Beam Attenuation

Conclusion

• No significant differences in O/P factors was detected.
• The beam attenuation is found to be a fixed amount for each energy.
• IQM has little effect on PDD beyond Dmax or on profiles <20x20cm.
• For 6 & 10 MV modelling at larger radial distances needed review.

Step2: Comparison with existing clinical models

Step3: Initial validation

Conclusions

• The build up region is modified by the presence of the IQM, reducing ‘skin sparing’.
• Smaller field PDDs & Profiles are not affected by IQM.
• Larger fields have shoulders drop, more notably at shallower depths.
• Adjustment of arbitrary fluence profile improved agreement – but not observable clinical benefit.
• Clinical models acceptable at 6MV & 10MV.
• Simple correction factor to change MUs was all that was required in Pinnacle 9.8.
• Validation of your clinical models necessary when integrating the IQM into your planning system, but modifications are likely to be small.

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.

Test results

Pearson’s product–moment correlation coefficient ρ indicates a positive linear correlation between surface dose and the radiation field size for all studied flattened and unflattened beams, however, except for the 20 cm x 20 cm field size for the 10 MV FFF beam, the change in surface dose did not exceed 3.3%.

Conclusion

This study demonstrates clinically negligible changes in the surface dose for all investigated beams. A modest increase of surface dose when IQM is in the beam path can be considered acceptable and not a limiting factor for its clinical use.
Therefore, the time needed to implement the IQM system can be shortened since no additional commissioning beyond taking the beam attenuation into account (as a transmission factor or modified output factor) is needed regarding beam properties such as beam hardening and surface dose.

Test results

The presence of IQM caused a small but measurable and statistically significant beam hardening effect. Beam hardening was most pronounced for the 6 MV FFF beam where it increased to 0.53%. Statistical significance of the beam hardening effect was demonstrated (p < 0.01) for all investigated beams except for 15 MV where no statistically
significant difference was found.

Conclusion

As beam quality changes were always lower than 0.53%, our conclusion is that this effect has no considerable clinical relevance.

This study demonstrates clinically negligible changes in beam quality for all investigated beams. Therefore, the time needed to implement the IQM system can be shortened since no additional commissioning is
needed regarding this beam property.

Conclusion

IQM beam attenuation can be considered to be homogeneous in both X and Y directions and the machine-specific beam attenuation percentage could be used to rescale treatment plan dose for clinical IQM use.