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

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/ρair (water), 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 First, to study the relationship between TPR20,10 and L̄/ρair (water) for FFF beams, where the flattening filter has been replaced by a metal plate as in most clinical FFF beams.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/ρair (water) by adding another beam-quality metric, TPR10,5. The relationship between L̄/ρair (water) 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/ρair (water), TPR20,10, and TPR10,5 using the EGSnrc Monte Carlo package. The relationship between L̄/ρair (water) 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/ρair (water) 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/ρair (water) 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/ρair (water) 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/ρair (water) predicted using IAEA TRS-398 and Monte Carlo calculations. An additional parameter for determining L̄/ρair (water) 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/ρair (water) ratio for beams both with and without a flattening filter at the investigated energies.