Photon beam convolution using polyenergetic energy deposition kernels

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

doi:10.1088/0031-9155/39/4/002

Cited by 31 publications

(32 citation statements)

References 9 publications

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“…The "effective mean kernel" method ignores spectral changes of the photon beam throughout the patient geometry (e.g., beam hardening and off-axis softening) as well as spectral changes for different field sizes, and can lead to dose calculation inaccuracies. [4][5][6] In fact, several studies have found that higher accuracy was achieved when beam hardening of the kernels was taken into account in the convolution calculation. Dose errors caused by using nonhardened kernels were found to be as high as 2%-5% on the central axis.…”

Section: Introductionmentioning

confidence: 99%

“…Dose errors caused by using nonhardened kernels were found to be as high as 2%-5% on the central axis. [4][5][6] Although Liu et al 6 suggested that their kernel interpolation method could be used to take into account the effects of offaxis softening, there have been no studies that have quantified the effects of kernel softening with larger field sizes and greater off-axis distances for a clinical photon beam.…”

Section: Introductionmentioning

confidence: 99%

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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: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

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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“…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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“…Two polyenergetic kernels are created by Terma-weighting monoenergetic energy deposition kernels 10,11 . One kernel is formed using the primary dose deposition kernels and the other is formed using the scatter dose deposition kernels.…”

Section: Introductionmentioning

confidence: 99%

“…One kernel is formed using the primary dose deposition kernels and the other is formed using the scatter dose deposition kernels. Kernel-hardening correction factors are computed and applied to Terma based on the Kerma (kinetic energy released per unit mass) to Terma ratio at depth and at the surface 10 . The dose is computed by convolving the polyenergetic Terma with the primary and scatter polyenergetic energy deposition kernels.…”

Section: Introductionmentioning

confidence: 99%

Investigation of photon beam models in heterogeneous media of modern radiotherapy

Ding

Johnston

Wong

et al. 2004

Australas. Phys. Eng. Sci. Med.

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This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%. 24.9% and 27.7% for the MGS. FFTC and Clarkson algorithms. respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%. 4.9% and 6.9% for the MGS. FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%. 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than +/-4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within +/-7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.

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Photon beam convolution using polyenergetic energy deposition kernels

Cited by 31 publications

References 9 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

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

Investigation of photon beam models in heterogeneous media of modern radiotherapy

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