Why is the energy constancy factor in myQA Daily of different sensitivity for different energies?
The myQA Daily device has four dedicated ionization chambers with integrated absorber material for automatic verification of photon/electron energy constancy (see 3a-d below):
Here the details of the absorber material:
|3a||Stainless steel||0.4 ± 0.01 + 0.2 ABS|
|3b||Air (Drill)||0.4 ± 0.01 + 0.2 ABS|
|3c||Aluminium||0.4 ± 0.01 + 0.2 ABS|
|3d||Tungsten||0.4 ± 0.01 + 0.2 ABS|
|Average of 9 central chambers||ABS||0.6|
For each beam quality (type and energy) there is an ideal combination of absorbers for which the measured ratio is most sensitive to slight changes in beam energy. These ideal combinations were first determined theoretically, later confirmed in experiment.
Here the results for photon energies:
|E < 5 MV||Tungsten||Stainless steel|
|5 MV ≤ E < 8 MV||ABS||Tungsten|
|8 MV ≤ E < 12.5 MV||Tungsten||ABS|
|12.5 MV ≤ E < 19.5 MV||Stainless steel||ABS|
|19.5 MV ≤ E||ABS||Tungsten|
And here the results for electron energies:
|E < 7 MeV||Aluminum||ABS|
|7 MeV ≤ E < 12.5 MeV||Stainless steel||Aluminum|
|12.5 MeV ≤ E < 19.5 MeV||Stainless steel||ABS|
|19.5 MeV ≤ E||Tungsten||Stainless steel|
The energy constancy check calculates those ideal ratios as energy factor.
The following table shows the reference energy factors, the energy factors when the PDD is shifted for 1 mm and the relative deviation between the two values for different energies:
|Energy||Typcial Energy Factor||Energy Factor for shifted PDD of 1 mm||Deviation [%]|
|6 MV FFF||1.15689||1.19138||2.98|
This table shows that the myQA Daily device energy check is of different sensitivity for different energies, e.g. most sensitive for 8 MeV.
Please adjust the tolerances in the myQA Daily software according to measurements in your water phantom in a meaningful way.
Your tolerances may be different for different energies.