Effect of Finite Size of Ionization Chambers on Measurements of Small Photon Sources

Kondo, Sohei; Randolph, M. L.

doi:10.2307/3570872

Cited by 100 publications

(67 citation statements)

References 18 publications

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“…An Exradin model A3 3.6 cm 3 spherical ion chamber with a 2.25 mm C552 buildup cap and a precision electrometer were used for all measurements presented in this work. Although an NEL 2571 graphite Farmer chamber was also used by Goetsch et al, 2 the Exradin model A3 was chosen for this work as it is widely used as a reference standard and the Kondo-Randolph factors 7 are well known. The 2.5 mm buildup cap is used to ensure charged particle equilibrium for 137 Cs.…”

Section: Iic Seven-distance Technique Overviewmentioning

confidence: 99%

Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

et al. 2011

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Purpose: To perform a comparison of the interim air-kerma strength standard for high dose rate (HDR) 192 Ir brachytherapy sources maintained by the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) with measurements of the various source models using modified techniques from the literature. The current interim standard was established by Goetsch et al. in 1991 and has remained unchanged to date. Methods: The improved, laser-aligned seven-distance apparatus of the University of Wisconsin Medical Radiation Research Center (UWMRRC) was used to perform air-kerma strength measurements of five different HDR192 Ir source models. The results of these measurements were compared with those from well chambers traceable to the original standard. Alternative methodologies for interpolating the 192 Ir air-kerma calibration coefficient from the NIST air-kerma standards at 137 Cs and 250 kVp x rays (M250) were investigated and intercompared. As part of the interpolation method comparison, the Monte Carlo code EGSnrc was used to calculate updated values of A wall for the Exradin A3 chamber used for air-kerma strength measurements. The effects of air attenuation and scatter, room scatter, as well as the solution method were investigated in detail. Results: The average measurements when using the inverse N K interpolation method for the Classic Nucletron, Nucletron microSelectron, VariSource VS2000, GammaMed Plus, and Flexisource were found to be 0.47%, À0.10%, À1.13%, À0.20%, and 0.89% different than the existing standard, respectively. A further investigation of the differences observed between the sources was performed using MCNP5 Monte Carlo simulations of each source model inside a full model of an HDR 1000 Plus well chamber. Conclusions: Although the differences between the source models were found to be statistically significant, the equally weighted average difference between the seven-distance measurements and the well chambers was 0.01%, confirming that it is not necessary to update the current standard maintained at the UWADCL.

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Section: Iic Seven-distance Technique Overviewmentioning

confidence: 99%

Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

et al. 2011

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“…Even though the chamber was placed at the centre of an arrangement of four source positions, because the exposure was made by a single source, which moved from one place to another, rather than by an arrangement of sources that were exposed simultaneously, a dose gradient did exist across the chamber. Kondo and Randolph (1960) have analysed this situation and their results suggest that for an ionization chamber of the type that we used, that is 6 mm diameter, 18 mm long at 50 mm from the source, the chamber readings should be increased by 1%. The results presented here have been corrected by this amount.…”

Section: Calculation Of Resultsmentioning

confidence: 99%

Calibration of a Microselectron HDR iridium 192 source

Flynn¹,

Workman²

1991

The British Journal of Radiology

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A method for the calibration of the output, in terms of an air kerma rate, of the high activity miniature iridium 192 sources used in the Microselectron HDR afterloading unit is described. An air kerma rate is measured using a calibrated thimble chamber in an "in-air" calibration jig. The results are compared with an air kerma rate derived from the manufacturer's test certificate. In some cases, the ionization chamber measurements have been followed by a further calibration check using thermoluminescent dosimetry. Other checks carried out when a new source is received are also briefly described.

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“…Subsequently for all the measurements, the source was made to dwell in the maximum response position. The measured charge for a time interval t (sec) was corrected for ambient temperature and pressure, air attenuation, ion recombination, non uniform electron fluence within the air cavity [11][12][13] and applicator attenuation. No correction for transit time was made as the electrometer readings were obtained in the current mode.…”

Section: Methodsmentioning

confidence: 99%

Determination of contributions of scatter and distance error to the source strength of ¹⁹²Ir HDR brachytherapy source

Shwetha

Ravikumar

2016

Polish Journal of Medical Physics and Engineering

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High dose rate (HDR) brachytherapy commonly employs a 192 Ir encapsulated source to deliver high dose to the malignant tissues. Calibrations of brachytherapy sources are performed by the manufacturer using a well-type chamber or by in-air measurement using a cylindrical ionization chamber. Calibration using the latter involves measurements to be carried out at several distances and room scatter can also be determined. The aim of the present study is to estimate the scatter contribution from the walls, floor and various materials in the room in order to determine the reference air kerma rate of an 192 Ir HDR brachytherapy source by in-air measurements and also to evaluate the error in the setup distance between the source centre and chamber centre. Air kerma measurements were performed at multiple distances from 10 cm to 40 cm between the source and chamber. The room scatter correction factor was determined using the iterative technique. The distance error of -0.094 cm and -0.112 cm was observed for chamber with and without buildup cap respectively. The scatter component ranges from 0.3% to 5.4% for the chamber with buildup cap and 0.3% to 4.6% without buildup cap for distances between 10 to 40 cm respectively. Since the average of the results at multiple distances is considered to obtain the actual air kerma rate of the HDR source, the seven distance method and iterative technique are very effective in determining the scatter contribution and the error in the distance measurements.

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Effect of Finite Size of Ionization Chambers on Measurements of Small Photon Sources

Cited by 100 publications

References 18 publications

Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

Calibration of a Microselectron HDR iridium 192 source

Determination of contributions of scatter and distance error to the source strength of ¹⁹²Ir HDR brachytherapy source

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Effect of Finite Size of Ionization Chambers on Measurements of Small Photon Sources

Cited by 100 publications

References 18 publications

Comparison of air‐kerma strength determinations for HDR 192Ir sources

Comparison of air‐kerma strength determinations for HDR 192Ir sources

Calibration of a Microselectron HDR iridium 192 source

Determination of contributions of scatter and distance error to the source strength of 192Ir HDR brachytherapy source

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Product

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Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

Determination of contributions of scatter and distance error to the source strength of ¹⁹²Ir HDR brachytherapy source