Effect of Off-axis Ion Recombination Factor on the Beam Profile in Flattening Filter-free Photon Beams

抄録

The ion recombination factor (k_s) of a beam without a flattening filter differs from that of a filtered beam. In this study, we examined the effect of changing the measurement conditions on k_s in the off-axis direction, and clarified the effect on the beam profile. We calculated ks using the Jaffe plot and two-voltage method (TVM) by varying the measurement conditions, adding k_s,_rel, _off-ax to the beam profile, and comparing the changes via local gamma analysis. The central value of k_s increased with X-ray energy, and the effect become more pronounced when the measurement depth is varied. For the beam profile with high energy and a field size of 40 × 40 cm², the results of the local gamma analysis are lower than the reference value. At the maximum dose depth, the results are poor, even when the field size is 30× 30 cm². At 40× 40 cm², the results are lower than the reference value even when the criteria are further relaxed. Our results indicate that k_s differs depending on the measurement method, and thus, k_s,_rel,_off-ax should be considered when measuring beam profiles with field sizes larger than 30 × 30 cm².