Beam hardening of 10 MV radiotherapy X-rays: analysis using a convolution/superposition method

Metcalfe, Peter E; Hoban, Peter; Murray, David C.; Round, W. Howell

doi:10.1088/0031-9155/35/11/008

Cited by 27 publications

(24 citation statements)

References 18 publications

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“…The effective depth of penetration of charged particlesz for the polyenergetic kernels increased by 21%, going from a depth of 1.5 cm to a depth of 20 cm for a 10 × 10 cm 2 field size (Table I). Metcalfe et al 17 also found that the effective range of charged particles increased for depth-hardened kernels in comparison to the surface kernel. Although the use of depth-dependent polyenergetic kernels has already been studied and shown to improve dose accuracy, no studies, to our knowledge, have investigated the effects of beam softening for larger field sizes and greater offaxis distances.…”

Section: Discussionmentioning

confidence: 91%

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Investigation of various energy deposition kernel refinements for the convolution/superposition method

Huang¹,

Eklund²,

Childress³

et al. 2013

Medical Physics

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Purpose: Several simplifications used in clinical implementations of the convolution/superposition (C/S) method, specifically, density scaling of water kernels for heterogeneous media and use of a single polyenergetic kernel, lead to dose calculation inaccuracies. Although these weaknesses of the C/S method are known, it is not well known which of these simplifications has the largest effect on dose calculation accuracy in clinical situations. The purpose of this study was to generate and characterize high-resolution, polyenergetic, and material-specific energy deposition kernels (EDKs), as well as to investigate the dosimetric impact of implementing spatially variant polyenergetic and material-specific kernels in a collapsed cone C/S algorithm. Methods: High-resolution, monoenergetic water EDKs and various material-specific EDKs were simulated using the EGSnrc Monte Carlo code. Polyenergetic kernels, reflecting the primary spectrum of a clinical 6 MV photon beam at different locations in a water phantom, were calculated for different depths, field sizes, and off-axis distances. To investigate the dosimetric impact of implementing spatially variant polyenergetic kernels, depth dose curves in water were calculated using two different implementations of the collapsed cone C/S method. The first method uses a single polyenergetic kernel, while the second method fully takes into account spectral changes in the convolution calculation. To investigate the dosimetric impact of implementing material-specific kernels, depth dose curves were calculated for a simplified titanium implant geometry using both a traditional C/S implementation that performs density scaling of water kernels and a novel implementation using material-specific kernels. Results: For our high-resolution kernels, we found good agreement with the Mackie et al. kernels, with some differences near the interaction site for low photon energies (<500 keV). For our spatially variant polyenergetic kernels, we found that depth was the most dominant factor affecting the pattern of energy deposition; however, the effects of field size and off-axis distance were not negligible. For the material-specific kernels, we found that as the density of the material increased, more energy was deposited laterally by charged particles, as opposed to in the forward direction. Thus, density scaling of water kernels becomes a worse approximation as the density and the effective atomic number of the material differ more from water. Implementation of spatially variant, polyenergetic kernels increased the percent depth dose value at 25 cm depth by 2.1%-5.8% depending on the field size, while implementation of titanium kernels gave 4.9% higher dose upstream of the metal cavity (i.e., higher backscatter dose) and 8.2% lower dose downstream of the cavity. Conclusions: Of the various kernel refinements investigated, inclusion of depth-dependent and metalspecific kernels into the C/S method has the greatest potential to improve dose calculation accuracy. Implementation of spatially vari...

