## Question

Why is the energy constancy factor in myQA Daily of different sensitivity for different energies? [Software version 1.3 and newer]

## 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 a certain combination of absorbers, so that the measured ratio makes most sense for practical purposes.

Here the combinations 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 combinations for electron energies:

Energy E |
Numerator |
Denominator |

E < 7 MeV | Aluminum | ABS |

7 MeV ≤ E < 9.5 MeV | Aluminium | Air (Drill) |

9.5 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, scale factors for deviation calculation and the relative deviation between the two values for different energies:

Energy |
Typcial Energy Factor |
Energy Factor for shifted PDD of 1 mm |
Scale Factor for Deviation Calculation |
Deviation [%] |

6 MV | 1.09686 | 1.11859 | 1.0 | 2.0 |

6 MV FFF | 1.15689 | 1.19138 | 1.0 | 3.0 |

10 MV | 1.11177 | 1.06953 | 1.0 | -3.8 |

15 MV | 1.32806 | 1.26361 | 1.0 | -4.9 |

6 MeV | 0.92403 | 0.90244 | 1.0 | -2.3 |

8 MeV | 1.06421 | 1.05109 | 1.7* | -2.1 |

10 MeV | 0.49292 |
0.46659 |
0.6* | -3.2 |

12 MeV | 0.69124 | 0.67005 | 0.6* | -1.8 |

15 MeV | 0.75291 | 0.73456 | 1.0 | -2.4 |

* Introduced with myQA Daily 1.3 to allow meaningful sensitivities.

This table shows that the myQA Daily device energy check is of **different sensitivity for different energies.**

**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.**