ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1992.tb36352.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596855

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method for photon dose calculation in radio therapy planning using pencil beam energy deposition kernels is presented. It is designed to meet the requirements of an algorithm for 3‐D treatment planning that is general enough to handle irregularly shaped radiation fields incident on a heterogeneous patient. It is point oriented and thus faster than a full 3‐D convolution algorithm and uses the same physical data base to characterize a clinical beam as a full 3‐D convolution algorithm. It is shown that photon therapy beams can be characterized with great accuracy from a combination of precalculated Monte Carlo energy deposition kernels and dose distributions measured in a water phantom. The data are used to derive analytical pencil beam kernels that are approximately partitionated into the dose from (i) primary released electrons and positrons, (ii) scattered, bremsstrahlung, and annihilation photons, (iii) contaminating photons, and (iv) charged particles from the collimator head. A semianalytical integration method, based on triangulation of the field, is developed for dose calculation using the analytical kernels. Dose is calculated in units normalized to the incident energy fluence which facilitates output factor calculation. For application in heterogeneous media, a scatter correction factor is derived using monodirectional convolution along the ray path. In homogeneous media results are compared with measurements and in heterogeneous media with Monte Carlo calculations and the Batho method.

ISSN:0269-3127    

DOI:10.1118/1.596856

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Many manufacturers of modern linacs provide collimators with jaws that can be set asymmetrically. In the present work, the authors propose the use of a product of four independent output factors related to the four jaws to calculate the reference dose rate of asymmetrical fields. Each of these factors is normalized to the maximum opening of the collimator. A general algorithm for symmetrical and asymmetrical fields is proposed for dose calculation. Calculations have been checked by measurements at the reference depth in a 4‐MV x‐ray beam.

ISSN:0269-3127    

DOI:10.1118/1.596857

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The radiation dose outside the radiotherapy treatment field can be of clinical concern and, therefore, a method of accurately predicting the peripheral doses received by tissues would be beneficial. This paper describes a semiempirical method developed for calculating the peripheral dose received at points outside the collimated field edge for incident photon beams with energies below neutron production thresholds (<10 MV). The dependence of the peripheral dose upon depth, distance, field shape and size, azimuthal angle about the central axis, external contour variations, and tissue heterogeneities are accounted for by this calculation. Predictions by this algorithm are compared with measurements and it is shown that the method is capable of reproducing the measured peripheral dose values usually to within the statistical uncertainties of the data.

ISSN:0269-3127    

DOI:10.1118/1.596858

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The output factor for a therapy photon beam is defined as the dose per monitor unit relative to the dose per monitor unit in a reference field. Convolution models for photon dose calculations yield the dose in units normalized to the incident energy fluence with phantom scatter intrinsically modeled. Output factors calculated with the convolution method as the dose per unit energy fluence relative to the calculated dose per unit energy fluence in a reference field could deviate as much as 5% if corrections are not made for perturbations due to treatment head scatter. Significant perturbations are particles backscattered from the collimators to the monitor and photons forward scattered from the filter and collimators in the treatment head. The forward scatter adds an “unmonitored” contribution to the total energy fluence of the beam. A model is developed that describes the field size dependence of these perturbations for conversion of output factors, calculated with the convolution method, to machine output factors as an integrated part in treatment planning. The necessary machine characteristics are derived from measurements of the output in air for a limited set of field sizes. The method has been tested using five different multileaf collimated irregular fields at 6 MV and for a large set of rectangular fields at 5, 6, and 18 MV and found to predict output factors with an accuracy better than 1%.

ISSN:0269-3127    

DOI:10.1118/1.596859

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Fortran 77 Monte Carlo source code built from the EGS4 Monte Carlo code system has been used for timing benchmark purposes on 29 different computers. This code simulates the deposition of energy from an incident electron beam in a 3‐D rectilinear geometry such as one would employ to model electron and photon transport through a series of CT slices. The benchmark forms a standalone system and doesnotrequire that the EGS4 system be installed. The Fortran source code may be ported to different architectures by modifying a few lines and only a moderate amount of CPU time is required ranging from about 5 h on PC/386/387 to a few seconds on a massively parallel supercomputer (a BBN TC2000 with 512 processors).

ISSN:0269-3127    

DOI:10.1118/1.596860

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This article presents the considerations and efforts involved to install and fully support the EGS4 radiation‐transport Monte Carlo simulation code on an 80386‐based microcomputer. It also presents some EGS4 benchmark timing comparisons between this and other computer architectures.

ISSN:0269-3127    

DOI:10.1118/1.596907

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electron beams in the MM50 racetrack microtron are generated by computer controlled scanning of a well‐focused electron pencil beam. The treatment head is optimized to give a minimum of scatter between the source position and the collimator plane by a general minimization of all scattering material in the beam and by replacement of the air in the treatment head by helium, which has a much lower linear scattering power than air. A double‐focused multileaf collimator with a 31‐cm collimator to patient distance is used both for electron and photon collimation. In general, no extra electron collimation is needed for the standard SSD of 100 cm. To make irregular field collimation at a distance this far from the patient possible, a number of requirements have to be fulfilled regarding the virtual source position and the spatial and angular distribution of the initial electron beam. The virtual source position has been found to be at a fixed position for different irradiation parameters. This is important for the use of the light field in electron beam treatment but also for achieving a high degree of accuracy in the dosimetry. Scatter from the multileaf collimator has not been found to give any significant contribution to the radiation field or to the monitor output factor of the MM50. Experimental dose distribution data on the MM50 have been compared to data both from other types of treatment units and to Monte Carlo simulations.

ISSN:0269-3127    

DOI:10.1118/1.596933

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Film and thermoluminescent dosimetry (TLD) are investigated in heterogeneous phantoms irradiated by high‐energy electron beams. Both film and TLD are practical dosimeters for multiple and moving beam radiotherapy. The accuracy and precision of these dosimeters for radiation dose measurements in homogeneous water‐equivalent phantoms has been discussed in the literature. However, film and TLD are often used for dose measurements in heterogeneous phantoms. In those situations perturbations are produced which are related to the density and atomic number of the phantom material and the physical size and orientation of the dosimeter. In our experiments the relative dose measurements in homogeneous phantoms were the same regardless of dosimeter or dosimeter orientation. However, significant differences were observed between the dose measurements within the inhomogeneity. These differences were influenced by the type and orientation of the dosimeter in addition to the properties of the heterogeneity. These differences could be reproduced with Monte Carlo calculations and modeling of the experimental conditions.

ISSN:0269-3127    

DOI:10.1118/1.596861

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY