Beam quality correction factors for linear accelerator with and without flattening filter Damian Czarnecki1,3, Philip von Voigts-Rhetz1, Björn Poppe3, Klemens Zink1,2 1University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Gießen, Germany 2University Hospital Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany 3University Hospital for Medical Radiation Physics, Pius-Hospital, Medical Campus, Carl von Ossietzky Universität Oldenburg, BACKGROUND RESULTS As the use of flattening filter free linear accelerators (FFF) increases there is an urgent need to revise the existing dosimetry protocols which are based on linear accelerators with flattening filter (WFF). This study deals with this problem, comparing current dosimetric concepts to determine the beam quality correction factor kQ for different ionization chambers. Figure 1: Monte Carlo calculated beam quality correction factor kQ for the PinPoint ionization chamber PTW31014 as a function of current beam quality specifiers and photon spectrum based beam quality specifiers for WFF (black) and FFF beams (red). The polynomial fit for WFF beams publish by Muir et al. [2] is shown in the upper panels. The statistical uncertainties are within 0.1%. Figure 2: Monte Carlo calculated beam quality correction factor kQ for the ionization chamber NE 2571 as a function of current beam quality specifiers and photon spectrum based beam quality specifiers for WFF (black) and FFF beams (red). The polynomial fit for WFF beams publish by Muir et al. [2] is shown in the upper panels. The experimentally determined kQ for WFF beams published by Krauss and Kapsch [3] are symbolized by open circles. The statistical uncertainties are within 0.1%. OBJECTIVES The aim of this study was two determine the uncertainties when using the well known beam quality specifiers or energy based beam specifiers as predictor for the beam quality correction factor kQ when removing the flattening filter. MATERIALS & METHODS Monte-Carlo simulations were performed using the EGSnrc/BEAMnrc code system [1]. Nine virtual linear accelerators from three major vendors (Siemens, Elekta and Varian) were modelled based on data provided by them using BEAMnrc (see Fig. 1 & 2) [2]. The nominal energy of the LINACs was in the range of 4 MV to 20 MV. FFF radiation fields were implemented by replacing the flattening filter by a 2 mm thick aluminum layer. The PinPoint ionization chamber PTW31014 and Farmer- type ionization chamber NE2571 were modeled in detail according to manufacturer drawings, to calculate the beam quality correction factor kQ. The transport threshold/cutoff energies in all Monte Carlo simulations were set to AE = ECUT = 521 keV (electron) and AP = PCUT = 10 keV (photon). CONCLUSIONS REFERENCES The results show that removing the flattening filter led to a change in the relationship between kQ and all investigated beam quality specifiers. This change is caused not only by the different relationship between the beam quality specifier and the stopping-power ratio water-to-air for FFF beams but may also depend on the geometry of the ionization chamber. Moreover, the results suggest that the large volume chamber NE 2571 is not suitable for dosimetry in FFF beams. Apart form the ionization chambers NE2571, the results confirm that %dd(10)x can be used with an acceptable bias (< 0.5%) for FFF beams within the whole clinical used energy range . Restricting the x-ray beam energies of clinical FFF electron accelerators to the commonly used energy of 6 MV x-rays, TPR20,10 is still an eligible beam quality specifier with an bias of at most 0.3%. [1] I. Kawrakow, E. Mainegra-Hing, D. W. O. Rogers, F. Tessier, and B. R. B. Walters, “The egsnrc code system: Monte carlo simulation of electron and photon transport,” National Research Council of Canada, Report 350 PIRS-701 (2010). [2] B. R. Muir and Rogers, D. W. O., “Monte Carlo calculations of kQ, the beam quality conversion factor,” Medical Physics 37, 5939 (2010). [3] A. Krauss and R. P. Kapsch, “Experimental determination of kQ factors for cylindrical ionization chambers in 10 cm × 10 cm and 3 cm × 3 cm photon beams from 4 MV to 25 MV,” Physics in medicine and biology 59, 4227– 4246 (2014). CONTACT INFORMATION: Damian Czarnecki damian.czarnecki@lse.thm.de