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Why is the energy constancy factor in myQA Daily of different sensitivity for different energies?

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Question

Why is the energy constancy factor in myQA Daily of different sensitivity for different energies?

Answer

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):

mceclip0.png

Here the details of the absorber material:

Absorber Material Thickness [cm]
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:

Energy E Numerator Denominator
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:

Energy E Numerator Denominator
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 1.09686 1.11859 1.98
6 MV FFF 1.15689 1.19138 2.98
10 MV 1.11177 1.06953 -3.80
15 MV 1.32806 1.26361 -4.85
6 MeV 0.92403 0.90244 -2.34
8 MeV 0.34111 0.32233 -5.51
10 MeV 0.57914 0.55336 -4.45
12 MeV 0.69124 0.67005 -3.07
15 MeV 0.75291 0.73456 -2.44

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.