myQA Daily - 2D Detector Array

The myQA Daily has an array comprised of 125 vented ionization chambers, distributed in a 5 mm grid over a 25 cm × 25 cm area. The ionization chambers are individually connected to high resolution current-todigital converters, measurement values are digitally transferred to the myQA Daily application servers via integrated wired and wireless networking interfaces.

Chamber placement has been optimized to suit the measurement of 10 cm × 10 cm and 20 cm × 20 cm radiation fields and groups of ionization chambers are used for individual measurements:

1. High-resolution centerline for field size and penumbra measurement

   28 ionization chambers for each centerline (1). The chambers are higher resolution in vicinity of standard 20 cm × 20 cm (A) or smaller 10 cm × 10 cm (B) penumbra regions to achieve higher measurement accuracy for the field size and penumbra measurements of standard 20 cm × 20 cm or smaller 10 cm × 10 cm beams.

2. High-resolution center-area for output measurement (2)

3.  Energy constancy checks 

   

   For energy constancy tests, the SW calculates an energy factor as the ratio of two energy chambers. The two energy chambers are predefined such that said energy factor is most sensitive to slight spectral changes of the selected beam quality.

   Four dedicated ionization chambers with integrated absorber material (3) for automatic verification of photon / electron energy constancy. The materials and thickness of the absorber are listed in Section 9.3.

   
   
   
   
    

4. Symmetry and flatness tests

   For symmetry and flatness measurements/tests the following number of chambers are used:

   • Field size of 10 cm × 10 cm: 25 chambers
   • Field size of 20 cm × 20 cm: 61 chambers

   

Table 4.1. Number of chambers from different QA tests

Beam Symmetry

Beam symmetry S_x and S_y is calculated from the uniformity- and k_TP -corrected measurement signals M_i,j of
the chambers for the two reference fields. Indices i and j denote the chamber position in X and Y direction,
with i= j = 0 at the center position.

For crossline symmetry, the ratio R_x,i,j = M_i,j / M_−i,j is calculated from the measurement signals of each chamber and its mirrored counterpart. The maximum ratio in percent is the measurement value for field symmetry in crossline direction: S_x = 100% · max{ Rx,i,j }. Similarly, the field-symmetry in inline direction S_y = 100% · max{ R_y,i,j }, where R_y,i,j = M_i,j / M_i,−j.

In the SW General Settings (Section 6.6.3.4) the user can select between the available symmetry algorithms:

• IEC 60976 original: The minimal possible value is 100% and every deviation from an ideally symmetric
  beam leads to an increase of this number.

• IEC 60976 directional: DSx,y is defined as DS_x,y = 100% · Rx,y,max where R_x,y,max is the ratio which has the largest difference to one: max{|1-R_x,y,i,j|}.

  An ideally symmetric beam results in a DS_x,y value of 100%. A tilted beam corresponds to values above or below 100%, depending on the direction of tilt.

• Mean difference directional:

  Crossline symmetry (DScross ) is defined as:

  

  Inline symmetry (DSin) is defined as:

  

  where Dose values = D_l (left), D_r (right), D_u (up), and D_d (down)

Note: If the symmetry algorithm has been changed, the reference measurement may no longer be valid and is it recommended to re-measure the baseline (see Section 7.1.4.2).

Beam Flatness

Beam flatness F is calculated from the uniformity- and k_TP - corrected measurement signals M_i,j of the chambers for the two reference fields. Indices i and j denote the chamber position in X and Y direction, with i = j = 0 at the center position.

F = 100.0% ▪ max{ M_i,j } / min{ M_i,j }

Note: Beam flatness is also calculated for FFF beams. In the case of FFF beams, it is normal to obtain unusually large values for the flatness parameters that also may be very sensitive to variations in beam center offsets. If this raises a problem, please increase the acceptance tolerance for the flatness parameter in the test-setup.

In the SW General Settings (Section 6.6.3.4) the user can select between the available FFF - Fieldsize algorithms.