Comparison of <i>k</i>_Q factors measured with a water calorimeter in flattening filter free (FFF) and conventional flattening filter (cFF) photon beams.

Recently flattening filter free (FFF) beams became available for application in modern radiotherapy. There are several advantages of FFF beams over conventional flattening filtered (cFF) beams, however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. Beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger. Despite several studies describing FFF beam characteristics, only a limited number of studies investigated their effect on <i>k</i>_Q factors. Some of those studies predicted significant discrepancies in <i>k</i>_Q factors (0.4 % up to 1.0 %) if TPR_20,10 based codes of practice (CoPs) would be used. This study addresses the question to which extent <i>k</i>_Q factors, based on a TPR_20,10 CoP, can be applied in clinical reference dosimetry. It is the first study that compares <i>k</i>_Q factors measured directly with an absorbed dose to water primary standard in FFF-cFF pairs of clinical photon beams. This was done with a transportable water calorimeter described elsewhere. The measurements corrected for recombination and beam radial non-uniformity were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers (8 serial numbers). The ratio of measured <i>k</i>_Q factors of the FFF-cFF beam pairs were compared with the TPR_20,10 CoPs of the NCS and IAEA and the <i>%dd</i>(10)_x CoP of the AAPM. For the TPR_20,10 based CoPs differences less than 0.23 % were found in <i>k</i>_Q factors between the corresponding FFF-cFF beams with standard uncertainties smaller than 0.35 %, while for the <i>%dd</i>(10)_x these differences were smaller than 0.46 % and within the expanded uncertainty of the measurements. Based on the measurements made with the equipment described in this study the authors conclude that the <i>k</i>_Q factors provided by the NCS-18 and IAEA TRS-398 codes of practice can be applied for flattening filter free beams without additional correction. However, existing codes of practice cannot be applied ignoring the significant volume averaging effect of the FFF beams over the ion chamber cavity. For this a corresponding volume averaging correction must be applied.