P288] Use of organic plastic scintillators for determination of the beam quality correction factor for flattening filter free photon beams

Abstract

Purpose The purpose was to use organic plastic scintillators for determination of the beam quality correction factor ( k Q , Q 0 ) for flattening filters free (FFF) photon beams increasingly used in modern radiotherapy. The values were compared with those recommended in the recent TRS-483 code of practice from the International Agency of Atomic Energy. Methods The formalism in TRS-483 covers not only small conventional photon beams with flattening filter (WFF), but also flattening filter free (FFF) beams. TRS-483 splits the k Q , Q 0 into two correction factors for FFF beams: k Q WFF , Q 0 f ref and k Q FFF , Q WFF f ref . An IBA FC-65G ionization chamber, calibrated for conventional MV photon beams, was used to experimentally determine the k Q FFF , Q 0 . The fiber-coupled scintillator (1 mm diameter; 2 mm length) was used as a water-point surrogate detector for determination of the k Q , Q 0 from quality Q FFF with respect Q WEF . Given the high degree of water-equivalence of this detector, and in line with the studies presented in TRS-483, it was assumed that the detector was perturbation free. Corrections were, however, made for ionization quenching based on published parameters for the used scintillator and Birks formalism implemented in the Monte Carlo toolkit EGSnrc. Results The spectral difference between WFF and FFF resulted in changes in the ionization quenching which in turn affected the FFF/WFF-ratio in the TRS-483 formalism by about 0.6% for 6 MV and by 0.35% for 10 MV, respectively. After correcting for this, the beam quality correction factor for the FFF 6 MV and 10 MV beam was found to differ insignificantly, 0.3%, compared with those recommended in TRS-483 when given that uncertainty of the ionization quenching correction factor alone was about 0.15% and that additional uncertainty components (>0.5%) are associated with the measurements. Conclusions An experimental method for determining the beam quality correction factor for FFF to WFF was carried out. The study supports that organic plastic scintillators can be used as a perturbation-free detector in MV photon beams. However, considerations should be given to the change in quenching when spectrally different beams are involved such as when computing the dose ratio for WFF and FFF beams in the TRS-483 formula for beam quality correction factor.