Dose perturbations at interfaces in photon beams: Secondary electron transport

Werner, Barry L.; Das, Indra J.; Salk, William N.

doi:10.1118/1.596500

Cited by 29 publications

(19 citation statements)

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“…Radiation dose perturbations near media interfaces have long been a subject of investigation. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] If two media of different atomic number ͑Z͒ are irradiated in contact geometry by a photon beam, there will exist a transition zone in which the electron fluence will be composed of electrons generated in both media. This transition zone may extend from the interface to a few millimeters or even centimeters depending on the energy of the photon beams.…”

Section: Introductionmentioning

confidence: 99%

“…In this region of electronic disequilibrium, significant perturbations in the absorbed dose can occur. Analytic models, 4,5,9 Monte Carlo techniques 2,8,15 and experimental measurements 3,[6][7][8][12][13][14] have been reported to determine these perturbations. Scrimger 16 has measured a 10% dose enhancement in the entrance side ͑backward͒ of tissue-titanium interface for the 60 Co beam and 12% for a 8 MV photon beam using a cylindrical chamber, whereas Tatcher et al 17 have reported a 25% to 40% enhancement at the exit side ͑forward͒ using films.…”

Section: Introductionmentioning

confidence: 99%

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Dose enhancement by a thin foil of high‐Z material: A Monte Carlo study

Chu

Chen

et al. 1999

Medical Physics

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The purpose of this work is to study the dose enhancement by a thin foil (thickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbations on the beam spectra due to the presence of the foils, and dose enhancement dependence of photon-beam quality, beam incident angle, atomic number (Z), the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectra indicates that the dose enhancement near an inhomogeneity interface is primarily due to secondary electrons. A calculation for 1-mm-thick planar lead foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 and 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17%, respectively, for a 15 MV beam. Calculations for a variety of planar foils and photon beams show that dose enhancement: (a) increases with Z; (b) decreases with decreasing foil thickness when the foils are thinner than a certain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing incident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary electrons. The dose enhancement calculation is also performed for a cylindrically shaped lead foil irradiated by a four-field-box. The dose enhancement of 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interface for a 15 MV four-field-box.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Dose enhancement by a thin foil of high‐Z material: A Monte Carlo study

Chu

Chen

et al. 1999

Medical Physics

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“…Previous studies [7,8,10,12,14,15,17,18] have shown that the dose distribution in the build-up region depends on several factors such as source to skin distance (SSD), field size, presence of secondary blocking tray and the distance between the skin and the blocking tray. A number of studies [2-4, 7, 14, 15] has been carried out by many authors for the early generation linear accelerators and different model cobalt-60 machines.…”

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confidence: 99%

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Ravikumar¹,

Ravichandran

2000

Strahlenther Onkol

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The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.

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“…However, the atomic number of Gamma Putty is high (90% bismuth, Z=83), which enhances pair production and changes the spectral distribution of the secondary charged particles. The high atomic number of Gamma Putty also enhances local variation in the angular distribution of secondary electrons, as suggested by several reports [22][23].…”

Section: Discussionmentioning

confidence: 57%

Analytical Derivation and Validation of Dosimetric Parameters for Gamma Putty Attenuators with Megavoltage Photon Beams

Gloi¹

2017

Am. J. Biomed. Sci.

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Background: Several descriptors are used to characterize the dosimetric parameters of a photon beam through an attenuator. This study evaluates these descriptors analytically through various thicknesses of Gamma Putty. Specifically, we measure percent ionization depth doses as a function of depth and field size, and fit three models to the data. Materials and Methods: Measurements of percent ionization at various depths along the central axis were generated from 6 MV and 18 MV photon beams at a source-axis distance of 100 cm with an ionization in solid water phantom. We fit analytic models to the data to determine linear attenuation, beam hardening, beam quality, and electron contamination. Results: We report best-fit parameters for all the analytical models. All models yielded a root mean square (RMS) error of less than 1% with respect to the data. At depths below 5cm in the phantom, the largest attenuation coefficients (μ) were observed for the 6 MV beam, regardless of Gamma Putty thickness. Also, the smallest field sizes (4×4 and 5×5 cm 2 ) have the largest attenuation coefficients at these depths, for both beam energies. At a depth of 10 cm, the variation in μ was negligible for both beam energies. Conclusions: By fitting parametric models to axial ionization profiles, it is possible to characterize the dosimetric parameters of any attenuator as a function of thickness and field size, without knowing the precise spectral distribution of the beam. Parameters such as attenuation coefficients, beam hardening, and electron contamination can then be calculated accurately for any combination of field size and attenuator thickness.

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Dose perturbations at interfaces in photon beams: Secondary electron transport

Cited by 29 publications

References 0 publications

Dose enhancement by a thin foil of high‐Z material: A Monte Carlo study

Dose enhancement by a thin foil of high‐Z material: A Monte Carlo study

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Analytical Derivation and Validation of Dosimetric Parameters for Gamma Putty Attenuators with Megavoltage Photon Beams

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