[P190] Investigation of beam quality correction factors for flattening filter free photon beams

Purpose It has previously been shown that the use of a dual beam quality specifier consisting of two tissue-phantom ratios, TPR 20/10 and TPR 10/5 , provides an accurate prediction of stopping power ratios (s w,air ) for flattening filter free (FFF) photon beams with different filtration, using a bi-linear relationship between the two TPRs and s w,air . For the current project, the intention is to evaluate this dual beam quality specifier on k Q factors used in radiotherapy reference dosimetry, calculated using Monte Carlo modelling. Methods Two different ionisation chambers were used in the simulations, NE2571 and FC65-G. Calculations of k Q factors, TPR 20/10 and TPR 10/5 were performed using EGSnrc, and the chamber geometry was acquired from technical specifications from the manufacturer and from previously published articles. A total of 18 photon beams (10 with flattening filter (FF) and 8 flattening filter free) were used. Results For the FFF beams, the calculated beam quality correction factors were found to be 0.1–0.3% lower than for the beams with a flattening filter at the same TPR 20/10 . Using data from TRS398 overestimated k Q by up to 1% for FFF beams but also up to 0.9% for FF beam for the FC65-G chamber. The corresponding numbers for the NE2571 were 0.6% for both FF and FFF beams. Using the dual beam quality metric fitted to the calculations, k Q factors could be predicted within 0.12% for all beams. Conclusions Beam quality conversion factors for NE2571 and FC65-G farmer type ionisation chambers have been calculated, for beams with and without a flattening filter. Differences between beams with and without a flattening filter were found to be 0.1–0.3%. An overestimation of k Q factors for conventional photon beams of up to 0.9% was found using TRS398. Using a dual beam quality measure, consisting of both TPR 20/10 and TPR 10/5 , K Q factors could be predicted to within 0.12%. Although the difference between FF and FFF beams is small, it is a systematic difference which can be accounted for.