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Section: Discussionmentioning

confidence: 91%

“…These primary photon spectra were used to calculate the TERMA for each energy bin, which was then used as weighting factors for combining our highresolution, monoenergetic water kernels into polyenergetic kernels that are specific to a given depth, field size, or off-axis distance. 17 …”

Section: B Calculation Of Polyenergetic Kernelsmentioning

confidence: 99%

Investigation of various energy deposition kernel refinements for the convolution/superposition method

Huang¹,

Eklund²,

Childress³

et al. 2013

Medical Physics

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“…The extension of the source (focus) 28 is approximated by a 2D Gaussian function. In order to take into account the beam hardening with increasing depth 29,30 , the attenuation coefficient μ of the corresponding depth is considered.…”

Section: Determination Of the Primary Fluencementioning

confidence: 99%

Development of a 3-D convolution / superposition algorithm for precise dose calculation in the skull

Mack

Weltz²,

Scheib

et al. 2006

Australas. Phys. Eng. Sci. Med.

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In this paper an algorithm for calculating 3-D dose distributions within the brain is introduced and adapted to the demands of modem radiosurgery. The dose calculation with this model is based on a 3-D distribution of the primary photon intensity which is calculated with a ray casting algorithm. A prelocated matrix takes into account field sizes as well as modifying elements as collimator positions (MLC), blocks, wedges and compensators. Monte Carlo precalculated monoenergetic kernels from 0.1 MeV to 50 MeV were at our disposal. The components of the spectrum were either determined by deconvoluting depth dose curves measured in water or analyzed with a Ge-Li detector system in the case of 60Co. The calculated fluence distribution has to be superposed to the complete kernel containing the spatial energy deposition. Inhomogeneities and tissue interface phenomena (rhoe, Z) have been investigated. The divergence of the rays and the curved surface of the patient are taken into account. Assuming homogenous media, it is possible to shorten the computation time by using the Fast Fourier Transformation (FFT) delivering a first overview within seconds. The algorithm was evaluated and verified under specific conditions of small fields as used in radiosurgery and compared to dose measurements and Monte Carlo calculations. In using both the fast algorithm (FFT) for mainly homogenous conditions on one hand and the very precise superposition for inhomogeneous cases on the other, this algorithm can be a very helpful instrument especially for critical locations in the skull.

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“…One is the single polyenergetic method [3][4][5][6][7][8][9][10][11][12], where the convolution is performed with the use of a single-dose kernel that is formed by use of the incident X-ray spectrum. The other is the components method [3,7,[9][10][11][12][13][14][15], where a separate convolution is performed that uses each of the multi-dose kernels that are usually formed with the use of effective X-ray spectra, by which the number of energy bins can be decreased.…”

Section: Introductionmentioning

confidence: 99%

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: a theoretical study on 10-MV X-ray dose calculations in thorax-like phantoms

Iwasaki¹,

Kimura

Sutoh

et al. 2011

Radiol Phys Technol

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A convolution/superposition method is proposed for use with primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. It should be noted that the number of energy bins is usually about ten, and that the reconstructed X-ray spectra can reasonably be applied to media with a wide range of effective Z numbers, ranging from water to lead. The study was carried out for 10-MV X-ray doses in water and thorax-like phantoms with the use of open-jaw-collimated fields. The dose calculations were made separately for primary, scatter, and electron contamination dose components, for which we used two extended radiation sources: one was on the X-ray target and the other on the flattening filter. To calculate the in-air beam intensities at points on the isocenter plane for a given jaw-collimated field, we introduced an in-air output factor (OPF(in-air)) expressed as the product of the off-center jaw-collimator scatter factor (off-center S (c)), the source off-center ratio factor (OCR(source)), and the jaw-collimator radiation reflection factor (RRF(c)). For more accurate dose calculations, we introduce an electron spread fluctuation factor (F (fwd)) to take into account the angular and spatial spread fluctuation for electrons traveling through different media.

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Beam hardening of 10 MV radiotherapy X-rays: analysis using a convolution/superposition method

Cited by 27 publications

References 18 publications

Investigation of various energy deposition kernel refinements for the convolution/superposition method

Investigation of various energy deposition kernel refinements for the convolution/superposition method

Development of a 3-D convolution / superposition algorithm for precise dose calculation in the skull

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: a theoretical study on 10-MV X-ray dose calculations in thorax-like phantoms

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