摘要:

A new model for calculating electron beam dose has been developed. The algorithm is based on a two‐ or three‐dimensional geometry defined by computerized tomography (CT) images. The Monte Carlo technique was used to solve the electron transport equation. However, in contrast to conventional Monte Carlo models (EGS4) several approximations and simplifications in the description of elementary electron processes were introduced reducing in this manner the computational time by a factor of about 35 without significant loss in accuracy. The Monte Carlo computer program does not need any precalculated data. The random access memory required is about 16 Mbytes for a 1282×50 matrix, depending on the resolution of the CT cube. The Voxel Monte Carlo model (VMC) was tested in comparison to calculations by EGS4 and the “Hogstrom algorithm” (MDAH) using several fictive phantoms. In all cases a good coincidence has been found between EGS4 and VMC, especially near tissue inhomogeneities, whereas the MDAH algorithm has produced dose underestimations of up to 40%.

ISSN:0269-3127    

DOI:10.1118/1.597673

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Starting from the Boltzmann–Fokker–Planck transport equation, we have developed a new theory of multiple scattering which incorporates the advances already made with our Gaussian multiple‐scattering theory for electron dose calculation. This incorporation has been accomplished in a natural way, by modifying the scattering powerTand by adding a convolution term to the distribution‐function equation of the Gaussian theory. Our previous results concerning increasing the accuracy of the small‐angle approximation used and dealing with localized tissue inhomogeneities have thus been maintained, and we have arrived at a complete distribution function in both transverse spatial and angular variables. When integrated over the transverse angular variables, for a first‐order small‐angle approximation this distribution function for a pencil beam is essentially the same as the Molière multiple‐scattering distribution, which includes large‐angle single scattering. For a water phantom, we have used comparisons with EGS4 Monte Carlo calculations to demonstrate the greatly increased accuracy of our new multiple‐scattering theory over the Gaussian theory, which includes the usual Fermi–Eyges theory. We have also presented a fairly accurate Gaussian approximation to the pencil‐beam dose profiles given by our new theory, which can be used in order to maintain the mathematical simplicity of the predictions of the Fermi–Eyges theory.

ISSN:0269-3127    

DOI:10.1118/1.597777

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The superposition/convolution method and the transport of pregenerated Monte Carlo electron track data have been combined into theSuper‐MonteCarlo(smc) method, an accurate 3‐D x‐ray dose calculation algorithm. The primary dose (dose due to electrons ejected by primary photons) is calculated by transporting pregenerated (in water) Monte Carlo electron tracks from each primary photon interaction site, weighted by the terma for that site. The length of each electron step is scaled by the inverse of the density of the medium at the beginning of the step. Because the density scaling of the electron tracks is performed for each individual transport step, the limitations of the macroscopic scaling of kernels (in the superposition algorithm) are overcome. This time‐consuming step‐by‐step transport is only performed for the primary dose calculation, where current superposition methods are most lacking. The scattered dose (dose due to electrons set in motion by scattered photons) is calculated by superposition. In both a water–lung–water phantom and a two lung‐block phantom,smcdose distributions are more consistent with Monte Carlo generated dose distributions than are superposition dose distributions, especially for small fields and high energies—for an 18‐MV, 5×5‐cm2beam, the central axis dose discrepancy from Monte Carlo is reduced from 4.5% using superposition to 1.5% usingsmc. The computation time for this technique is approximately 2 h (depending on the simulation history), 20 times slower than superposition, but 15 times faster than a full Monte Carlo simulation (on our platform).

ISSN:0269-3127    

DOI:10.1118/1.597679

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The development of capillary neutron optics permits a new technology for neutron capture therapy involving the application of a focused thermal neutron beam at the medically optimal location within the patient. A subthermal neutron beam begins to converge as it travels through a neutron “lens,” reaching a narrow focus within a tube that allows it to pass directly to the treatment region. This technique results in a substantially lower dose to untreated parts of the patient and a substantially weaker radiation field in the treatment room generally. Additional advantages include the relative ease of thermal neutron generation and the ability to shield the patient completely and effectively from fast neutrons or gamma rays originating at the neutron source. This work describes the application of capillary optics to neutron capture therapy, along with Monte Carlo calculations of the neutron flux profiles within a patient for an optimized system design. Specific dose profiles for the case of boron neutron capture therapy within the brain are also provided.

