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

Abstract

Purpose It has previously been shown that the use of a dual beam quality specifier consisting of two tissue-phantom ratios, TPR20/10 and TPR10/5, provides an accurate prediction of stopping power ratios (sw,air) for flattening filter free (FFF) photon beams with different filtration, using a bi-linear relationship between the two TPRs and sw,air. For the current project, the intention is to evaluate this dual beam quality specifier on kQ 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 kQ factors, TPR20/10 and TPR10/5 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 TPR20/10. Using data from TRS398 overestimated kQ 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, kQ 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 kQ factors for conventional photon beams of up to 0.9% was found using TRS398. Using a dual beam quality measure, consisting of both TPR20/10 and TPR10/5, KQ 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.