| Together with surgery and chemotherapy, radiotherapy plays an important role in oncology, both in definitive and palliative aspects of treatment. The accuracy of dose calculation within±5% dose error is the basis of radiotherapy quality. Dose calculation is the kernel of the treatment planning system. The paper concerns dose calculation for three-dimensional conformal radiotherapy based on radiation dose.It is well known that Monte Carlo (MC) dose calculation method is widely accepted as the most accurate dose calculation method by the experts in radiation physics. The Monte Carlo method, namely statistic testing method or the technique of statistical sampling, is a statistical technique for solving a variety of stochastic problems, which is applied to nuclear weapons at the end of the second world war. The driving forces for the initial appear to have been Stanislaw Ulam and John von Neumann who saw the development of ENIAC, the first electronic computer.The distance to the next interaction, interaction type (photoelectric effect, compton effect or pair production for X ray) and secondary particle energy/direction change are statistical variables when the particle interacts with the matter. Its principle is to simulate a large amount of single particle, such as 106, interaction during transport, consequently calculating dose or influence. However, in the past decades despite its unparalleled accuracy, the Monte Carlo method is not widely used for treatment planning due to the large amount of computer calculation time required to produce a reasonably accurate result.With the development of computer parallel technology and graphic processing unit, the variety of MC codes, EGS, MNCP, PENELOPE and ETRAN, are most frequently used for modeling of radiotherapy beams. Although the correction is necessary for accurate simulations of certain types of geometries, it is generally not needed for typical radiotherapy calculation. So, dose calculation based Monte Carlo in radiotherapy are attracted attention from research scholars and several commercial vendors.Dose planning method (DPM) is applied to perform electron and photon beam dose calculation as fast codes.DPM utilizes step size independent multiple-scattering theory and a number of approximation algorithms. It is available to apply DPM for clinical dose calculation.However, DPM1.1 can only calculate dose with parallel square beam and quadric surface geometry. In practice, the beam is delivered by linear accelerator (linac) and the phantom is acquired by CT. The paper concerns linac treatment head simulation, dose calculation based CT and the verification in homogeneous and inhomogeneous tissues.The structure of linac is various with the vendors and the type, even positioning and machining of the same type. The factors can influence the physical characteristics of the final photon or electron beams used in treatment, and the dose distribution in the patient. It is necessary to simulate linac treatment head to implement accurate dose calculation. The paper is based on BEAMnrc/EGSnrc code system.BEAM/BEAMnrc is a Monte Carlo simulation system for modeling radiotherapy sources which was developed as part of the ottawa madison electron gamma algorithm (OMEGA) project to develop three-dimensional treatment planning for radiotherapy. BEAMnrc is built on the EGSnrc Code system. The transport physics in EGSnrc is greatly improved over than in EGS4. The accuracy of EGSnrc is over EGS4, especially at lower energies. BEAMnrc can not only run on Unix/Linux systems, but also on Windows-based system unlike another Monte Carlo code systems.The main output of the simulation is the phase space file which stores the position, direction and energy of the particles achieving the scoring plane. The BEAMnrc/EGSnrc code system was used to simulate 6MV photon beam produced by a Varian 600C accelerator with 10 cm×10 cm field, SSD=100 cm. The paper studies the effect of mean energy and the full-width half maximum (FWHM) of incident electron beam intensity distribution (assumed Gaussian distribution) on dose distribution and derives a most optimal combination of energy and FWHM of incident electron beam intensity distribution. It is an iterative process to determine a set of simulation parameters (i. e, electron beam energy and radial distribution of the incident beams). First, the simulation is run by starting with manufacture's specifications or suggestions for two parameters. Assuming that this dose not lead to a satisfactory match of central-axis relative percent depth dose (PDD) or off-axis ratio (OAR), one adjusts the incident electron energy to match PDD. Once this is matched, the radius of the incident radial distribution is varied to get a match with OAR. If a match cannot be achieved, it may be necessary to re-adjust the energy to achieve agreement with OAR. Once this agreement is satisfactory, it is essential to verify that PDD is still matched adequately since incident energy has been changed.The material and density of a voxel are needed to prepare to cross sectional data when DPM simulate the transport of the particles. There are complicated formulas accounting for elemental weights. Using experimentally determined parameters, the density of any Hounsfield number were obtained via linear interpolation. To compared with the value of dose using DOSXYZnrc code system easily, four major types of tissues are adopted:air (H<50), lung tissue (50<H≤300), soft tissue (300<H<1125), bone (1125<H<3000). The mass density is obtained via linear interpolation of the values set at the boundaries of the Hounsfield number bin (0.001 g/cm3 at H=0,0.043 g/cm3 at H=50,0.032 g/cm3 at H=300,1.101 g/cm3 at H=1125, 2.088 g/cm3 at H=3000).Several experiments are used to verify the accuracy of dose distribution. DPM is applied to calculate①depth dose curves and off-axis ratios at a depth of 10.0 cm in water using a 6 MeV photon beam with a 3 cm×3 cm field and phase space file simulated Varian 600C medical linear accelerator with a 10 cm×10 cm field at SSD=100 cm;②depth dose curves using 6 MeV photon beam in inhomogeneous tissues, such as water(6 cm)/lung(6 cm)/water(8 cm) with a 3 cm×3 cm field and water(6 cm)/bone(2 cm)/water(12 cm)with a 10 cm×10 cm field;③depth dose curves using 6 MeV photon beam based on the CT data of a patient's head and abdomen. The dose calculated by DPM is compared to with the dose calculated by DOSXYNnrc/EGSnrc under the same condition. The error is within 3% in water phantom while the error is within 3% in inhomogeneous tissues, except a few points. DPM can accurately predict the dose to homogenous and inhomogeneous tissue.The end part of the paper gives an overview and puts forward the issues to do in the future. |
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