Combining tissue-phantom ratios to provide a beam-quality specifier for flattening filter free photon beams

Authors

  • Dalaryd Mårten,

    1. Department of Clinical Sciences, Medical Radiation Physics, Lund University, P.O. Box 117, Lund SE-221 00, Sweden and Radiation Physics, Skåne University Hospital, Lund SE-221 85, Sweden
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  • Knöös Tommy,

    1. Department of Clinical Sciences, Medical Radiation Physics, Lund University, P.O. Box 117, Lund SE-221 00, Sweden and Radiation Physics, Skåne University Hospital, Lund SE-221 85, Sweden
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  • Ceberg Crister

    1. Department of Clinical Sciences, Medical Radiation Physics, Lund University, P.O. Box 117, Lund SE-221 00, Sweden
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Abstract

Purpose:

There are currently several commercially available radiotherapy treatment units without a flattening filter in the beam line. Unflattened photon beams have an energy and lateral fluence distribution that is different from conventional beams and, thus, their attenuation properties differ. As a consequence, for flattening filter free (FFF) beams, the relationship between the beam-quality specifier TPR20,10 and the Spencer–Attix restricted water-to-air mass collision stopping-power ratios, L̄/ρairwater, may have to be refined in order to be used with equivalent accuracy as for beams with a flattening filter. The purpose of this work was twofold. First, to study the relationship between TPR20,10 and L̄/ρairwater for FFF beams, where the flattening filter has been replaced by a metal plate as in most clinical FFF beams. Second, to investigate the potential of increasing the accuracy in determining L̄/ρairwater by adding another beam-quality metric, TPR10,5. The relationship between L̄/ρairwater and %dd(10)x for beams with and without a flattening filter was also included in this study.

Methods:

A total of 24 realistic photon beams (10 with and 14 without a flattening filter) from three different treatment units have been used to calculate L̄/ρairwater, TPR20,10, and TPR10,5 using the EGSnrc Monte Carlo package. The relationship between L̄/ρairwater and the dual beam-quality specifier TPR20,10 and TPR10,5 was described by a simple bilinear equation. The relationship between the photon beam-quality specifier %dd(10)x used in the AAPM's TG-51 dosimetry protocol and L̄/ρairwater was also investigated for the beams used in this study, by calculating the photon component of the percentage depth dose at 10 cm depth with SSD 100 cm.

Results:

The calculated L̄/ρairwater for beams without a flattening filter was 0.3% lower, on average, than for beams with a flattening filter and comparable TPR20,10. Using the relationship in IAEA, TRS-398 resulted in a root mean square deviation (RMSD) of 0.0028 with a maximum deviation of 0.0043 (0.39%) from Monte Carlo calculated values. For all beams in this study, the RMSD between the proposed model and the Monte Carlo calculated values was 0.0006 with a maximum deviation of 0.0013 (0.1%). Using an earlier proposed relationship [Xiong and Rogers, Med. Phys. 35, 2104–2109 (2008)] between %dd(10)x and L̄/ρairwater gave a RMSD of 0.0018 with a maximum deviation of 0.0029 (0.26%) for all beams in this study (compared to RMSD 0.0015 and a maximum deviation of 0.0048 (0.47%) for the relationship used in AAPM TG-51 published by Almond et al. [Med. Phys. 26, 1847–1870 (1999)]).

Conclusions:

Using TPR20,10 as a beam-quality specifier, for the flattening filter free beams used in this study, gave a maximum difference of 0.39% between L̄/ρairwater predicted using IAEA TRS-398 and Monte Carlo calculations. An additional parameter for determining L̄/ρairwater has been presented. This parameter is easy to measure; it requires only an additional dose measurement at 5 cm depth with SSD 95 cm, and provides information for accurate determination of the L̄/ρairwater ratio for beams both with and without a flattening filter at the investigated energies.

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