## 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):

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.**