Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry

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Abstract

Purpose

The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well-known beam quality specifiers or energy-based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter.

Methods

Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios Symbol, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors kQ, this factor for detailed ionization chamber models was calculated by Monte Carlo simulations.

Methods

Stopping power ratios Symbol and kQ values for different ionization chambers as a function of Symbol and %dd(10)x were calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated.

Results

The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10)x and Symbol, whereby the relationship between Symbol and Symbol changed up to 0.8% for high energy photon beams. However, Symbol was a good predictor of Symbol for both types of linear accelerator with energies Symbol 10 MeV with a maximal deviation between both types of accelerators of 0.23%.

Results

According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of Symbol and kQ to merge the dosimetry of both linear accelerator types. It was possible to derive Symbol using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between kQ for linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for Symbol and %dd(10)x respectively.

Conclusion

The results of this study have shown that removing the flattening filter led to a change in the relationship between the well-known beam quality specifiers and dosimetric quantities at the point of measurement, namely Symbol, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams.

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