Experimental derivation of wall correction factors for ionization chambers used in high dose rate source calibration

Abstract

At present there are no specific primary standards for 192Ir high dose rate sources used in brachytherapy. Traceability to primary standards is guaranteed through the method recommended by the AAPM that derives the air kerma calibration factor for the 192Ir gamma rays as the average of the air kerma calibration factors for x-rays and 137Cs gamma-rays or the Maréchal et al. method that uses the energy-weighted air kerma calibration factors for 250 kV x rays and 60Co gamma rays as the air kerma calibration factor for the 192Ir gamma rays. In order to use these methods, it is necessary to use the same buildup cap for all energies and the appropriate wall correction factor for each chamber. This work describes experimental work used to derive the A(W) for four different ionization chambers and different buildup cap materials for the three energies involved in the Maréchal et al. method. The A(W) for the two most common ionization chambers used in hospitals, the Farmer NE 2571 and PTW N30001 is 0.995 and 0.997, respectively, for 250 kV x rays, 0.982 and 0.985 for 192Ir gamma rays, and 0.979 and 0.991 for 60Co gamma rays, all for a PMMA build-up cap of 0.550 gm cm(-2). A comparison between the experimental values and Monte Carlo calculations shows an agreement better than 0.9%. Availability of the A(W) correction factors for all commercial chambers allows users of the in-air calibration jig, provided by the manufacturer, to alternatively use the Maréchal et al. method. Calibration laboratories may also used this method for calibration of a well-type ionization chamber with a comparable accuracy to the AAPM method.

Comparison of192Ir air kerma calibration coefficients derived at ARPANSA using the interpolation method and at the National Physical Laboratory using a direct measurement

“…This source is currently the only one available at NPL for calibrations and is a reasonable match to the Nucletron microSelectron V2 source which was used to determine correction factors for the jig at the Peter MacCallum Cancer Centre. Air kerma calibration factors for the Farmer chamber were then measured at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) to obtain an interpolated calibration coefficient for 192 Ir gamma rays according to the methods of Goetsch et al 2 (modified to use 60 Co instead of 137 Cs by Marechal et al 6,7 ) and Mainegra-Hing and Rogers 8 . Correction factors for scatter and non-uniformity were then determined following recommendations of the International Atomic Energy Agency (IAEA) TecDoc 1274 1 , and the results compared.…”

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

“…Brought to you by | MIT Libraries Authenticated Download Date | 5/12/18 10:30 PM · A w is the chamber wall correction factor [17] to account for scattering and attenuation in the wall of the ionization chamber. A w is determined using a formula A w ≈ 1 -γt, where γ is the attenuation and scattering fraction per wall thickness (cm 2 /g) and taken as 0.0277 and t is the total thickness (g/cm 2 ) of wall material.…”

Avaliação da dose no reto em pacientes submetidas a braquiterapia de alta taxa de dose para o tratamento do câncer do colo uterino

“…The thermoluminescent powder samples were calibrated at a 6.0 cm-distance from the 192 Ir source. The absorbed dose values in water in the points where the samples of thermoluminescent material were placed, was calculated using the mathematical for-mulas developed in the Report 51 of the American Association of Physicists in Medicine (AAPM) (12) , based on measurement of air kerma rates at 10.0 cm from the source using the Farmer chamber with the calibration factor determined by Maréchal (13)(14)(15) . After TLDs calibration for the energy spectrum of gamma radiation from 192 Ir, the method was tried with patients undergoing treatment at INCA.…”