ISSN:0269-3127    

DOI:10.1118/1.597680

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A combination of Monte Carlo, convolution, and experimental techniques have been used to investigate bremsstrahlung production at 50 MeV in full‐range targets to produce narrow elementary photon beams for scanning. Calculations using the ITS 3.0 Monte Carlo system for various target designs, including particle transport through the treatment head of an MM50 racetrack microtron and a water phantom, have been compared to experimental dose profiles from narrow photon beams at 10‐cm depth in water. A reduction in the ITS 3.0 default substep size has been found necessary even for incomplete agreement, in consistency with the findings of Faddegon and Rogers [Nucl. Instrum. Meth. A327, 556–565 (1993)] for a different experimental setup and energy using the previous version of ITS. Results show that the calculated shape of the tail of dose distributions from narrow photon beams agrees well with measurements, but CYLTRAN/ITS 3.0 fails to reproduce the central part of the distribution. The discrepancy at small angles, reported previously for EGS4 and ITS 2.1 simulations, possess a limitation to Monte Carlo simulations of narrow photon beams used in scanned systems of clinical accelerators. Radial dose profiles have been calculated by convolution of the energy fluence at the exit of the target with one polyenergetic Monte Carlo calculated dose kernel and also a database consisting of ten different dose kernels corresponding to different monoenergetic photon pencil beams for comparison. The agreement with the much slower fully detailed Monte Carlo calculations was better when using the database kernels than the polyenergetic kernel. Results for the mean energy, mean polar angle, and energy fluence at different depths within various targets have been obtained. These are discussed in the context of the design characteristics of bremsstrahlung targets with emphasis on their utilization for scanning photon beam techniques.

ISSN:0269-3127    

DOI:10.1118/1.597681

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A semiempirical method to characterize the pencil‐beam dose kernel is presented. Results from measurements are described by mathematical models of the applicable physical processes. The measurements were made with 6 and 25 MV x‐ray beams from a linear accelerator. Broad‐beam notations were used consistently, and the pencil‐beam quantities were obtained by differentiation. The results were compared to pencil‐beam kernels calculated by Monte Carlo techniques. The analysis of the measured data included a number of approximations. It was assumed that all the constituent pencil beams in the field are parallel, i.e., the divergence is ignored. Furthermore, the lateral variations of the incident photon fluence and the energy spectrum were disregarded. Monte Carlo calculations, on the other hand, are based on an average energy spectrum over the field, and are free from divergence and variations in the incident photon fluence. Measured and Monte Carlo calculated pencil beams nevertheless agreed well, and the approximations mentioned caused at maximum 2.7% discrepancies for the largest field size at 6 MV.

ISSN:0269-3127    

DOI:10.1118/1.597807

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Quality assurance methods used in association with radiosurgery must include all aspects of the radiosurgery process: visualization and localization of the target, treatment and dose planning and dose delivery. Presented here is a quality assurance method that utilizes an anthromorphic head phantom and a micro‐ionization chamber to demonstrate precise target localization and accurate dose delivery. This micro‐ionization chamber method offers an immediate readout which is both accurate and reproducible. Additionally, this method allows unlimited repetition of the dose measurement process without repeated radiographic localization studies as is necessary with the conventional methods of TLD, film, and Fricke gels. The method and techniques presented can be used in the acceptance testing and routine quality assurance of both linac‐based and Gamma Knife radiosurgery units.

ISSN:0269-3127    

DOI:10.1118/1.597682

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597809

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effects of the mechanical loss of a stainless steel primary scattering foil on a 12‐MeV electron beam from a dedicated intraoperative electron accelerator are discussed. Routine quality assurance tests, including dose output constancy, energy constancy, and beam uniformity (flatness and symmetry), were used to determine the nature of the malfunction when it occurred. It is concluded that these quality assurance checks, if done with the frequencies recommended by the AAPM Task Group 40 Report [Med. Phys.21, 581–619 (1994)] and repeated at the time of occurrence, are sufficient to detect loss of an electron scattering foil.

ISSN:0269-3127    

DOI:10.1118/1.597683

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Recently, the mechanical failure of one of the upper collimator mechanical trimmers on a cobalt‐60 unit resulted in large beam asymmetries and unacceptable flatness characteristics. This malfunction was not detected using currently accepted schedules for quality assurance tests. The incident suggests that the frequency of routine beam profile constancy checks should be increased to weekly for cobalt‐60 units.

ISSN:0269-3127    

DOI:10.1118/1.597808

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY