Why is the energy constancy factor in myQA Daily of different sensitivity for different energies? [Software version 1.2 and older] – IBA Dosimetry Service & Support

Question

Why is the energy constancy factor in myQA Daily of different sensitivity for different energies? [Software version 1.2 and older]

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):


Previous Hardware Platform	Current Hardware Platform

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.