Monte Carlo determination of k_Q and k_Qmsr values for the exradin A26 ionisation chamber for the Varian TrueBeam.

We have calculated conversion factors, k(Q) for the A26 micro ionisation chamber along with machine specific reference beam quality factors, k(Qmsr), for a number of field sizes and beam qualities for the Varian TrueBeam accelerator. The A12 ionisation chamber was simulated alongside the A26, so as to validate against known literature values. Both ionisation chambers were modelled from manufacturer data sheets and schematics. The egs_chamber Monte Carlo user code was used to simulate each absorbed dose relevant to the beam quality conversion factors k(Q) and k(Qmsr). Tabulated spectra for beam energies of 4 through 25 MV were used in the k(Q) calculations for both investigated chambers. Varian TrueBeam phase space files for 6 MV flattened as well as 6 and 10 MV unflattened beams were used in the simulations of the A26 chamber in field sizes from 2 x 2 cm square to 20 x 20 cm square in order to determine k(Qmsr) values. The PDD(10) x values of the tabulated spectra were found to be within variation between studies, with an average deviance of 0.4% from one prior study. The simulated A12 k(Q) values matched the accepted literature values with an average variation of <0.1%. The A26 k(Q) values match the manufacturer provided values to within 0.5%. For all investigated field sizes the k(Qmsr) values are within 0.006 of unity. There is no published data for this chamber for a direct comparison, but there is similarity between these results and results from other chambers regularly used in similar circumstances. Furthermore, the agreement of the simulated k(Q) values to knowns, and the agreement of the PDD(10) x factors would suggest the correctness and accuracy of the study.