Calculated beam quality correction factors for ionization chambers in MV photon beams

The beam quality correction factor, kQ, which corrects for the difference in the ionization chamber response between the reference and clinical beam quality, is an integral part of radiation therapy dosimetry. The uncertainty of kQ is one of the most significant sources of uncertainty in the dose determination. To improve the accuracy of available kQ data, four partners calculated kQ factors for 10 ionization chamber models in linear accelerator beams with accelerator voltages ranging from 6 MV to 25 MV, including flattening-filter-free (FFF) beams. The software used in the calculations were EGSnrc and PENELOPE, and the ICRU report 90 cross section data for water and graphite were included in the simulations. Volume averaging correction factors were calculated to correct for the dose averaging in the chamber cavities. A comparison calculation between partners showed a good agreement, as did comparison with literature. The kQ values from TRS-398 were higher than our values for each chamber where data was available. The kQ values for the FFF beams did not follow the same TPR20,10, kQ relation as beams with flattening filter (values for 10 MV FFF beams were below fits made to other data on average by 0.3%), although our FFF sources were only for Varian linacs.