The determination of beam quality correction factors: Monte Carlo simulations and measurements

Abstract

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine the absorbed dose at a user's beam contains the well-known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user's radiation. The dosimetry protocols describe the whole ionization chamber calibration procedure and include tabulated beam quality correction factors which refer to 60Co gamma radiation used as calibration quality. They have been calculated for a series of ionization chambers and radiation qualities based on formulae, which are also described in the protocols. In the case of high-energy photon beams, the relative standard uncertainty of the beam quality correction factor is estimated to amount to 1%. In the present work, two alternative methods to determine beam quality correction factors are prescribed-Monte Carlo simulation using the EGSnrc system and an experimental method based on a comparison with a reference chamber. Both Monte Carlo calculations and ratio measurements were carried out for nine chambers at several radiation beams. Four chamber types are not included in the current dosimetry protocols. Beam quality corrections for the reference chamber at two beam qualities were also measured using a calorimeter at a PTB Primary Standards Dosimetry Laboratory. Good agreement between the Monte Carlo calculated (1% uncertainty) and measured (0.5% uncertainty) beam quality correction factors was obtained. Based on these results we propose that beam quality correction factors can be generated both by measurements and by the Monte Carlo simulations with an uncertainty at least comparable to that given in current dosimetry protocols.

Measured and Monte Carlo calculated kQ factors: Accuracy and comparison

“…The Monte Carlo calculated k Q values for the Exradin A12 are up to 0.6% lower than the measured k Q factors of Seuntjens et al4 and show agreement with the less precise Monte Carlo calculated factors of Tantot and Seuntjens 26 well within the statistical uncertainties of that work (up to 0.5%). As mentioned above, Monte Carlo calculations of k Q agree within the statistical uncertainty (up to 1%, although within 0.7% for most chambers) of calculated k Q factors provided by González-Castaño et al18 for all chambers except for the IBA CC01. We performed calculations of the IBA CC01 with the same specifications of theFIG.…”

“…However, by analyzing the results for the large number of different chambers as a single data set, one can make stronger statements than would otherwise be possible. central electrode as used by González-Castaño et al 18 (private communication) but no improvement in agreement between the two sets of calculations was observed. The major difficulty with simulating small ionization chambers is that obtaining tight statistical uncertainty requires a significant amount of computing time.…”

“…Other publications 17,18 calculate k Q factors using the same equation [Eq. (3)] and a very similar approach but for a small subset of chambers and, in the case of the work of González-Castaño et al, 18 with much larger statistical uncertainty.…”

“…(3)] and a very similar approach but for a small subset of chambers and, in the case of the work of González-Castaño et al, 18 with much larger statistical uncertainty. Calculated k Q factors were presented in terms of a high-precision fit, with the % RMS deviation generally <0.1%, as a function of %ddð10Þ x for each chamber.…”

Comparison of AAPM Addendum to TG‐51, IAEA TRS‐398, and JSMP 12: Calibration of photon beams in water

“…Further, since the publication of AAPM TG‐51 and IAEA TRS‐398, Monte Carlo (MC) simulation methods8, 9 have been developed and accurately benchmarked10, 11, 12 for the calculations of detailed chamber geometries 13, 14, 15. Moreover, studies in current literature have provided ionization chamber perturbation correction factors15, 16, 17 and beam quality conversion factors 14, 15, 18.…”