Monte Carlo calculations of the ionization chamber wall correction factors for¹⁹²Ir and⁶⁰Co gamma rays and 250 kV x-rays for use in calibration of¹⁹²Ir HDR brachytherapy sources

Abstract

As in the method for the calibration of 192Ir high-dose-rate (HDR) brachytherapy sources, the ionization chamber wall correction factor A(w), is needed for 192Ir and 60Co gamma rays and 250 kV x-rays. This factor takes into account the variation in chamber response due to the attenuation of the photon beam in the chamber wall and build-up cap and the contribution of scattered photons. Monte Carlo calculations were performed using the EGS4 code system with the PRESTA algorithm, to calculate the A(w) factor for 51 commercial ionization chambers and build-up caps exposed to the typical energy spectrum of 192Ir and 60Co gamma rays and 250 kV x-rays. The calculated A(w) correction factors for 192Ir and 60Co sources and 250 kV x-rays agree very well to within 0.1% with published experimental data (the statistical uncertainty is less than 0.1% of the calculated correction factor value). For the 192Ir sources, A(w) varies from 0.973 to 0.993 and for the 250 kV x-rays the minimum value of A(w) for all chambers studied is 0.983. The calculated A(w) correction factors can be used to calculate the air kerma calibration factor of HDR brachytherapy sources, when interpolative methods are considered, contributing to the reduction in the overall uncertainties in the calibration procedure.

Comparison of TLD calibration methods for dosimetry

“…( 20 ) However, it is expected this value will be less for , but the uncertainty in this value will be negligible compared with other factors. For the remaining factors, we used a combination of values published by Tolli and Johansson, ( 19 ) Ferreira et al, ( 21 ) Bielajew, ( 16 ) and Butler et al ( 11 ) and assumed their published uncertainty for each of these factors, resulting in an overall uncertainty as shown in Table 3. In addition to the uncertainty in deriving absorbed dose from the ionization chamber reading, we must also include the uncertainty in dose due to uncertainty in distance from the source (2%), as described in Method 1.…”

“…For the perturbation due to the chamber wall , Tolli and Johansson quote a value of 1.018 for the A‐150 chamber and 1.001 (at 50 mm) for the graphite wall chamber. Ferreira et al ( 21 ) quotes a value for (defined as the ionization chamber wall correction, which includes the PMMA cap) of 0.990 for the PTW chamber. We therefore used this value (0.990) in our calculations, though assumed an uncertainty of , as the chamber was placed directly in water and the cap not used.…”

A Feasibility Study of Fricke Dosimetry as an Absorbed Dose to Water Standard for 192Ir HDR Sources

“…The quantity S k for the 192 Ir source was determined using a Farmer-type cylindrical chamber calibrated by the Brazilian Accredited Dosimetry Calibration Laboratory, as proposed by Marechal et al [10] and Ferreira et al [11] and recommended by the IAEA [12] . The microSelectron HDR 192 Ir Alpha Omega source was used for the simulations and measurements.…”

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.…”