for relative off-axis dose distribution by a 6 cm * 6 cm initial monoenergetic proton beam are compared with the results from the published Monte Carlo simulations. The excellent level of agreement between the results of these two methods of dose calculation (< 2%) demonstrates that the off-axis dose distribution from rectangle proton beam may be computed with high accuracy using this algorithm. The results also prompts the necessity to consider the off-axis distribution when the proton is applied to clinical radiotherapy since the penumbra is significant at the distal of its range (about 0.6 cm at the Bragg-peak depth).

Phoenix 2000, A 3D conformal Treatment Planning System

Phoenix 2000 is a 3D conformal treatment planning system developed by the center for radiation physics of Sichuan University cooperating with the Sichuan Tumor Hospital. This paper gives a brief account of the system, which meets the needs of modern radiation therapy. In the aspect of medical image technology, the system can reconstruct a stereoscopic display of the anatomy structure and the target volumes directly by means of CT data. And it can conveniently provide users with automatic, semi-automatic or manual-operated way to sketch contour of the target volumes and the anatomy, and with a flexible tool to reconstruct the 3D images of them. The dose algorithms in this system include photon beam algorithm and electron beam algorithm. For the former we use the improved Convolution/ Superposition method, in which the concept of equivalent radiation source is applied to carefully treat the radiation field perturbation induced by bone. For the latter the bipartition model for electron transport, which has been extended from 1D inhomogeneous media to 3D inhomogeneous media, is used.