Test purpose
The influence of the IQM transmission detector on photon beam properties was evaluated in a pre-clinical phase, using data from nine participating centers. The beam attenuation due to IQM was measured and the dependence of the transmission factors on beam energy and field size was investigated.
Test method
The transmission factor was calculated by means of measured absorbed dose at 10 cm depth for all available energies and field sizes.
Test results
The mean transmission factors of the IQM detector showed statistically significant energy dependence for all investigated flattened and FFF beams. It ranged from 0.9412 to 0.9589 for standard beams 6 MV to 18 MV, while for 6 MV FFF and 10 MV FFF beams it was 0.9440 and 0.9533, respectively. While it might be argued that transmission factors close to 1 are preferable, the authors emphasize that every transmission factor has to be adequately incorporated into the TPS. On the other hand, variations in the range of 2% or more of the transmission factors versus field size can be considered as a shortcoming for the straightforward use within TPS. In that case no single value can be recommended and a new beam model needs to be introduced to the TPS. The maximal dispersion of the aggregated measurements from all participating centres and for all
field sizes from 1 cm × 1 cm to 20 cm × 20 cm was less than 0.49%, with a maximal cumulative uncertainty of 0.58%.
Conclusion
The transmission factors calculated for the attenuation due to the presence of IQM show energy dependence but only weak dependence on field size, with the maximum dispersion of measurements to be < 0.5% across 9 treatment machines, 6 energies and 9 field sizes.
The mean transmission factors of the IQM detector can be used either as tray factors within TPS for the particular linear accelerator energies or alternatively as modified outputs for the respective beam energies.
