Specific absorbed fractions for photon point sources within a scattering medium

Ellett, W.H.

doi:10.1088/0031-9155/14/4/004

Cited by 21 publications

(11 citation statements)

References 3 publications

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“…An unbounded phantom with a 30 cm radius has been assumed, considered as such because the photons scattered at a distance of rϭ30 cm from the source do not have any influence on the dose at rϭ10 cm. For 137 Cs and 192 Ir sources, the phantom radii selected are 10,15,20,25,30,35,40,45, and 50 cm. In this case we are interested in the kerma values at distances of less than 20 cm from the source.…”

Section: Methodsmentioning

confidence: 99%

Phantom size in brachytherapy source dosimetric studies

2004

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An important point to consider in a brachytherapy dosimetry study is the phantom size involved in calculations or experimental measurements. As pointed out by Williamson [Med. Phys. 18, 776-786 (1991)] this topic has a relevant influence on final dosimetric results. Presently, one-dimensional (1-D) algorithms and newly-developed 3-D correction algorithms are based on physics data that are obtained under full scatter conditions, i.e., assumed infinite phantom size. One can then assume that reference dose distributions in source dosimetry for photon brachytherapy should use an unbounded phantom size rather than phantom-like dimensions. Our aim in this paper is to study the effect of phantom size on brachytherapy for radionuclide 137Cs, 192Ir, 125I and 103Pd, mainly used for clinical purposes. Using the GEANT4 Monte Carlo code, we can ascertain effects on derived dosimetry parameters and functions to establish a distance dependent difference due to the absence of full scatter conditions. We have found that for 137Cs and 192Ir, a spherical phantom with a 40 cm radius is the equivalent of an unbounded phantom up to a distance of 20 cm from the source, as this size ensures full scatter conditions at this distance. For 125I and 103Pd, the required radius for the spherical phantom in order to ensure full scatter conditions at 10 cm from the source is R = 15 cm. A simple expression based on fits of the dose distributions for various phantom sizes has been developed for 137Cs and 192Ir in order to compare the dose rate distributions published for different phantom sizes. Using these relations it is possible to obtain radial dose functions for unbounded medium from bounded phantom ones.

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Section: Methodsmentioning

confidence: 99%

Phantom size in brachytherapy source dosimetric studies

2004

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“…9 Meli et al 8 based their conclusion as to the water equivalence of PMMA, polystyrene, and solid water on the results of experiments and MC simulations without studying the impact of varying phantom dimensions. The influence of phantom size on radial dose distributions around 192 Ir sources in water was investigated by Ellett 10 and later by Pérez-Calatayud et al, 11 with extensions to various tissue materials by Melhus and Rivard. 12 The influence of phantom dimensions on experimental 192 Ir dosimetry was pointed out by Williamson 13 who found improved agreement between MC and experimental results when the MC simulations were performed for the measurement geometry used.…”

Section: Introductionmentioning

confidence: 99%

Influence of phantom material and dimensions on experimental dosimetry

Tedgren

Carlsson

2009

Medical Physics

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In treatment planning of brachytherapy, absorbed dose is calculated by superposing predetermined distributions of absorbed dose to water in water for the single source according to the irradiation pattern ͓i.e., placement of the source͑s͒ or dwelling position͑s͔͒. Single-source reference water data are derived from Monte Carlo ͑MC͒ simulations and/or experiments. For reasons of positional accuracy, experimental brachytherapy dosimetry is most often performed in plastic phantoms. This work investigates the water equivalence of phantoms made from polystyrene, PMMA, and solid water for 192 Ir dosimetry. The EGSnrc MC code is used to simulate radial absorbed dose distributions in cylindrical phantoms of dimensions ranging in size from diameter and height of 20 cm to diameter and height of 40 cm. Water equivalence prevails if the absorbed dose to water in the plastic phantom is the same as the absorbed dose to water in a water phantom at equal distances from the source. It is shown that water equivalence at a specified distance from the source depends not only on the size of the plastic phantom but also on the size of the water phantom used for comparison. Compared to equally sized water phantoms, phantoms of polystyrene are less water equivalent than phantoms of PMMA and solid water but compared to larger water phantoms they are the most water equivalent. Although phantom dimension is the most important single factor influencing the dose distributions around 192 Ir sources, the effect of material properties is nonnegligible and becomes increasingly important as phantom dimensions increase. The importance of knowing the size of the water phantom whose data underlies treatment planning systems, when using such data as a reference in, e.g., detector evaluation studies, is discussed. To achieve the highest possible accuracy in experimental dosimetry, phantom-specific correction factors should be used.

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“…The computer calculations have been performed for a point isotropic emitter at the centre of a sphere of radius 25 cm. It has been shown by Ellett (1969) that the dose at any distance r is the same in any spherical absorber provided the boundary R of that absorber is at least one photon mean free path A from the point of interest, i.e. (R-r)>A.…”

Section: Resultsmentioning

confidence: 99%

“…In calculating the dose-rate around specific radionuclide point sources we have extended similar calculations for monoenergetic point sources by Ellett et al (1968) and Ellett (1969). It is possible to compare their analysis with the present one by estimating the dose-rate at 1 cm for a 137 Cs source using their table of point specific absorbed fraction which includes data at 662 keV and 30 keV.…”

Section: •0)mentioning

confidence: 99%